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SCIENTIFIC REPORT

FINDINGS OF WESTERN VICTORIAN LAKES EEL DEATH INVESTIGATION 2004—06 PAUL LEAHY, ALEX LEONARD, RENEE JOHNSON

Publication 1114 March 2007

1. SUMMARY

Since spring 2004 EPA Victoria has been investigating short-finned eel (Anguilla australis) and fish death events in 10 water bodies across Victoria. These water bodies include three sub-catchments of the Yarra River, two freshwater lakes in the Greater Melbourne area and two estuarine embayments. All events have involved multiple fish, with the majority of the deaths involving fewer than 100 fish. The common factor in all deaths is 10 years of drought, from 1997 onwards. Although fish populations have suffered, they should recover given sufficient rainfall. Birds are still abundant and diverse at the lakes, with wading birds utilising the mudflats along the receding shoreline.

The largest event occurred in lakes within Victoria’s Western District and constituted more than 95 per cent of all eels that have died in the last two summers. At Lake Modewarre there have been deaths in two separate seasons, totalling 30,000 carp and 50,000 eels. At Lake Bolac approximately 5000 eels have died. As a result of the deaths mainly being associated with the western lakes, the investigative effort and the content of this report is also focused on these lakes.

As of November 2006, the exact numbers of eels remaining in the Lake Modewarre and Lake Bolac is not known. The only estimates of stock size are anecdotal, based on the quantity of eels moved into these water bodies by commercial eel fishers. It is estimated that Lake Bolac has over 90 per cent of its eels surviving. However, the number of eels surviving in Lake Modewarre is likely to be low, given the pH had risen to over 10 in October 2006.

EPA has undertaken detailed investigations at the sites of the largest eel deaths, Lake Modewarre and Lake Bolac, both of which have been important recreational and commercial eel fisheries that have been stocked with large quantities of eels in the past. EPA has collected information to investigate hypotheses on the deaths. After construction of a conceptual model based on expert opinion and review of the data, a number of hypotheses can now be ruled out or are not of a likelihood that justifies further investigation at this time.

This includes the effect of ammonia, sulfide, eel parasites, overpopulation, association with worldwide declines, and toxicity from metals (aluminium excepted) and organic contaminants.

Figure 1: Eels on the shore of Lake Bolac.

The results of the investigation indicate that multiple factors influenced by the persisting drought have led to the fish deaths. Monitoring of Lake Modewarre and Lake Bolac in the summer of 2005—06 showed a common set of conditions that preceded the eel deaths, all related to 10 years of below-average rainfall. This included a 30—40 per cent rise in salinity for the three-to-six-week period preceding the deaths, alkaline pH (>8.5) and an air temperature of 35 °C or greater on the day of the deaths or the day before the deaths.

FINDINGS OF WESTERN VICTORIAN LAKES EEL DEATH INVESTIGATION 2004—06

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Further information needs to be collected before causality can be attributed to these factors.

At Lake Modewarre and Lake Bolac other fish were associated with the eel death events, indicating that whatever killed the fish is not specific to eels in these events. Further information on other fish species present in these water bodies will be collected in the summer of 2006—07.

The investigation in the summer of 2006—07 will attempt to resolve the remaining key questions surrounding the deaths.

In conjunction with this report, EPA has released A review of historic western Victorian lake conditions in relation to fish deaths (EPA Victoria 2007), which reviews historical and anecdotal evidence for fish kills in a number of Western District lakes, and A review of eel biology: knowledge and gaps (McKinnon 2006), which reviews the state of knowledge of eel biology in Victoria.

2. INTRODUCTION

In the summers of 2004—05 and 2005—06 eel deaths occurred in Victoria (Figure 1). These events occurred in southern Victoria and were most severe in the Western District of the state (Figure 2).

The deaths have occurred during a period of prolonged drought, the effects of which are apparent on the lakes (Figure 3). The cause of the deaths is clearly complex, but drought is a key part of the puzzle.

In May 2006 the Eel Death Investigation Reference Group was set up to guide EPA in its investigations into the cause of the eel and fish deaths. As part of this, a major scientific review of all information collected was undertaken with the help of external scientific experts. This report presents the results of this review.

The approach taken in this review is a risk-based one following US EPA (1998). The risk-based approach allows for the consideration of a multitude of possible causes of the deaths, but weights each according to its likelihood and the consequence if it is the cause. The study has largely employed a qualitative hazard screen of all hypotheses to increase the focus of the study. Risk-based assessments were adopted by EPA Victoria in the State Environment Protection Policy (Waters of Victoria) (EPA 2003). This study sought to examine hypotheses surrounding the causes of eel deaths, focusing on the large events (more than 100 deaths) that have occurred in the stock-enhanced lakes of the Western District since the summer of 2004.

Figure 2: Map of eel kills 2004—06.


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Figure 3: Water level decline at Lake Modewarre (upper photo September 2005, lower photo September 2006).

3. BACKGROUND

3.1 Victorian eel deaths

3.1.1 2004—05 deaths

Between October 2004 and March 2005 eel deaths were recorded in 10 different waterways, mostly during the summer (EPA Victoria 2005a). Although most of the deaths in this period were of fewer than 100 eels, there were large deaths at Lake Modewarre and the Royal Botanic Gardens, Melbourne (EPA Victoria 2005a).

The largest fish death event of the 2004—05 summer consisted of over 30,000 carp at Lake Modewarre in December 2004 (EPA Victoria 2005b). Coinciding with these deaths, anecdotal reports indicate brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) disappeared from recreational catches, and the deaths of small native fish including common galaxias (Galaxias maculatus) were also observed in Lake Modewarre. These deaths were followed by the appearance of 5000 dead eels in Lake Modewarre in January 2005.


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3.1.2 2005—06 deaths

In the period between October 2005 and February 2006, EPA investigated eel deaths in five waterways. Another death event occurred in Lake Martin — another Western District Lake — during this period, although it was not reported at the time of the deaths. Five of the six events in 2005—06 occurred in the summer season and all but one were in lakes. In 2004—05, most deaths occurred in lowland rivers.

In the 2005—06 season, deaths recurred at Lake Modewarre. The death of 5000 eels in January 2005 was followed in January and February 2006 by 20 tonnes of dead eels (an estimated 50,000 individuals). Lake Modewarre is the only waterway to have experienced a repeat of the eel deaths over the two consecutive summers. There are now doubts amongst commercial fishers whether there are any eels surviving in this lake, let alone commercially viable quantities.

Another large eel death event was recorded at Lake Bolac, where over 5000 eels died in January 2006. Fifty redfin (Perca fluviatilis) also died, along with small numbers of some larger native fish including yellowbelly or golden perch (Macquaria ambigua) and Murray cod (Maccullochella peelii peelii). Smaller native fish (galaxias, gudgeons), eels and redfin have been caught regularly in eelers’ nets since these deaths.

In January and February 2006, a local landholder observed more than 200 dead eels (40—80 cm long) on the shore of Lake Martin, with some redfin carcasses also observed (John Darcy pers. comm., local landholder). In September 2006, EPA testing showed salinity in Lake Martin was 25 per cent higher than seawater and the maximum water level depth was less than 50 cm.

Much smaller events (fewer than 100 eels) occurred at a small golf course wetland in Dromana, the Eumerella River and at Lake Colac.

The eel deaths in the Western District lakes mostly involved large females that appeared to be in good condition.

A summary of all large death events (more than 100 eels) since 2004 is presented in Table 1. Small eel death events (fewer than 100) were left out, as many of these eel deaths may have been due to natural mortality or possibly from the poor maintenance of eelers’ nets, set for eel capture. This excludes Lake Colac, where the death of fewer than 100 eels was included to allow comparison of conditions at this lake to lakes Modewarre and Bolac, where the largest events occurred. As the deaths were predominantly a summer phenomenon (Figure 4), the deaths are referred to as the 2004—05 and 2005—06 summer seasons.

Table 1: Details of significant (>100) eel kill events in Victoria since October 2004 and types of information collected by the EPA.

* Numbers of surviving eels are anecdotal estimates based on eels stocked in the lake by commercial eel fishers (Bill Allen pers. comm., 88 Golden Eels).

** Lake Colac is included for comparison with other Western District lakes.

Number of short-finned eels Other fish species

Lake Kill date

Dead Survivors* Dead Survivors

Macroinvertebrates

In situs

Water m

etals

Soluble water m

etals

Sediment m

etals

Water organics

Sediment organics

Water chem

istry

Histopathology

Gross pathology

Gross morphology

Virology

Bacteriology

Tissue organics

Tissue metals

Algal

23/12/04 1

30,000 carp ? 1 1

31/01/05

5,000

3 Galaxiids ? 1 1 1 11 1 1 1 1 1

27/01/06 5000 1 1 1 ## ## 11 1 1 1

17/02/06 1 1 1 1 1 1

Modewarre

24/02/06 >30,000

?0 ?

1 1 1 1

09/01/06 >50 000 50 Redfin, 2 Yellowbelly 1 1 1 1 1 1 1 1 1Bolac

10/01/06 5,000

1 Murray cod ? 1 1 1 1 1 1 1 1 1 1 1 1 1 1

21/01/06 Martin

7/02/06

>300 In tributaries? Redfin? ?

Colac** 23/01/06 <100 >150 000

? 1 1 1 1 1 1 1 1 1 1 1 1 1 1


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3.2 Worldwide and interstate eel populations

Eel populations worldwide have been in decline but there are no consistent explanations for these declines. Declines in the northern hemisphere have seen slow reductions in catch sizes and in the recruitment of juveniles (Dekker et al 2003). Theories on the declines include pollution, particularly by polychlorinated biphenyls (PCBs) (Palstra et al 2006), climate change (Knights 2003) and overfishing (ICES 2005). The decline of eels over such large parts of the world is a cause of concern and ongoing research (e.g., ICES 2005).

Declines in the New Zealand long-finned eel fishery have also been noted and seem related to overfishing and blockage of riverine migratory routes by impoundments (Bruno David, pers. comm., NZ Department of Conservation).

With a few exceptions, overseas declines have not involved large numbers of eels dying suddenly, unlike in Victoria. The notable exception is Lake Balaton in Hungary where around 300 tonnes of eels were found dead in 1991, and subsequently 30 tonnes of eels were found dead in 1995 (Bálint et al 1997). The suggested cause of the deaths at Lake Balaton was the insecticide deltamethrin, used for mosquito control, although other factors included high summer water temperatures, low dissolved oxygen, a parasitic nematode, Anguillicola crassus (Bálint et al 1997). In Victoria this pesticide is used in termite control, household insect sprays and in aerial spraying of cereal crops during wet years for armyworm control.

Other examples of large eel death events have involved infectious diseases, particularly those of bacterial or viral origin, and have usually been restricted to intensive aquaculture (eg Amaro et al 1992).

Figure 4: Eel kills and air temperature at Melbourne Airport (compiled from BOM data).


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Three cases of eel deaths have occurred interstate in the last two years. These have occurred in NSW in the Myall Lakes (near Newcastle), Centennial Park in Sydney (NSW DPI, pers. comm.) and in Queensland in a pond at a hydroelectric operation (FRC Environmental 2006). The cause of the NSW deaths remains unknown, while the Queensland deaths were suspected to be due to a combination of poor water quality and an unidentified bacterial infection (FRC Environmental 2006).

3.3 Historical eel deaths

3.3.1 Western District lakes

Victoria’s Western District lakes are important commercial and recreational fisheries. Over the last 10 years, several important trout and eel fisheries have been lost, including those at Lake Murdeduke and Lake Gnarput.

There are historical reports of the deaths of large numbers of eels in Victoria (EPA 2006). In particular, historical accounts of Lake Bolac show that early explorers to the region encountered thousands of dead eels at Lake Bolac in 1841, attributed to low water levels.

‘The water in the lake was nearly dried up… Dead eels [were] 3 feet and a half long and 3 inches thick… Dead eels lay in mounds, thousands of dead eels, and very large too’, according to GA Robinson (1841).

It appears then that eel deaths may not be a new phenomenon. A number of lakes that would have contained fish have been documented as drying up since European arrival (e.g., Lake Modewarre), yet historical records of fish deaths are scarce (EPA 2006). It is highly likely that fish death events have occurred in the past at several Western District lakes but that they were not noticed, were unreported or the reports have since been lost.

3.3.2 Estuaries

Although there are very few reports of eel deaths in estuaries before 2004, it is a commonly held belief of commercial eel fishers that a small number of eels die each year in estuaries while preparing to migrate to sea. Accurate natural mortality levels are not known. Reports of small (fewer than 100) eel death events in Victorian estuaries since October 2004 have included the Yarra, Eumerella, Tarwin and Snowy Rivers.

Figure 5: Lifecycle of short-finned (A.australis) and long-finned (A.reinhardtii) eels. (Adapted from McKinnon 2006).


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3.4 Eels in Victoria

The following summarises a report to EPA by McKinnon (2006) on the state of knowledge with regard to eel biology.

3.4.1 Eel life cycle

The genus Anguilla comprises 15 species, of which two species — the short-finned eel (Anguilla australis) and the long-finned eel (Anguilla reinhardtii) — are found in Victoria. The great majority of deaths reported to EPA have been short-finned eels. The natural distribution of the long-finned eel within Victoria is restricted to the east of Melbourne, whereas the short-finned eel has a wider distribution, being common in all coastal draining waterways.

Anguillid eels are catadromous, which means they spend most of their complex life cycles in lakes, rivers or estuaries before migrating to sea to spawn and dying shortly afterwards. The life cycle of Victorian eels is summarised in Figure 5. Spawning is thought to occur in the Coral Sea, north-east of Australia, from October to March for short-finned eels, and from July to October for long-finned eels (Allen et al 2002).

Newly hatched larvae or leptocephali are transported by the south equatorial current towards Australia and metamorphose into glass eels at 50—60 mm in size. Glass eels then move into estuaries along the east coast of Australia. When the glass eels are between one and five years old and still less than 30 cm long, they become pigmented and change from a saltwater to a freshwater fish. They move up into lakes and rivers and are called elvers (McKinnon 2002). For much of their early lives the sex of the eels remains undetermined. Although females tend to dominate, it is uncertain what determines the male-to-female sex ratio — but population density or habitat probably play significant roles (McKinnon 2006).

As the elvers become larger they are known as yellow eels. Full sexual maturity is usually reached at 10—20 years of age. In preparing to migrate back to sea, eels undergo large morphological changes, initially with a reduction in the size of the stomach and development of sexual organs, and then a silvering of the body colour and an enlargement of the eyes. Eels cease to feed and rely on fat stores to sustain them during migration, which is undertaken between January and March (McKinnon 2006). It has been

Figure 6: Eel catch data from western Victoria from 1979 to 2005. Rainfall is an average derived from Terang, Lismore and Buckley.

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Short finned eels

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Rainfall

Average rainfall (1919-1996)


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reported that eels in landlocked lakes will undergo preparations to migrate but, if migration is not possible, they will return to feeding and delay their migration (Van der Thillart et al 2005 cited in McKinnon, 2006). The effect of being unable to migrate for a sustained period of time is unknown.

At maturation, female short-finned eels range from 80—130 cm in length and can weigh over 6 kg, with males being much smaller at around 50 cm in length and weighing 250 g (McKinnon 2002). Long-finned eels are generally much bigger than short-finned eels.

3.4.2 Commercial eel fishery

The Victorian eel fishery is the largest producer of eels in Australia, generating 54 tonnes in 2005—06, worth $550,000 (DPI Fisheries Production Bulletin, in press). The eel fishery is dominated by catches of short-finned eels, comprising 70 per cent of the total eel catch.

A number of lakes are operated as stock-enhanced waterways. Stock enhancement involves the translocation of elvers and yellow eels from rivers into lakes and dams for extensive growing, and makes up just under half the Victorian short-finned harvest. The majority of stock-enhanced waters are lakes located in

western Victoria and include the lakes where eel deaths have occurred (Lakes Modewarre, Colac and Bolac). Wild caught long-finned and short-finned eels make up the remainder of the eel fishery and are commonly caught as migrating adult eels from estuaries and rivers.

Prolonged drought conditions since the mid-90s have had a dramatic impact on Victorian eel fisheries (McKinnon 2006), with harvests of both wild and stock-enhanced short-finned eels declining markedly from a peak of 267 tonnes in 1993 to an average of 55 tonnes in the last five years (DPI Fisheries Production Bulletin, in press). Figure 6 shows that wet years tend to have larger eel harvests. This underlines the critical link between eel numbers and climate conditions.

In contrast, the long-finned eel fishery, based in eastern Victoria, has maintained harvests of 10—50 tonnes annually (DPI Fisheries Production Bulletin, in press) and appears to be increasing in recent years. Eastern Victoria has been less severely affected by the last 10 years of drought (Figure 7), and the catches of eels seem to reflect this.

Figure 7: Rainfall deciles in Victoria since 1996 (source: BOM).


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3.5 Limnology of Western District lakes

Studies of the limnological conditions of Western District lakes suggest that these lakes are relatively homogenous with respect to physical and chemical conditions. These lakes were formed as a result of volcanic activity and are surrounded by basaltic soils, which result in the lakes being naturally saline and alkaline (Pollard 1971). The lakes are generally sinks for nutrients and salt. Discharges from the lakes only occur during wet years.

The affected lakes are shallow (less than 2 m in depth) and, as a result, are well mixed and show no significant variation in water quality with increasing depth. Due to their shallow nature, sediments are easily resuspended by wave action, which can lead to very high suspended solid concentrations. Water levels are maintained primarily through groundwater and the evaporation/ precipitation balance. The intermittent nature of natural inflows means they tend to fluctuate seasonally due to high evaporation rates in summer.

The affected Western District lakes have large nutrient loads within the sediments that can result in nutrient release into the water column. Phosphorus is the limiting nutrient to primary productivity in lakes Bolac and Modewarre (Conder 1998, Bostock 2000), whereas

nitrogen is the limiting nutrient in Lake Colac (Khan 2003).

Although some work on the influence of saline groundwater on loads of salt and consequently concentrations of salt in these lakes has been done (e.g., Nicholson et al 2003), its role is not well understood.

The aquatic macrophyte communities vary between lakes, with Lake Modewarre historically dominated by submerged water milfoil (Myriophyllum sp.), with seagrass (Ruppia sp.) also present (Conder 1998). Seagrass is now the dominant macrophyte in Lake Modewarre due to its greater salt tolerance, and is increasing rapidly in its extent. In recent times Lake Bolac and Lake Colac have only had small stands of emergent macrophytes, like Phragmites and Triglochin. Recent reports suggest more salt-tolerant macrophytes, including Ruppia sp. and Lamprothamnium sp., are starting to appear at Lake Bolac (Gervasi 2006). Macrophytes provide refugia for aquatic life and prevent resuspension of sediment. The relative lack of macrophytes in lakes Bolac and Colac may be contributing to the overall decline of lake health. Macrophytes can improve water quality by their roots stabilising soft sediments and through reducing nutrient concentrations in waters that may otherwise produce harmful algal blooms.

Figure 8: Conceptual model of eel kills in Western District lakes.


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4. METHODS

EPA has undertaken sampling for a broad range of parameters in water, sediments and fish tissues in response to significant eel deaths (more than 100 individuals — refer to Table 1). This has included sampling for fish pathology, algae, some algal toxins, metals, pesticides, industrial chemicals, macroinvertebrates, nutrients, dissolved oxygen, pH, salinity, suspended sediments and water temperature.

In addition to spot measurements taken in response to eel death events, EPA installed water quality loggers at lakes Modewarre, Colac, Bolac and Tooliorook (another eel fishery lake) in January and February 2006, to record a range of water quality parameters hourly during the warmer months.

Much of this sampling was conducted while the eels were dying, particularly during the 2005—06 season, or just after the fish had died.

In addition to sampling in affected water bodies during deaths, EPA has also undertaken sampling in three water bodies with unaffected eels present (Lake Tooliorook, Burrumbeet Creek and Mill Swamp).

Before the information was analysed, a conceptual model of the cause of the fish deaths was developed by external experts and EPA scientists (Figure 8). This expertise included fish biologists, fish pathologists, limnological chemists and a professional eel fisher who operates in the Western District lakes. The conceptual model was used to generate a number of hypotheses on which stressor(s) (such as high pH) explain the mass eel mortalities. The complexity of the conceptual model (Figure 8) can be reduced to three groups of hypotheses: those associated with the environment, fish biology or pathogens — or overlaps between these three groups.

Information collected on fish deaths in the Western District lakes has been used to categorise the potential (low or high) for stressor(s) to explain the fish deaths. The data review assessed multiple lines of evidence including field observations of exposure and fish mortality, pathology and toxicology. This is in addition to laboratory-derived tolerances and physiological information.

Figure 9: Gauge height versus rainfall at Lake Bolac. Rainfall is an average derived from Terang, Lismore and Buckley.

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Hypothesis Evidence for Evidence against Stressor potential

Comment

Drought-related stress Historical fish kills at Lakes Murdeduke, Gnarput, and Colongulac. Recent kills at Modewarre and Bolac.

High Still under investigation

Declining water quality due to drought and climate change

Reduced water quality impacts on the gill, vital to both osmoregulation and respiration.

Eel specific? High Still under investigation

Salinity (conductivity) Conductivity rose steeply before deaths and for Lakes Modewarre and Bolac were within the range known to cause mortality in eels, carp and Redfin. Tolerance to conductivity 'may' be reduced by high pH and turbidity.

Eels "maybe" physiologically capable of dealing with observed increases in conductivity in the laboratory. Conductivity in Lake Colac was below those levels asscoiated with mortality.


pH Observed pH (9-9.5) associated with toxicity and physical symptoms observed in dead eels. pH causes gill damage making fish more susceptable to both conductivity and DO.


Dissolved oxygen The effects of pH, conductivity and high temperatures may cause DO to have played a role in the eel deaths.

Dissolved oxygen has never been recorded below 47% saturation which normally would not kill eels


NH3 and H2S Dissolved oxygen not low enough to cause high H2S. Measured levels of NH3 not high enough to cause toxicity.

Low Not being actively investigated

Gill clogging from sediment High turbidity in Lake Colac and Bolac, major organ potentially impacted is again the gill.

Low turbidity in Lake Modewarre during kills High Still under investigation.

High air and water temperature

Clear correlation in summer 2005-06 Water temperatures recorded not normally lethal to eels High Still under investigation

Fish killing toxins from Golden Algae

Some of the characteristics of the deaths are what would be expected with Golden algal toxins. Have often been hard to detect overseas

As yet no common suspect found in numerous samples. No conclusive pathological changes in fish to match a suspect against

High Algae can be difficult to detect due to short bloom duration or they are difficult to find in the environment (eg located in sediment). Still under investigation.

Infectious disease Isolation of infectious disease agents detected in a small number of cases

Infectious disease agents have been rare High Only isolated cases, perhaps is more symptomatic of fish under stress than a cause. Monitored, but not high priority

Not eel-specific Kills occurred in waters stocked with eels, so dominance of eels expected, but other dead fish species have been found in association with dead eels.

Dominance of eels in kills. Environmental tolerance of life stages associated with migration 'may' be more sensitive to stress. Still under investigation.

High The great majority of reports have been eels, possible that other fish are less visible or eaten by birds first. Still under investigation

Overpopulation of eels There are large populations of eels, particularly in the Western District Lakes

The eels were in good condition so there was enough food & the low proportion of male eels suggests overpopulation is not an issue


Trace metal pollution Some high values for aluminium and selenium Unlikely to be cause of deaths over such a large area Low Only aluminium is under active consideration at Lake Bolac

Organic contaminants None None found Low Not being actively investigatedAssociation with worldwide declines

In Australia and overseas, the conclusive cause of eel declines remains unknown

Mass mortalities have not been common, losses of eels overseas seem to have different characteristics.


Table 2: Assessment of proposed hypotheses of stressors contributing to the eel deaths in the Western District lakes.


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5. FINDINGS

5.1 Overview

The summary of the evidence for and against which stressors are contributing to the eel deaths is presented in Table 2. The potential for stressors to contribute to the eel deaths was categorised as either high or low, consistent with a hazard screening approach. The information in Table 2 is a summary of the detailed reviews of each stressor presented in the subsequent sections. The first section describes those stressors with the highest potential to account for the fish deaths.

5.2 Likely cause of the fish deaths

5.2.1 Impact of falling water levels on the Western District lakes

The effect of the current drought on Lake Modewarre and Lake Bolac has been associated with dramatic changes in these shallow lakes (Figure 3). Although changes in rainfall are apparent in the Western Districts (Figure 9), the most important change has been the decline in run-off into the lakes. This has resulted in declining water levels in Lake Bolac (Figure 9) and Lake Modewarre (Figure 3).

Despite close to average rainfall last year, stream flows in the Western District were less than 40 per cent of the average flow (DSE, 2006). The reduction in water reaching water bodies may be due to changes in the rainfall pattern, particularly low autumn rainfall (Greg Day pers. comm., DSE), or a decrease in the frequency of larger rainfall events. Stream flows since 1997 are about a quarter of the long-term average, exemplified by flows in the Moorabool River (Figure 10) directly east of the Western District lakes.

An understanding of the water budget within each lake catchment is required to fully understand the management options that would be required to sustain these lakes as viable fisheries. In some cases, land use change, water usage and a reduction in catchment area could be compounding the effect of drought (Clifton et al 2006).

5.2.2 Declining rainfall, rising salinity

Although the 1982—83 drought saw much lower annual rainfall than the previous 10 years before 2006, the effect on Lake Bolac was not as severe as conditions in 2006, particularly in relation to salinity (Figure 11). This is because of the cumulative effect of below-average rainfall. Water levels have declined gradually during the last 10 years, while groundwater and stream flow have contributed salt, with no flushing of the lakes by overflow. This decline in water level has caused changes in water quality, including increases in salinity

at Lake Modewarre and Lake Bolac that have accelerated in recent years (Figure 11).

Historical climate and water quality data show that electrical conductivity (a measure of salinity) has risen almost exponentially at Lake Modewarre and is rapidly increasing at Lake Bolac (Figure 11). Electrical conductivity in Lake Colac has been less influenced by the drought, due to higher inputs of freshwater (Figure 11).

Although rainfall is a key element of the salinity increase in the Western District lakes, other factors such as the timing and size of rainfall events, evaporation, groundwater input and changes to catchments’ surface drainage are likely to have played a role in the salinity increase.

5.2.3 Drought and fish deaths in the Western District lakes

Over the last 10 years a number of Western District lakes have dried up, at least seasonally during summer, including Lake Gnarput and Lake Colongulac. It is possible that Lake Modewarre and Lake Bolac will also dry up in the near future if the current water level declines continue. Repeated fish deaths (of more than 100 individuals) in the western lakes were observed at lakes Murdeduke, Gnarput and Colongulac in recent years and these no longer support commercial eel fisheries due to extremely high salinities.

It has been suggested that deaths in these lakes were the forerunners to the deaths seen elsewhere in 2004—06. The large rises in salinity are therefore consistent factors associated with these deaths and those at lakes Modewarre and Bolac. However, few data exist on the water conditions in lakes Murdeduke, Colongulac and Gnarput at the time of the fish deaths, so the potential causes remain anecdotal.

Summary of the effect of drought

It is highly likely that the eel deaths in the Western District lakes were related to drought-related stress across these water bodies. The question remains whether the deaths are due to drought-related factors alone or whether some other agent is acting in combination with drought.

5.2.4 Water quality

Data on the in situ water quality at the time of the deaths are presented for Western District lakes and for other water bodies in Table 3. Only data taken within 48 hours of the deaths are presented, as these best reflect the water conditions at the time of the deaths.

The analysis is focused on the Western District lakes due to better reliability of data and because over 95 per cent of all eel deaths occurred in these water bodies.


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Figure 10: Annual run-off for the Moorabool River, western Victoria (source: DSE 2006).

Figure 11: Annual rainfall and average rainfall in comparison to electrical conductivity at lakes Colac, Bolac and Modewarre. Average rainfall is an average derived from Terang, Lismore and Buckley.

0

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s/cm

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fall (

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Rainfall

Average rainfall (1977-1997)

Commencement of current

drought

1982-1983 drought


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Table 3: Minima and maxima for in situ water quality parameters recorded at the time of the eel kills. ‘n’ is the number of eel deaths or samples taken.

Lake Data type Start End n Temperature (°C) pH Electrical conductivity

at 25 °C (μS/cm)

Dissolve oxygen (%)

Turbidity (NTU)

Min Max Min Max Min Max Min Max Min Max

Impacted Modewarre Eel kill 17/02/06 24/02/06 30000

Logger 17/02/06 21/02/06 216 19.28 23.22 9.47 9.57 29529 32492 109.1 169.5 1 447

Spot-measures 17/02/06 24/02/06 2 19.7 25.0 9.50 9.64 30400 34416 80.0 110.0

Modewarre Eel kill 26/01/06 5000

Logger 27/01/06 28/01/06 95 22.83 25.21 9.12 9.29 25274 26988 105.5 140.6

Spot-measures 24/01/06 28/01/06 10 21.5 25.3 9.24 9.33 26300 26803 97.0 140.2 6 8

Modewarre Eel kill 27/01/05 27/01/05 5000

Spot-measures 31/01/05 31/01/05 3 22.6 23.6 18300 19320 47.5 80.3

Bolac Eel kill 9/01/06 10/01/06 5000

Logger 10/01/06 11/01/06 95 20.88 21.99 8.42 8.65 16553 16937 74.5 105.1 341 629

Spot-measures 10/01/06 12/01/06 5 20.8 28.5 8.4 8.5 15524 15898 85.5 115.5 512 563

Colac Eel kill 23/01/06 <100

Logger 23/01/06 24/01/06 96 21.0 24.0 8.85 9.01 7559 8106 98.0 123.0 92 203

Spot-measures 4 18.9 24.0 8.55 8.70 7102 7119 68.3 89.0

Reference Burrumbeet Spot-measure 13/01/06 13/01/06 1 20.3 7.72 1654 63.0 54

Mill Swamp Spot-measure 12/01/06 12/01/06 1 21.2 9.2 330 83.1 2

Tooliorook Spot-measure 12/01/06 8/02/06 2 25.3 9.15 18138 72.1 12


15

During the eel deaths in the Western District lakes in early 2006, water quality parameters were measured in situ using both water quality loggers and hand-held water quality meters for spot measurements. The data loggers were deployed in the centre of the lakes, taking measurements every 30 minutes. The spot measures were taken in the middle of the lakes and at the lake margins. Although there were differences in the number of measurements taken at each lake, there was generally good agreement between the two datasets for measures taken within 48 hours of the deaths (Table 3).

Spot measurements taken at the time of the deaths are now discussed in relation to other measurements, variables and the tolerances of the species involved in the fish deaths.

Salinity

Fish are able to maintain a constant internal salt balance despite small fluctuations in the salinity of the surrounding water by the process of osmoregulation.

However, when sudden increases in salinity occur, fish may not be able to maintain their internal salt balance, possibly resulting in fish deaths. Sudden changes in salinity may be driven by a period of hot weather increasing evaporation. In support of this hypothesis, hot days (warmer than 35 °C) are correlated with the occurrence of the eel deaths (Figure 12).

Rate of increase in salinity

At Lake Modewarre salinity has risen sharply in the last two summers directly prior to each of the four major fish death events (Figure 13i). In the 2003—04 summer, salinity rose much less sharply and no fish deaths were observed. In December 2004, salinity increased by 13 per cent in the four weeks preceding the death of 30,000 European carp (Cyprinus carpio), which is an extremely rapid change in salinity. In the four weeks prior to the three eel death events in January 2005, January 2006 and February 2006, stepped increases in salinity were even more rapid, with increases ranging from 20 to 35 per cent (Figure 13i).

Figure 12: Eel deaths and air temperature in the Western Districts, 2005—06. Arrows denote eel death days.


16

Figure 13: Conductivity (at 25 oC) before and after fish kills at lakes (i) Modewarre, (ii) Bolac and (iii) Colac. All scales are standardised.

0

10000

20000

30000

1-Oct-00

28-Jul-01

24-May-02

20-Mar-03

14-Jan-04

9-Nov-04

5-Sep-05Date

Con

duct

ivity

@25

C (u

S/cm

)

0

5000

10000

15000

20000

25000

30000

35000

1-Oct-00

28-Jul-01

24-May-02

20-Mar-03

14-Jan-04

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5-Sep-05

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duct

ivity

@25

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S/cm

)

0

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10000

15000

20000

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Con

duct

ivity

@25

C (u

S/cm

)

Eel Kill carp kill spot measures loggers


17

Over the summers of 2000—01 to 2004—05, electrical conductivity (a measure of salinity) in Lake Bolac rose by about 5000 μS/cm (Figure 13ii). In the 2005—06 summer the rise in electrical conductivity was over 10,000 μS/cm and was associated with a major fish death event. Salinity rose by up to 34 per cent in the four-week period preceding the January 2005—06 deaths.

Lake Colac is influenced by freshwater inputs from the Barwon Water Sewage Treatment Plant and its tributaries, resulting in a much lower salt concentration than both Lake Modewarre and Lake Bolac. Nonetheless, its electrical conductivity rose from about 6000 to 9000 μS/cm over the spring—summer 2005—06 period in association with the small eel death event (fewer than 100 deaths) in January 2006 (Figure 13iii).

For comparison, data were also collected in other water bodies that have not had eel deaths in the summer of 2005—06. The most comparable lake was Lake Tooliorook. The salinity of Lake Tooliorook was similar to that associated with the first eel deaths at Modewarre, but no deaths were reported (Table 3). Further work is required to determine the differences between Lake Tooliorook and the lakes that have experienced fish deaths.

The rates of change in salinity have large implications for fish. Fish can adapt to slow changes in water quality, but acute changes may result in mortality, particularly if the fish is nearing its tolerance limits for a particular stressor. The body condition of moribund eels appears to be similar to that of healthy eels, suggesting the deaths were due to sudden changes (Figure 14).

Measured salinity in comparison with laboratory studies

Information on eel physiology and tolerance to salinity indicates freshwater-acclimatised eels are likely to be stressed by rapid changes in salinity, particularly if their ability to osmoregulate is compromised in any way.

The effect of the observed changes in salinity in the western lakes cannot be determined from existing literature on the toxicity of salt to eels (Anguilla spp.). This literature describes the mortality of eels following direct transfer from freshwater to constant concentrations of sodium chloride (e.g., Hinton and Eversole, 1979; Buchmann et al, 1992). The range of electrical conductivity in which mortality was observed varied from 13,000 to 19,500 μS/cm (Table 4). The eel

Figure 14: Eel length plotted against eel weight for eels submitted to Attwood in 2006 (not recorded for all eels).


18

Table 4: Reported salinity tolerances of fish species associated with kills in the Western District lakes

Common name Species Stage Exposure time

(days) Transfer Salt

Electrical Conductivity

(μS/cm-1 ) Reference

min max

Redfin Perca fluviatilis adults 1 direct & slow sea salt 12,500 22648* Craig, 1986 cited in Hart et al. (1991); Cadwallader and Backhouse (1983) cited in

Ryan and Dixon, 2006

European carp Cyprinus carpio adults ND direct ND 10,735 Alderman et al. (1976) cited in Hart et al. (1991)

American and Japanese eel

Anguilla rostrata/japonica juvenile 1 direct NaCl 13,000 19,500 Hinton & Eversole, 1979; Buchmann et al., 1992

Trout/salmon Salmonidae adults ND slow sea salt 54,700 Hoar and Randell (1969) cited in Hart et al. (1991)

Common Galaxiid Galaxia maculatus adults 7 direct & slow various 70,300 96,900 Chessman and Williams, (1975) cited in Hart et al. (1991)

‘ND’ is Not Described. ‘Slow’ indicates a period of acclimatisation before the period of exposure (‘Exposure time’) began. ‘Direct’ means the test fish were directly transferred to the test conductivity with no period of acclimatisation. * Conductivity measured in Lake Bolac on 31/03/06, Redfin were still being caught in Fyke nets in September 2006.


19

LOCATION Sample date pH Ion Parameter n mg/L

Proportional difference

from seawater (%)

Lake Bolac 9-Jan-06 8.49 Cation Magnesium 4 623 45Sodium 1 360 -53Calcium 4 173 7Potassium 1 43 0

Anion Carbonate 1 120 3Bicarbonate 1 230 5Chloride 1 2100 -6Sulphate 2 490 3

Lake Colac 24-Jan-06 8.78 Cation Sodium 1 1000 -4Magnesium 1 150 2Calcium 1 92 2Potassium 1 36 0

Anion Carbonate 1 140 3Bicarbonate 1 300 6Chloride 1 2400 0Sulphate 2 99 -3*Phosphate (filtered) 4 0.38

Lake Modewarre 28-Jan-06 9.25 Cation Sodium 2 6950 5Magnesium 2 800 -2Potassium 2 80 -2Calcium 2 37 -2

Anion Carbonate 2 740 6*Bicarbonate 2 300 6*Chloride 2 11000 -1Sulphate 2 115 -4*Phosphate (filtered) 4 0.0095

deaths at Lake Modewarre and Lake Bolac all occurred between 17,000 and 30,000 μ S/cm (Figure 13i and 13ii). However, the much smaller death event (fewer than 100 deaths) at Lake Colac occurred at 8000 μS/cm (Figure 13iii), well below the observed toxicity range for eels (Table 4).

Despite this reasonable match between the toxicity data and observed responses, the laboratory toxicity data are limited in their application to the field situation by differences in the rate of salinity increase, eel species, life-stages, salt composition and other unmeasured differences between the laboratory and the field. This emphasises the need for information on the salinity tolerance of short-finned eels (A. australis), including the effect of migratory condition.

Other species of fish were also observed dead in association with eels, particularly at Lake Modewarre and Lake Bolac. At Lake Modewarre in December

2004, the recorded electrical conductivity was 15,000 μS/cm, when 30,000 carp died. This is above the expected lethal electrical conductivity level for carp (Table 4). In Lake Bolac in the summer of 2005—06 about 50 redfin were reported to have died (Bill Allan, pers. comm., Lake Bolac eel fisherman).

The observed salinity during the deaths (electrical conductivity of 16,800 μS/cm) is within the range of salinity known to kill redfin (electrical conductivity of 12,500—17,500 μS/cm, Table 4). However, redfin were known to have survived salinities exceeding 24,000 μS/cm in the lake in spring 2006, as they have been regularly caught in eelers’ nets (Bill Allan, pers. comm., Lake Bolac eel fisherman).

These observations substantially exceed the previously reported salinity tolerance thresholds for redfin (Craig (1986) cited in Hart et al 1991;

Table 5: Average concentrations of major ions and pH in surface waters of the Western District lakes during the fish kills and compositional differences from seawater

* % differences from seawater were determined using non-coincident April 2006 data. Proportional differences from seawater were based on data from Atkinson and Bingman (1998) and Riley and Skirrow (1975) for seawater at 35ppt, differences between these two references were <0.3%.


20

Cadwallader and Backhouse (1983) cited in Ryan and Dixon 2006). Anecdotally, redfin are no longer present in Lake Bolac.

Based on these patterns, if salinity continues to rise as expected, it is reasonable to predict that large numbers of eels are likely to die in Lake Bolac in the summer of 2006—07. Common galaxias, brown trout and rainbow trout are much more resistant to changes in salinity (Table 4). However, anecdotal evidence suggests the trout at Lake Modewarre disappeared at the same time as the European carp in the lake. If this is true, the trout are either more sensitive than the toxicity data indicate or another factor is compromising their osmoregulatory capacity or causing their mortality via a different mechanism.

Differences in salt composition between the lakes were examined, with differences in the composition of Lake Bolac noted. Salt composition refers to the major ions (which make up more than 98 per cent of salinity). The potential toxicity of trace metals is considered separately (section 5.3.2). In addition to the rate of salinity change, large differences in the percentage composition of salts relative to seawater may impact on the eels’ survival, given eels migrate from and to seawater.

Lakes Colac and Modewarre are similar to seawater in their composition, with all their major ions in similar proportion to seawater (Table 5).

Lake Bolac is different from seawater, though, with magnesium 45 per cent higher than its percentage composition (by molar concentrations) in seawater. Sodium is 53 per cent lower in Lake Bolac than in seawater.

The high concentrations of magnesium and other divalent ions in Lake Bolac should be able to be regulated by the kidneys of the short-finned eels. The kidneys of the Japanese eel (Anguilla japonica) can switch instantaneously to rapid magnesium secretion if water concentrations increase (Bijvelds et al 1998). This would suggest that the difference in ionic composition at Lake Bolac should not lead to differences in the response of eels to salinity.

Wong and Chan (1998) demonstrated immature freshwater-acclimatised individuals (weighing 500—600 g) of Anguilla japonica acclimatise rapidly to seawater. These individuals were fully acclimatised in two to four days using two increments — from tap water to 50 per cent and then to 100 per cent seawater. Unfortunately Wong and Chan (1998) did not indicate if any mortalities occurred during this process or report the pH of the test solutions. However, the gills of the surviving eels had chloride cells characteristic of fully seawater-acclimatised individuals.

The osmoregulatory ability of the eels indicates that, if the correlation between the rate of salinity change is the cause of the eel deaths, the osmoregulatory

capacity of the eels must be compromised by another factor. Other factors reducing osmoregulatory capacity by causing structural gill damage include pH and algal concentrations. The likelihood of this occurring is discussed in the following sections.

Summary of salinity

Summarising the evidence for and against salinity being related to fish deaths in the Western District lakes:

• data on salinity before the eel deaths indicate sharp rises of salinity of up to 40 per cent in the three-to-six-week period before the deaths

• high evaporative rates prior to the eel deaths are further supported by the correlation between air temperatures and the occurrence of eel deaths

• laboratory toxicity data support eels and other fish being near their upper tolerance limits for salinity at Lake Bolac and Lake Modewarre and so they may be susceptible to rapid rises in salinity

• little loss of overall body condition of the eels supports an acute event such as a rapid rise in salinity

• the osmoregulation system can be compromised by both pH and suspended particulate matter, including algal cells (Tang and Au 2004)

• Evidence against salinity being related to the eel deaths at Lake Modewarre and Lake Bolac is based on their potential ability to increase their osmoregulation faster than the observed rates of salinity increase; however, the literature is unclear in this regard.

• Salinity remains a key focus of the investigation (Table 2).

Lake water pH

Levels of pH and salinity are positively correlated in the lakes because, as water evaporates, basic metal salts (including hydroxides, silicates and phosphates) concentrate. Steady increases in pH have been recorded over the last two years, particularly at Lake Modewarre.

Initial comparisons of the ionic composition with seawater show the cation composition is close to that of seawater in lakes Modewarre and Colac (see Table 5). However, the pH of the lakes is higher than seawater (pH 7.5 to 8.4), suggesting that changes in anion composition are responsible for the increasing pH. Some alkaline ions, including hydroxides and silicates, were not measured during previous analysis and they may contribute to the rising pH.

The most likely explanation for the high pH level is that increased primary productivity of the lakes may be driving pH upwards by consumption of inorganic carbon. Inorganic carbon buffers further increases in


21

pH, by the formation of acidic and bicarbonate ions from carbonic acid, produced by carbon dioxide dissolving in water. With evaporation, the pH may rise to 9.5, leading to bicarbonate ions dissociating to more acidic ions and carbonate, so resisting further increases in pH until the bicarbonate ions become exhausted (Wetzel, 2001). If this occurs, as more inorganic carbon is used up by the plants and algae the pH will become more alkaline.

This may provide the most likely explanation for the gradually increasing pH in Lake Modewarre. Aquatic plants have substantially increased their coverage of Lake Modewarre over the last two years. The high salinity of Lake Modewarre has contributed to much lower suspended sediment concentrations, as salt removes suspended inorganic material from the water column. This has been to the advantage of aquatic plants that thrive in better light conditions.

Lakes Bolac and Colac had pH values (greater than 8.4) at the time of the eel deaths (Table 3, Figure 15). Whilst the pH at Lake Modewarre was not recorded in the first eel death event, it was 9.2 in the second event and reached 9.6 at the time of the largest death event in February 2006. In October 2006, a pH of 10.5 was recorded in the north-east corner of this lake, which would be expected to be lethal to most fish (Alabaster and Lloyd, 1982).

Fish suffer from pH toxicity in gill chloride cells, the same cells that regulate salts in eels (Wong and Chang 1999; Pritchard 2003). Therefore, the levels of pH observed in the Western District lakes are likely to compromise the ability of the eels and other species of fish to meet the osmoregulatory demands of the increasingly saline water.

Literature suggests that, if pH values exceed 9.5 and dissolved oxygen is not supersaturated, American eels (Anguilla rostrata) attempt avoidance behaviour by seeking better water quality (Serafy and Harrell, 1993). Whether this avoidance behaviour was associated with the eels ‘crawling from the water’ in the last (February 2006) deaths at Lake Modewarre can only be speculated upon. However, it is possible that high pH may cause damage to the gills, impeding oxygen uptake and forcing the eels from the water.

Symptoms of gill damage by pH include an increased amount of mucus on the skin and on the inner side of the gill covers, and haemorrhages on the gills and the lower part of the body (Alistair Brown, pers. comm., Aquatic Veterinary Services). Excess amounts of mucus, often containing blood, are observed post-mortem on the skin and gills. The mucus is dull-coloured and watery. Alistair Brown (pers. comm., Aquatic Veterinary Services) described these effects, including skin erosion and haemorrhages, as common pathological findings in the eels from affected waterways (refer to Table 7). Of 23 moribund and dead eels submitted from Lake Modewarre, 15 had

suspected symptoms of alkalosis. Of the four moribund and dead eels submitted from Bolac, three had suspected symptoms of alkalosis. The prevalence of these symptoms supports the involvement of pH (alkalosis) in the eel death events.

Summary of pH

Evidence for pH contributing to the eel deaths in the Western District lakes includes:

• physical symptoms consistent with alkalosis were seen

• the observed pH levels potentially damage gills

• gill damage compromises osmoregulation in conditions requiring full osmoregulatory capacity

• pH, like salinity, was increasing at the time of the fish deaths.

Air and water temperature

The eel deaths in the western lakes were associated with a maximum air temperature of 35 °C or greater in the summer 2005—06 period (Figure 12). The maximum water temperatures during the time of the deaths in 2005—06 ranged between 18.9 and 28.5 °C (Table 3). The water temperatures do not appear to be exceptional compared with previous years’ data for Lake Bolac (Figure 16ii). The water temperatures alone would not normally be expected to kill eels, which have been shown to tolerate water temperature up to 39°C in laboratory experiments, in the absence of other stressors (Richardson et al 1994).

From a physiological perspective, increased temperatures lead to increased metabolic rates, and importantly place increased osmoregulatory demands on fish gills. As previously discussed, the osmoregulatory system of the eels would have already been under pressure in lakes Bolac and Modewarre during the time of the deaths due to the high salinity and the high pH.

Another less direct explanation for the association between deaths and high temperatures is the effect of high temperatures on evaporation rates. Faster evaporation during warm spells may have the effect of leading to a rapid deterioration in water quality, particularly salinity. Algae and bacteria too may respond to elevated temperatures.

The relationship between eel deaths and occurrence of 35 °C air temperatures is of particular concern, because it is projected that, in the Glenelg-Hopkins and Corangamite regions, there will be a ‘10 to 50 per cent increase in the number of hot summer days (over 35 °C) by 2030’ due to climate change (DSE 2004a, DSE 2004b).


22

Figure 15: Level of pH before and after eel kills at lakes (i) Modewarre, (ii) Bolac and (iii) Colac.

8.2

8.4

8.6

8.8

9

14-Dec-05

2-Feb-06

24-Mar-06

13-May-06Date

pH

8.40

8.60

8.80

9.00

14-Dec-05

2-Feb-06

24-Mar-06

13-May-06Date

pH

8.6

9.0

9.4

9.8

25-Oct-05

14-Dec-05

2-Feb-06

24-Mar-06

13-May-06Date

pH

Fish Kill loggers spot measures

ii. Lake Bolac

i. Lake Modewarre

iii. Lake Colac


23

Figure 16: Temperature before and after eel kills at lakes (i) Modewarre, (ii) Bolac and (iii) Colac.

10.0

15.0

20.0

25.0

30.0

19-Dec-04

29-Mar-05

7-Jul-05

15-Oct-05

23-Jan-06

3-May-06Date

tem

pera

ture

(deg

rees

cel

cius

)


5

10

15

20

25

30

35

1-Oct-00

28-Jul-01

24-May-02

20-Mar-03

14-Jan-04

9-Nov-04

5-Sep-05Date

tem

pera

ture

(deg

rees

C)

10.00

15.00

20.00

25.00

14-Nov-05

3-Jan-06

22-Feb-06

13-Apr-06Date

tem

pera

ture

(deg

rees

cel

cius

)

ii. Lake Bolac

i. Lake Modewarre

iii. Lake Colac


24

Summary of temperature effects

Temperature alone seems unlikely to explain deaths of eels in water bodies outside the western lakes, but may have contributed to the deaths within the western lakes by:

• increasing osmoregulatory and respiratory requirements on gills of fish possibly compromised by high pH and salinity

• being important in the rate of evaporation of the lakes.

Dissolved oxygen

Eels are known to tolerate low and temporarily anoxic conditions (McKinnon, 2006) and so low dissolved oxygen levels are not considered to be the sole cause of the large eel deaths in the western lakes, because the lowest recorded percentage of dissolved oxygen saturation was 47 per cent. These low levels may however be having an affect when the respiratory capacity of the gills is compromised due to undesirable conditions such as high pH, high suspended sediment concentrations, salinity and temperature.

However, supersaturated conditions with high dissolved oxygen (above 120 per cent) may cause a gas-bubble disease in eels (and other fish) similar to ‘the bends’ experienced by divers (Tucker, 1998, cited in McKinnon, 2006). These conditions may occur in the mid-afternoon during intense photosynthesis by algae and aquatic plants. Although supersaturated conditions occurred in the western lakes (up to 160 per cent, Figure 17), pathological symptoms of this disease were not found in dead eels.

Summary of dissolved oxygen

• Dissolved oxygen may have contributed to deaths in western lakes due to compromised gill function from high salinity, high pH and high temperature.

Suspended solids

Levels of suspended solids (particles such as clays in the water) were variable across the water bodies of concern (Table 3) and so it is unlikely to explain all the eel deaths in the western lakes. However, suspended solids were a possible contributor to the fish deaths occurring at Lake Bolac and Lake Colac. High

Table 6: Maximum recorded value for algae in Western District Lakes in 2005-06

Water body Time collected Algal parameter Maximum value Units

Chlorophyll a# <0.01 μg/L 28/01/2006 14:00

Cryptophyta 2365

Oscillatoria sp. 3 17/02/2006 12:30

Pennales <1

Chlorophyta 2080

Cyanophyta 1680

24/02/2006 0:00

Dinophyta 20 7/03/2006 16:00 Bacillariophyta 2615

MODEWARRE

January/February 2006 Plectonema wollei* >100000

Cell Count

9/01/2006 19:50 Chlorophyll a# < 0.01 ug/L

Bacillariophyta 990

Chlorophyta 795340

Cryptophyta 1110

Cyanophyta 4445

BOLAC

12/01/2006 17:30

Euglenophyta 495

Cell Count

14/12/2005 0:00 Microcystis sp. >200,000 Cell Count

24/01/2006 11:00 Chlorophyll a# 0.033 ug/L

Bacillariophyta 565

Chlorophyta 72070

8/02/2006 11:00

Cyanophyta 1510

COLAC

15/03/2006 10:00 Cryptophyta 250

Cell Count

# Chlorophyll a is a measure of algal productivity. * Difficult to give precise cell count because it forms dense mats.


25

Figure 17: Percentage dissolved oxygen (%DO) before and after eel kills at lakes (i) Modewarre, (ii) Bolac and (iii) Colac.

40.0

100.0

160.0

19-Dec-04

29-Mar-05

7-Jul-05

15-Oct-05

23-Jan-06

3-May-06Date

%D

O


i. Lake Modewarre

40.00

100.00

160.00

14-Nov-05

3-Jan-06

22-Feb-06

13-Apr-06Date

%D

O

iii. Lake Colac

40

100

160

17-Feb-05

28-May-05

5-Sep-05

14-Dec-05

24-Mar-06Date

%D

O

ii. Lake Bolac


26

suspended solid concentrations may reduce gill function by clogging gill filaments already struggling with potentially lethal levels of salinity, pH and temperature. In field situations suspended solids are most commonly measured as turbidity, which is a measure of the ability of water to pass light through it. Most fish tolerance data are given as total suspended solids (mg/L), not turbidity units, so it is difficult to assess the measured turbidity levels against published data. Without knowing the relationship between turbidity and suspended solids in the locations of the

deaths, the risks of exposure to suspended solids alone is difficult to assess.

Summary for suspended solids

• Extremely high turbidity was recorded at lakes Bolac and Colac.

• Suspended solids are a possible contributor to deaths by reducing gill function.

The role of suspended solids requires further assessment at lakes Colac and Bolac.

Table 7: Short-finned eels examined for pathology during the eel death investigation, 2004—06.

Waterway Date

caught State when collected

Leng

th (c

m)

Fem

ale

Alka

losi

s Vi

rus

Bact

eria

Pa

rasi

te

Diagnosis and comment

57 Undetermined cause of death. Focal dermatitis. 59 Undetermined cause of death. Focal dermatitis. 50 Undetermined cause of death. Focal dermatitis. 64 Undetermined cause of death. Focal dermatitis. 76 Undetermined cause of death. Focal dermatitis.

24/02/2006 Moribund

58 Undetermined cause of death. Focal dermatitis. 77 Focal dermatitis. 81 Undetermined cause of death. Minor unidentified bacterial growth in kidney.

17/02/2006 Moribund

76 Undetermined cause of death. 62 No diagnosis. Healthy eels. 66 No diagnosis. Healthy eels. 67 No diagnosis. Healthy eels.

7/02/2006 Healthy

75 No diagnosis. Healthy eels. Moribund 75 Pericloacitis and gill/skin haemorrhage. 6/02/2006 Healthy 71 Normal eel.

90 No lesions. Necrosis of gill epithelial cells. 71 Undetermined cause of death. Skin lesions. Good condition.

28/01/2006 Moribund

85 Undetermined cause of death. Minor bacterial growth in kidney (Vibrio anguillarum). No lesions. Good condition.

65 Undetermined cause of death. 27/01/2006 Dead 75 Undetermined cause of death. No lesions.

Undetermined cause of death. Mild hyperaemia on skin.

Undetermined cause of death. Minor bacterial growth found in spleen (Vibrio vulnificus).

1/02/2005 Moribund

Undetermined cause of death. Bacterial growth found in spleen and skin (Vibrio vulnificus).

No infectious disease. Mild gill lesions inflammatory response to mild irritant. Liver with hepatic oedema.

No infectious disease. Dilated bile ducts and pigmented accumulation in hepatocytes in liver.

No infectious disease. Small foci on gills. Dilated bile ducts and pigmented accumulation in hepatocytes in liver.

YesNo infectious disease. Diffuse congestion in gills. Pooling of bile. Pigment in spleen. Helminths in stomach.

Modewarre

28/01/2005 Moribund

No infectious disease. Diffuse congestion in gills. Mild gill lesions inflammatory response to mild irritant.


27

Waterway Date

caught State when collected

Leng

th (c

m)

Fem

ale

Alka

losi

s Vi

rus

Bact

eria

Pa

rasi

te

Diagnosis and comment

Rhabdovirus found. Pale gills/liver. No significant changes in histopathology. Healthy

Pale gills/liver.

Hyperaemia on ventral abdomen. White spots on gills. Heavy bacterial infection in kidney.

Gill haemorrhage and perivasculitis. Hyperaemia on ventral abdomen. Minor bacterial growth in kidney.

Dead

Gill haemorrhage and perivasculitis. Hyperaemia on ventral abdomen. Moribund Good condition. Moderate bacteria growth in kidney (Vibrio harveyi II).

Good condition. Good condition. Good condition. Yes Good condition. Minor unidentified bacterial growth in gills. ? Yes Suspected virus. Heavy bacterial growth in liver, gill and kidney (Vibrio harveyi II). Good condition.

11/01/2006

Healthy

Good condition. None — too autolysed.

Bolac

10/01/2006 Dead None — too autolysed.

Colac 23/01/2006 Dead 62 ? Yes

Necrotic cells in gills - Autolysis? Minor bacterial growth (Aeromonas schubertii) in kidney.

65 No diagnosis. Red tips on fins. Good condition. 64 No diagnosis. Pale scars on skin. Red fin tips. Good condition. 67 No diagnosis. Red tips on fins. Good condition. 72 No diagnosis. Good condition.

Tooliorook 2/02/2006 Healthy

75 Normal. Good condition. 67 Normal. Good condition. 78 Normal. Good condition. 76 Normal. Good condition. 83 Normal. Good condition. 82 Normal. Good condition.

Burrumbeet Ck

2/02/2006 Healthy

85 Normal. Good condition. 78 No diagnosis. Irregular growths in heart. Mill Swamp 17/01/2006 Healthy 90 Normal eel.

Colour code:

5.2.5 Algal toxins

Algae can kill fish through their influence on dissolved oxygen levels, as previously discussed. Algae can also be harmful to fish through the production of toxins and through physical irritation of gill tissues. Examples of fish-killing algae are golden algae (Prymnesiophyta), blue-green algae (cyanobacteria) and red-tide algae (e.g., dinoflagellates). These algae can produce toxins, which may kill fish in a range of ways, including nerve damage and tissue haemorrhage.

Results to date have not found any potential toxin-producing algae (Table 6), but they can be difficult to detect due to their small size.

Summary for algal toxins

• Testing to date has not indicated the presence of toxin-producing algae.

5.2.6 Fish health and lake ecology

Since 2004, the Department of Primary Industries (DPI) Attwood Laboratories have examined 88 eels and seven other fish in eel-related deaths (Table 7). Of the 88 eels collected, 26 per cent were dead, 31 per cent were moribund and 39 per cent were healthy and were collected for comparison with the moribund and dead eels. The great majority of the eels were short-finned eels (A. australis).

= Yes = Suspected = No = Not measured


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5.2.7 Infectious disease and parasites

No consistent pattern of infectious disease and parasites was found in the eels that were examined.

Parasites were found in six of the 88 eels analysed. However, there was little in common between the cases and no signs that the parasites were causing illness. The parasitic nematode Anguillicola crassus has been known to cause large eel mortalities, through affecting the swim bladder of eels (Kirk 2003). In one case an eel from Lake Modewarre in January 2005 was found to have an unidentified helminth in its stomach. Helminths are a group that include Anguillicola crassus, but the helminth was not identified beyond this. Apart from this possible case, Anguillicola spp. were not identified.

Of the 42 eels analysed for viruses, one apparently healthy eel was positively identified as having a rhabdovirus, and one moribund eel was identified as probably having a virus (Table 7). Both of these eels were collected from Lake Bolac in January 2006.

Bacteria were found in 25 out of 45 eels tested. However the same bacteria were rarely found from eel to eel, and only in three cases did the eel show pathological symptoms consistent with bacteria being the cause of death. The bacteria that were found are naturally occurring and not of concern for fish or human health. It is possible that the cases in which bacteria were found were a reflection of the eels being stressed by some other factor.

Summary for infectious disease and parasites

• For infectious disease to be responsible for the deaths a common set of symptoms would be expected. In cases where infectious disease or parasites were detected, they appeared to be incidental and not found generally in the population. The cases where disease has been detected are likely to be secondary infections, akin to a human getting a cold when they are subject to stress.

• Infectious disease and parasites are ruled out as the primary cause of the widespread deaths.

5.2.8 Deaths of other aquatic species

Although the great majority of deaths to date have only involved short-finned eels, the largest death events in the Western lakes have often included other fish species (Table 1). At Lake Modewarre there was an apparent sequence of deaths: European carp; common galaxias; the anecdotal disappearance of rainbow trout and brown trout; and finally the eels. At Lake Bolac around 50 redfin, one Murray cod and two yellowbelly died at the same time as the eels.

Observations of small native fish dying also raises the question on how their populations responded during the mass deaths of carp and eels at Lake Modewarre. Small native fish have been previously observed at Lake Modewarre and include flat-headed gudgeon (Philypnodon grandiceps), Australian smelt (Retropinna semoni), and common galaxias. It would be easy for these species to go unnoticed during death events because of their small size. Birds could also remove any evidence of the deaths of small fish.

In addition to fish populations, the prey of many of these species, aquatic macroinvertebrates (waterbugs), provide strong signals of aquatic health. Analysis of macroinvertebrates collected at Lake Colac and Modewarre prior to and after the January 2006 eel deaths show abundance and diversity comparable with other Western District lakes.

SIGNAL (Stream Invertebrate Grade Number Average Level) is a biotic index based on the tolerance of water bugs to water quality (Chessman 1995). Macroinvertebrates are ranked between 0 and 10 based on their tolerance of water. Lakes Modewarre, Colac and Bolac had reasonable SIGNAL scores with an average score of slightly higher than 5 (Table 8), with no large variation in macroinvertebrate communities occurring after the death events. The lack of any detectable response from the macroinvertebrates during the fish deaths may indicate their greater adaptation to these lakes. Many of the macroinvertebrates recorded are known to be particularly tolerant of adverse environmental conditions, particularly salinity.

Table 8: SIGNAL and average number of macroinvertebrate (waterbug) families pre and post-kill.

Water body Sample size SIGNAL Score Average number families per sample

Lake Bolac Jan 2006 post-kill 2 5.21 11.5

Lake Colac Dec 2005 5 4.74 9.8

Lake Colac Jan 2006 post-kill 2 4.94 13.5

Lake Modewarre Dec 2005 5 5.55 6.4

Lake Modewarre Jan 2006 post-kill 2 5.86 5.5


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Summary of deaths of aquatic species

• During the deaths in Lake Bolac and Lake Modewarre, other species of fish were found dead.

• Aquatic macroinvertebrate populations do not appear to be have changed significantly during fish death events in the Western District lakes.

5.2.9 Eel biology

It has been speculated that the dead eels were eels preparing to migrate to the ocean, perhaps triggered by the drought conditions. The precise migratory cues for eels are poorly understood (McKinnon, 2006).

Eels submitted for gross morphological and pathological examination to DPI Attwood were found to have high numbers of large females. Examination of 34 eels submitted to DPI Attwood for which sex was identified showed all were female (Table 7). Of the moribund and dead eels submitted to DPI Attwood since 2005—06 for which size data were recorded, the smallest eel was 50 cm long. Male short-finned eels tend to be smaller than 50 cm (McKinnon 2006), suggesting the eels were females.

The data collected suggest that it is possible that the deaths are a phenomenon associated with large female eels. However, this conclusion is complicated by:

• the naturally lower proportion of males in eel populations

• poor size and sex data, especially in 2004—05

• the possibility of small eels being missed due to predation or because they are less visible

• anecdotal evidence of eels ‘crawling’ from the water at Lake Modewarre and Lake Gnarput, which could also be interpreted as a sign of inhospitable water quality. This is supported by the apparent recovery of eels caught during the 9 January 2006 kill at Lake Bolac, when they were held in better quality water (Bill Allan, pers. comm., 88 Golden Eels).

Summary of knowledge gaps in eel biology

While many aspects of eel biology have been studied, many more remain unanswered. In particular the relationship between biology and the eel deaths requires further investigation by the scientific community.

Basic questions that remain unanswered include:

• the precise cues that eels need to begin migration. This is especially relevant given that the dead eels appear to be large female eels

• environmental tolerances of eels at the ‘yellow’, freshwater and ‘silvering’ migratory stages

• lakes Bolac, Colac and Modewarre have been landlocked for around 10 years. As a result eels may have been preparing to migrate out to sea annually for this time period and the long-term effect of eels being unable to migrate is unknown

• more needs to be done to understand the range of sizes of eels present in the waterways and their migratory state so that this can be compared with eels that are found dead.

5.3 Hypotheses discounted as a result of the investigation

5.3.1 Overpopulation

Water bodies that have had eel deaths other than the Western District lakes are not stock-enhanced and are all in close proximity to or are connected to the ocean, allowing migration and so are unlikely to be suffering from overpopulation. In the Western District, lakes Modewarre, Bolac and Colac have been stocked with eels over long periods and one suggestion is that the population of eels in these water bodies is no longer sustainable because of lower water levels. Eels are opportunistic feeders and will feed on a range of other animals, including macroinvertebrates (McKinnon 2005). As the eels get larger, they may tend towards larger prey such as fish (McKinnon 2005). Analysis of macroinvertebrate communities suggests they are plentiful. The eels found dead and dying appear to have been in good condition, which would suggest a lack of food is not the problem.

Populations of eels that are overstocked, as observed in aquaculture, have a relatively high proportion of males (Lachlan McKinnon, pers. comm.). The low proportions of male eels in the stocked lakes and the healthy condition of the eels would also suggest overpopulation in not a factor in the deaths.

Summary on overpopulation

• Water bodies outside the Western District lakes are not likely to be overpopulated.

• Although the Western District lakes are more prone to overpopulation, the morphology of the eels suggests they are well fed and the low proportion of males suggests the population is not overstocked.

5.3.2 Contaminants in water, sediment and fish

During the deaths in the summers of 2004—05 and 2005—06 water, sediment and tissue samples for metals and organic contaminants analysis were submitted to Ecowise, Water Ecoscience and the National Measurement Institute laboratories (refer to Table 1 for more detail).


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Table 9: Process for ruling out metals as significant stressors in relation to eel deaths

Concentrations of metals in surface waters

(mg/L)

Others Bolac

ANZECC (2000) Ecosystem Protection

Guidelines (mg/L) Fish toxicity (mg/L)

Metal Avg Max Avg Max Type Value Value Measure # species

Modfifying factors for toxicity to fish

Tissue (Gills, liver, muscles/skin) Concentrations

Comment

As (V) 0.010 0.035 0.018 0.019 0.95 0.221 16.3 96hrNOEC 7 Adjusted to min. hardness (840 mg/L) using algorithm.

Mostly (n=14) below DL (0.1 mg/kg fwt). No exceedances, unlikely to be a stressor.

B 0.634 2.150 0.578 0.620 0.95 0.370 27.6 3 Concentrations in seawater typically >5mg/L lakes. Unlikely to be a stressor.

Be 0.001 <0.01 0.002 0.002 IWL 0.00013 >13 96hrLOEC 3 Hardness, but no algorithm available. Not measured. Toxicity at >13 mg/L to the most sensitive of 3 fish species tested so unlikely to be a stressor to the eels.

Cd <0.001 0.001 0.002 0.95 0.004 15.5 96hrLC50 Anguilla rostrata Adjusted to min. hardness (840 mg/L) using algorithm.

Mostly (n=14) below DL (0.02 mg/kg fwt).

No exceedances, unlikely to be a stressor.

Co <0.01 0.007 0.009 LRTV 0.0028 2.74 8dLC50 1 Sorption to suspended particulate matter — no algorithm available.

Mostly (n=14) below DL (0.5 mg/kg fwt).Maximum values recorded at Bolac three orders of magnitude <concentrations causing toxicity to the one fish species tested. Unlikely to be a stressor to eels.

Cr (III) 0.010 0.026 0.068 0.140 0.95 0.051 213.6 96hrLC50 Anguilla rostrata Adjusted to min. hardness (840 mg/L) using algorithm.

Mostly (n=14) below DL (0.5 mg/kg fwt). No exceedances, unlikely to be a stressor.

Cu 0.008 0.024 0.015 0.017 0.95 0.024 101.9 96hrLC50 Anguilla rostrata Adjusted to min. hardness (840 mg/L) using algorithm.

Concentrations <90mg/kg (n=14). No exceedances, unlikely to be a stressor.

Hg <0.001 0.0001 0.0002 0.99 0.0010 2.4 96hrLC50 Anguilla rostrata Adjusted to min. hardness (840 mg/L) using algorithm.

All (n=14) <0.3 mg/Kg. Values >0.5mg/kg are diagnostic of effects.

No exceedances, tissue concentrations <0.5 mg/kg. Unlikely stressor to eels.

Mn 0.023 0.072 0.23 0.27 MRTV 1.7 >68 3 Toxicity to Salmo trutta halved from a hardness of 30 to 450 mg/L.

All (n=10) tissue concentrations 6.7 mg/kg.


Mo <0.01 0.007 0.018 LRTV 0.034 70 2 Even distribution between particulate and dissolved forms.

All (n=10) values below DL (0.5 mg/kg fwt).

All tissue concentrations <0.5 mg/kg. Unlikely stressor to eels.

Ni <0.01 0.014 0.028 0.044 0.95 0.187 220.8 96hrLC50 Anguilla rostrata Adjusted to min. hardness (840 mg/L) using algorithm.

All (n=14) values below DL (0.5 mg/kg fwt).


Pb 0.006 0.018 0.016 0.021 0.95 0.234 84.7 96hrLC50 7 Adjusted to min. hardness (840 mg/L) using algorithm.

Values (n=14) <DL (0.2 mg/kg fwt). No exceedances, unlikely to be a stressor.

Sb 0.008 0.053 0.004 0.005 IWL 0.009 9 96hrLC50 1 None described. Not measured. No value recorded above IWL. Unlikely stressor to the eels.

Sn <0.01 0.007 0.015 LRTV 0.003 78 24hrLC50 1 None described. All (n=10) values below DL (0.5 mg/kg fwt).

Maximum values were 3 orders of magnitude < than concentrations causing toxicity to the one fish species tested (Rainbow Trout). Unlikely to be a stressor to eels.

Tl <0.01 0.011 <0.01 LRTV 0.00003 0.04 96hrLC50 3 None described. Not measured. Maximum concentrations were four fold below those causing chronic toxicity in the most sensitive of the 3 fish spp tested. Unlikely stressor to the eels.

V 0.026 0.053 0.043 0.058 IWL 0.006 0.128 96hrLC50 7 Hardness, pH. Not measured. Maximum values were five times less than concentrations causing mortality in the seven fish species tested. Unlikely stressor to the eels.

Zn 0.013 0.043 0.154 0.420 0.95 0.041 14.5 96hrLC50 Anguilla rostrata Adjusted to min. hardness (840 mg/L) using algorithm.

Concentrations <140mg/kg (n=14). No exceedances, unlikely to be a stressor.

Note: ‘Fish toxicity’ consisted of data from ANZECC (2000) or from the USEPA (http://mountain.epa.gov/cgi-bin/ecotox_quick_search).All measurements were coincidental with eel kills. Barium, strontium and titanium were not assessed as there are no guidelines.

Abbreviations: Guideline types: 0.99 and 0.95=99% and 95% ecosystem protection values, MRTV=Medium reliability, LRTV=low reliability, IWL=Interim working level, refer to ANZECC (2000) for more detail. ‘#spp’ is the number of fish species for which toxicity data were available

(most sensitive value of fish toxicity is shown). If data on eels were available this is shown by itself, along with the species tested. 24hrLC50 = concentration causing 50% mortality over 24 hours; 96hrLC50=concentration causing 50% mortality over 96 hours;

8dLC50=concentration causing 50% mortality over 8 days; 96hrLOEC = Lowest Observable Effect Concentration over 96 hours; 96hrNOEC = No Observable Effect Concentration over 96 hours. Fwt = fresh weight of tissues.

http://mountain.epa.gov/cgi-bin/ecotox_quick_search


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Additional samples were submitted before and after deaths. Assessment of the metal concentrations followed the ANZECC/ARMCANZ (2000) risk-based investigative approach as recommended in EPA (2003).

Trace metals

Lake Bolac had significantly higher concentrations of trace metals than other water bodies. Concentrations of trace metals in other water bodies were generally low, with few elements exceeding the water quality guidelines (Table 9). As a result, the data were initially grouped into ‘Lake Bolac’ and ‘other water bodies’.

After assessment of these data using averages and maxima against ANZECC/AMRCANZ (2000) water quality guidelines and acute fish toxicity data it was not possible to eliminate aluminium, iron, selenium and silver as potential stressors to the eels in Lake Bolac (Table 10). However, this is because it was not possible to filter water samples at Lake Bolac because of technical issues related to high suspended sediment concentrations. Future testing will attempt to resolve this issue, but, given the results of testing from the other lakes, these metals are unlikely to be of concern.

Higher concentrations of selenium were recorded in the livers relative to the gills, suggesting that selenium was being taken into the body via the gill and dietary pathways (Table 11). However, the recorded concentrations of selenium in the liver are unlikely to be associated with fish mortality. Jarvinen and Ankley (1998) reviewed concentrations of selenium in tissues such as gills and livers associated with fish mortality (mainly salmonids). The highest recorded selenium concentration measured in the liver and gills of eels in EPA’s investigations was 12 mg/kg (see Table 11), below those associated with mortality of other species of freshwater fish (Jarvinen and Ankley, 1998).

The maximum selenium concentrations recorded by EPA in surface waters (0.032 mg/L, Table 10) were also below those shown to lead to mortality (Jarvinen and Ankley 1998). Despite the limitations of these laboratory data, it is unlikely that selenium was the cause of the eel and fish deaths.

Table 10: Aluminium and selenium concentrations in surface waters of Western Lakes during eel kill events.

Location Date Aluminium

(mg/L) Selenium (mg/L)

Lake Modewarre 27/01/06 Avg 0.06 0.023

Max 0.20 0.032

n 5 5

Lake Modewarre filtered 27/01/06 Avg 0.025 0.028

Max 0.025 0.028

n 2 2

Lake Colac 23/01/06 Avg 3.60 0.005

Max 3.60 0.005

n 1

Lake Colac filtered 23/01/06 Avg 0.025 0.008

Max 0.025 0.008

n 1 1

Lake Bolac 09/01/06 Avg 21.25 0.02

Max 27.00 0.02

n 4 4

Colour Key

Measurement below ANZECC/ARMCANZ 95—99% species protection guidelines.

Measurement between ANZECC/ARMCANZ 95—99% and 80% species protection guidelines.

Measurement above ANZECC/ARMCANZ 80% species protection guidelines.


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Table 11: Aluminium and selenium concentrations in tissues of dead eels collected from the western lakes 2004—06.

All concentrations are mg/Kg fresh weight tissue.

Table 12: Organics contaminants tested for during fish kills.

Parameter group Phase Units Detection Limit

Anionic surfactants water mg/l <0.01

Carbamate insecticides fish mg/kg 0.05

Halogenated volatiles water g/l 1

Semi-volatile industrial compounds* fish mg/kg 1

water g/l 1

Miscellaneous pesticides** fish mg/kg 0.05

Monocyclic aromatic hydrocarbons water mg/L 0.001

Organochlorine insecticide fish mg/kg 0.05

sediment mg/kg 0.1

water mg/L 0.001

Organophosphate insecticides fish mg/kg 0.05

sediment mg/kg 0.1

water mg/L 0.002

PCBs fish g/l 2

sediment mg/kg 0.1

water mg/L 0.001

Phenoxyacetic acid herbicides water mg/L 0.1

Plant growth retardants fish mg/kg 0.05

Polycyclic aromatic hydrocarbons (PAH) water g/l 1

Pyrazole insecticide fish mg/kg 0.05 Semivol, CHC water mg/L 0.001

Specific phenols water mg/L 0.001

Total petroleum hydrocarbons water mg/L 0.1

Triazine herbicide fish mg/kg 0.05

Uracil herbicide fish mg/kg 0.05

* Some possible industrial contaminants detected, but were not common across water bodies. ** >10 sub-groups.

Lake Kill date Eel tissue Metal Min Max n Bolac 9-Jan-06 Gills Al 14 120 3 Bolac 9-Jan-06 Liver Al 3.3 3.7 3 Colac 23-Jan-06 Liver & Gills Al 7.3 7.3 1 Modewarre 27-Jan-06 Liver & Gills Al 2.2 5.7 1 Bolac 09-Jan-06 Gills Se 2.1 2.5 3 Bolac 09-Jan-06 Liver Se 11.0 12.0 3 Colac 23-Jan-06 Liver & Gills Se 3.0 3.0 1

Modewarre 27-Jan-06 Liver & Gills Se 7.4 8.6 3


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Table 13: Free ammonia and sulfide concentrations in surface waters (top 1 m) within two days of eel death events.

* Filtered NH3

One cause for concern is that aluminium becomes more bioavailable with rising pH. Rising pH has been recorded in lakes Modewarre, Bolac and Colac. High aluminium concentrations were recorded in unfiltered water samples relative to filtered water samples at lakes Colac and Modewarre (Table 10), suggesting that, although total concentrations exceed ANZECC/ ARMCANZ (2000) ecosystem protection triggers, there is not enough bioavailable aluminium to be a potential risk to fish. At Lake Bolac it was not possible to filter water because of technical issues related to suspended sediment concentrations, so more testing is required this summer to determine if aluminium is a potential stressor there. Based on the amount of aluminium that is bioavailable at Lake Modewarre and Lake Colac, it is unlikely to be a cause for concern at Lake Bolac, but this requires confirmation.

Aluminium concentrations were higher in the gill filaments in comparison to the liver tissues (Table 11), which is likely to be due to aluminium-containing sediments (e.g., aluminium hydroxides) accumulating on the gill filaments.

Summary of metals

• Other than contributing to overall salt concentrations and osmotic stress to fish in the western lakes, trace metals are unlikely to be the major cause of the fish deaths.

• More testing is required to assess if aluminium, iron, selenium and silver are having an impact on fish in Lake Bolac Based on the other lakes, levels of bioavailable metals are likely to be low in Lake Bolac.

Organic contaminants

Organic contaminant groups were generally below detection limits in water, sediment and biota and are unlikely to have contributed to the fish deaths (Table 12). As a result, no further investigation of these contaminants will be conducted in the upcoming investigation (Table 2). As outlined in EPA (2005a), where organic contaminants were found, they were not common across waterways.

5.3.3 Ammonia and sulfide

Free ammonia and sulfide concentrations do not appear to be causes of the eel deaths in the Western District lakes (Table 13).

Sulfide forms in anoxic conditions and, as these have not been seen in the western lakes (dissolved oxygen was greater than 47 per cent saturated), is unlikely to be the cause of the eel deaths.

Free ammonia concentrations were not high enough to cause the fish deaths observed in the Western District lakes. Free ammonia concentrations were assessed against trigger values, using the ANZECC (2000) risk-based approach as recommended by EPA (2003).

Summary for ammonia and sulfide

• No evidence of anoxic preconditions required for sulfide toxicity.

• Measured ammonia concentrations were not of concern.

Location Date of Sample ParameterSample

sizeMin

(mg/L)Max

(mg/L)Temp (°C)

pHFree ammonia

(mg/L)Lake Bolac 09/01/06 Total Ammonia 3 <0.1 0.12 21–27 8.3–8.5 0.02Lake Colac 24/01/06 Total Ammonia 1 0.31 0.31 21–24 8.2–8.7 0.06Lake Modewarre 27/01/06 Total Ammonia 2 <0.1 <0.1 23–25 9.1–9.3 0.05Lake Modewarre 27/01/06 Sulfide 4 <0.1 <0.1 23–25 9.1–9.3Lake Modewarre 24/02/06 Total Ammonia 3 0.012 0.15 22* 9.6* 0.09Lake Modewarre 24/02/06 Total Free Ammonia 2 0.013 0.064 22* 9.6* 0.04


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6. CONCLUSIONS

The common factor in all eel deaths since 2004 has been drought. It is highly likely that a combination of water conditions reduces fish gill function, leading to the deaths. The fish appear to be near the edge of the their tolerances and it is possible that they are unable to cope with the rapid rates of change that have been recorded in the lakes. Further information needs to be collected to determine if these conditions were directly responsible for the deaths or if they brought about some other change in the lake.

Toxin-producing algae have not been detected but further monitoring will be undertaken. Another explanation is that the decline in water quality may trigger the eels into attempting a spawning migration.

Hypotheses excluded through the investigation include associations with worldwide declines, pesticides and other organic contaminants, metals (aluminium excepted), ammonia and sulfide, eel overpopulation and eel parasites.

A monitoring program over this summer will attempt to resolve any remaining questions surrounding the deaths. The program includes:

• deployment and maintenance of water quality data loggers through the summer period at lakes Modewarre, Bolac, Colac and Tooliorook

• fish surveys to get a better understanding of the fish species and population structures present in the lakes and the different migratory states of the lake’s eel populations

• understanding of physiological effects of water quality through improved histological analyses of gills

• the use of expert consultants in toxin-producing algae.

Deaths have been shown to be associated with periods of rapidly increasing pH and salinity in combination with high air temperatures. The weather conditions most likely to lead to deaths are spells of hot weather above 35 °C and high evaporation rates.

As the lakes have become shallower they have become more susceptible to rapid environmental change. Whether a stressor or combination of environmental stressors such as pH, temperature and salinity is causing the problem, the overall driver is the low rainfall over the last 10 years. While little can be done to alleviate this impact upon the lakes, investigations this summer will help to isolate the cause of the deaths even further. In the long term this may help to better manage the recreational and commercial fisheries that rely on these lakes.

7. ACKNOWLEDGEMENTS

The members of the Eel Death Investigation Reference Group (EDIRG) reviewed EPA Victoria’s investigation into eel and fish deaths. The authors thank them for their input and review of the report. The Eel Death Investigation Reference Group was chaired by Christine Forster and was comprised of Prof John Sherwood (Deakin University), Tom Ryan (Environment Victoria), Bill Allan (commercial eel industry), Peter Appleford (DPI Fisheries Victoria), Dr Mehdi Doroudi (DPI PIRVIC), Keith Jackson (Department of Sustainability and Environment), Peter Greig (Corangamite Catchment Management Authority), Dr Heather Builth (Glenelg-Hopkins Catchment Management Authority) and representatives from DPI Attwood.

The authors would also like to thank the group of experts who reviewed the technical aspects of the investigation. They were Prof John Sherwood (Deakin University), Tom Ryan (Environment Victoria), Bill Allan (commercial eel industry), Lachlan McKinnon (Audentes Pty Ltd), Alistair Brown (Aquatic Veterinary Services) and Dr Bruce Pease (NSW DPI).


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8. REFERENCES

Alabaster JS and Lloyd R (1982) Water quality criteria for freshwater fish. Second edition. London, FAO.

Allen GR, Midgley SH and Allen M (2002) Field Guide to the Freshwater Fishes of Australia. Western Australian Museum, CSIRO Publishing, Collingwood.

Amaro C, Biosca EG, Esteve C, Fouz B and Toranzo AE (1992) Comparative study of phenotypic and virulence properties in Vibrio vulnificus biotype 1 and 2 obtained from a European eel farm experiencing mortalities. Dis. Aquat. Org. 13, 29—35.

ANZECC/ARMCANZ (Australian and New Zealand Environment and Conservation Council/Agriculture and Resources Management Council of Australia and New Zealand) (2000) Australian and New Zealand Guidelines for Fresh and Marine Water Quality: Volume 1 The Guidelines.

Paper No. 4. ANZECC/ARMCANZ, Canberra.

Atkinson MJ and Bingman C (1997) Elemental composition of commercial sea salts. Journal of Aquariculture and Aquatic Sciences. 8, 39—43.

Balint T, Ferenczy J, Katai F, Kiss I, Kraczer L, Kufcsak O, Lang G, Polyhos C, Szabo I, Szegletes T and Nemcsok J (1997) Similarities and differences between the massive eel (Anguilla anguilla L.) devastations that occurred in Lake Balaton in 1991 and 1995. Ecotoxicology and Environmental Safety 37(1), 17—23.

Bijvelds MJC, Van der Velden JA, Kolar ZI and Flik G. (1998). Review of magnesium transport in freshwater teleosts. The Journal of Experimental Biology. 201, 1981—1990.

Bostock BM (2000) A limnological study of Lake Bolac, western Victoria. Deakin University, Honours thesis.

Buchmann K, Felsing A and Slotved HC (1992) Effects of metrifonate, sodium chloride and bithionil on a European population of the gill parasitic monogeneans Pseudodactylogyrus spp. And the host Anguilla anguilla. Bulletin of the European Association of Fish Pathologists 12(2), 57—60.

Clifton C, Daamen C and Sherwood J (2006) Water, land use change and ‘new forests’: what are the challenges for south-western Victoria Australian Forestry. 69(2), 95—100.

Chessman BC (1995) Rapid assessment of rivers using macroinvertebrates: A procedure based on habitat-specific sampling, family level identification and a biotic index. Australian Journal of Ecology, 20, 122—129.

Conder LJ (1998) The nutrient status and phytoplankton dynamics of Lake Modewarre, Victoria. Honours thesis, Deakin University, Warrnambool, Victoria, Australia.

Dekker W, Casselman JM, Cairns DK, Tsukamoto K, Jellyman D and Lickers H (2003) Worldwide decline of eel resources necessitates immediate action. Fisheries 28, 28—30

DPI (Department of Primary Industries). In press. Fisheries Victoria Commercial Fish production Information Bulletin. 2005—2006. State Government of Victoria.

DSE (Department of Sustainability and Environment) (2004a) Climate change in the Glenelg Hopkins region. State Government of Victoria.

DSE (Department of Sustainability and Environment) (2004b) Climate change in the Corangamite region. State Government of Victoria.

DSE (Department of Sustainability and Environment) 2006. Moorabool runoff data State Water Report 2004/05. A statement of Victorian water resources. State Government of Victoria.

EPA Victoria (2003) Variation to State Environment Protection Policy (Waters of Victoria). Victorian Government Gazette S 107.

EPA Victoria (2005a) Eel Deaths in Victoria, 2004-05. Available at: http://epanote2.epa.vic.gov.au/EPA/publications.nsf/ PubDocsLU/1013?OpenDocument

EPA Victoria (2005b) Lake Modewarre Water Quality 2004-05. Available at: http://epanote2.epa.vic.gov.au/EPA/publications.nsf/ PubDocsLU/1018?OpenDocument

EPA Victoria (2006) A Review of historic Western Victorian lake conditions in relation to fish deaths. Available at http://www.epa.vic.gov.au/publications

FRC Environmental (2006) An Investigation Into the Mass Mortality of Eels at Swanbank Power Station. Prepared for CS Energy.

Gervasi D (2006) Lake Bolac and Fiery Creek: Review of existing information on assets, current condition, threats, management options and information gaps. A report to Glenelg Hopkins Catchment Management Authority, P and P Design.

Hart B, Bailey B, Edwards R, Hortle R, James K, McMahon A, Meridith C and Swadling K (1991) A review of the salt sensitivity of the Australian freshwater biota. Hydrobiologia 210, 105—144

Hinton M J and Eversole AG (1979) Toxicity of ten chemicals commonly used in aquaculture to the black eel stage of the American eel. Proceedings of the World Mariculture Society 10, 554—560

ICES (International Council for the Exploration of the Sea) (2005) Report of the ICES/EIFAC Working Group on Eels. International Council for the Exploration of the Sea, Copenhagen.

Jarvinen AW and Ankley GT (1998) Linkage of effects to tissue residues: Development of a comprehensive


36

database for aquatic organisms exposed to inorganic and organic chemicals. Society of Environmental Toxicology and Chemistry, Pensacola, USA.

Khan TA (2003) Limnology of four saline lakes in western Victoria, Australia: 1 Physico-chemical parameters. Limnologica 33, 327—339.

Kirk RS (2003) The impact of Anguillicola crassus on European eels. Fisheries Management and Ecology. 10(6), 385—394.

Knights B (2003) A review of the possible impacts of long-term oceanic and climate changes and fishing mortality on recruitment of anguillid eels of the Northern Hemisphere. The Science of the Total Environment. 310, 237—244

McKinnon L (2002) Victorian Eel Fishery Management Plan. Marine and Freshwater Resources Institute, Queenscliff.

McKinnon LJ (2005) Investigation into the potential for shortfinned eel (Anguilla australis) to predate on carp. Report to the Glenelg Hopkins Catchment Management Authority. Report No. 58, Primary Industries Research Victoria, Queenscliff.

McKinnon LJ (2006) A Review of Eel Biology: Knowledge and Gaps. Consultants report to EPA Victoria by Audentes Investments Pty. Ltd.

Nicholson C, Dahlhaus PG, Anderson G, Stephens M and Tucker K (2003) Corangamite Salinity Action Plan 2003: Regional Draft. Salinity Action Plan Technical Report 2, Corangamite Catchment Management Authority.

Palstra AP, van Ginneken VJT, Murk AJ and van den Thillart G (2006) Are dioxin-like contaminants responsible for the eel (Anguilla anguilla) drama? Naturwissenschaften 93(3), 145—148.

Pollard DA (1971) Faunistic and environmental studies on Lake Modewarre, a slightly saline anthalassic lake in south-western Victoria. Aust. Soc. Limn. Bull. 4, 25—42.

Pritchard JB (2003) The gill and homeostasis; transport under stress.

Am J Physiol Regul Integr Comp Physiol 285, 1269—1271.

Ryan T and Dixon D (2006) Impacts of salinisation of freshwater fish and aquatic plants of the Glenelg River: literature review. Report for Glenelg-Hopkins Catchment Management Authority.

Richardson J, Boubée J and West D (1994) Thermal tolerance and preference of some native New Zealand freshwater fish. New Zealand Journal of Marine and Freshwater Research. 28, 399—407.

Riley JP and Skirrow G (1975) Chemical Oceanography. Second Edition, Volume 3. Academic Press.

Robinson GA (1841) Journals of George Augustus Robinson, March-May 1841. Presland, G (ed.) Records of the Victorian Archaeological Survey Number 6.

Serafy JE and Harrell RM (1993) Behavioural response of fishes to increasing pH and dissolved oxygen: field and laboratory observations. Freshwater Biology 30, 53—61.

Tang JYM, and Au DWT (2004) Osmotic Distress: A Probable Cause Of Fish Kills On Exposure To A Subbloom Concentration Of The Toxic Alga Chattonella Marina. Environmental Toxicology and Chemistry. 23, 2727—2736.

US EPA (1998). Guidelines for Ecological Risk Assessment. EPA/630/R-95/002F. Risk Assessment Forum, Washington DC.

Wetzel RG (2001) Limnology: Lake and River Ecosystems. Third Edition. Academic Press.

Wong CKC and Chan D K O (1999) Chloride cell subtypes in the gill epithelium of Japanese eel Anguilla japonica. Am J Physiol Regul Integr Comp Physiol 277: 517—522.

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