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Elasmobranchs in the Fitzroy River, Western Australia

Report to the

D.C. Thorburn, D.L. Morgan, A.J. Rowland and H. Gill

Centre for Fish and Fisheries Research

February 2004

ISBN: 0 642 553343 Disclaimer The views and opinions expressed in this publication are those of the authors and do not necessarily reflect those of the Australian Government or the Minister for the Environment and Heritage.

TABLE OF CONTENTS

1 INTRODUCTION 1

1.1 Background 1

1.2 Conservation status and distribution of elasmobranchs in the

Fitzroy River 1

1.3 Current threats to elasmobranchs in the Fitzroy River 5

1.4 Objectives 6

2 METHODS 7

2.1 Sampling strategy 7

2.2 Sample sites 7

2.3 Elasmobranch migration 11

2.4 Interpretation of abundance data 11

2.5 Tagging 11

3 RESULTS 13

3.1 Overall catch composition 13

3.2 Catch composition of replicate sampling and abundance 13

3.3 Tagging Data 16

4 DISCUSSION 17

4.1 The current survey 17

4.2 Elasmobranch migration in the Fitzroy River 17

4.3 Freshwater sawfish of the west Kimberley 19

4.4 Further research 24

4.5 Recommendations 25

5 REFERENCES 27

6 APPENDIX 1 29

1 INTRODUCTION

1.1 Background

In Australia, only two rays, Pristis microdon (freshwater sawfish) and Himantura

chaophraya (freshwater whipray), and three sharks Carcharhinus leucas (bull shark),

Glyphis sp. A (speartooth shark) and Glyphis sp. C (northern river shark), occur in

oligohaline environments of the upper reaches of rivers far from the coast. While

information on the occurrence, biology and distribution of these ‘freshwater’ species

remains limited, recent surveys of rivers throughout northern Australia by Thorburn et

al. (2003), and Morgan et al. (2002, in press) documenting the inland fishes of the

Fitzroy River, have identified the Fitzroy River, Western Australia, as a significant

habitat for several of these rare species.

1.2 Conservation status and distribution of elasmobranchs in the Fitzroy

River

Both Thorburn et al. (2003) and Morgan et al. (2002) recorded three of the above

elasmobranch species in the Fitzroy River, these being P. microdon, H. chaophraya

and C. leucas, with each being found in estuarine and freshwater sites throughout the

river (Figure 1). Furthermore, a single Glyphis sp. C was collected from the nearby

marine tidal creek, Doctors Creek (Figure 1). During a concurrently run study, a total

of six Glyphis sp. C were also collected from Doctors Creek and an additional survey

site south of Derby (Thorburn et al. 2004). The significance of the Fitzroy River

elasmobranch assemblage is further illustrated by the high conservation value of these

species (Table 1). For example, P. microdon and Glyphis sp. C are listed as

vulnerable and endangered, respectively, by the Commonwealth Government under

the Environmental Protection Biodiversity and Conservation Act 1999 (EPBC Act

1999). Another listed sawfish species, Pristis clavata (dwarf sawfish) has also been

encountered in the lower reaches of the Fitzroy River and in Doctors Creek (Morgan

et al. 2002, Thorburn et al. 2003) (Figure 1). Additionally, all five of the

aforementioned species have recently been listed in a report by the IUCN Shark

Specialist Group Australia and Oceania Regional Red List (IUCN 2003).

1

WesternAustralia

DerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerby

WesternAustralia


WesternAustralia


WesternAustralia


WesternAustralia


Pristis microdon Himantura chaophraya

Carcharhinus leucas Glyphis sp. C

Pristis clavata

Figure 1 Sites from which elasmobranchs have been recorded from the Fitzroy River

prior to this study (Morgan et al. 2002, Thorburn et al. 2003).

2

Table 1 Conservation listings of elasmbranchs encountered in the Fitzroy River

Species EPBC Act 1999 IUCN (2003)

Pristis microdon Vulnerable Endangered

Himantura chaophraya Vulnerable

Carcharhinus leucas Near Threatened

Glyphis sp. C Endangered Critically Endangered

Pristis clavata Endangered

Although some protection of P. microdon and Glyphis sp. C is provided by the listing

of these species by the Commonwealth, this legislation only applies in

Commonwealth waters, which in Western Australia begins three nautical miles from

the low water mark (under the Offshore Constitutional Settlement 1987, 1988 and

1995). The State Government is therefore responsible for the waters between this

three nautical mile limit and the coast (see Boulter 2002). Thus, even though P.

microdon and Glyphis sp. C are protected in Commonwealth waters, they are not

listed under Schedule 2 (Protected Fish) of the Fish Resources Management

Regulations 1995, which preclude them from protection in Western Australia.

Although Part 1 of Schedule 2, detailing ‘commercially protected fish’, states that “all

freshwater aquatic organisms other than sooty grunter and catfish of the family

Ariidae” are protected from commercial practices, it is unclear if P. microdon or

Glyphis sp. C are included in this freshwater classification, as they are more

accurately described as marine opportunists. Thus, in inshore water (including rivers)

none of the aforementioned species are protected.

Pristis microdon (freshwater sawfish)

Found throughout northern Australia, P. microdon is Australia’s largest fish found in

freshwater, reputed to reach 7 m (Last and Stevens 1994). Although accurate

distributional data are limited, and the taxonomy confused, it has been known for

some time that this species is encountered in the upper reaches of rivers, often several

hundred kilometres upstream from marine waters (Whitley 1940, Herre 1955, Merrick

and Schmida 1984). In the Fitzroy River, it is known from both fresh and salt water,

and as far inland as Margaret River Gorge, approximately 400 kilometres upstream

from the mouth (Morgan et al. 2002). Thorburn et al. (2003) also noted the high

3

abundance of this species in the Fitzroy River (as compared to all others sites

surveyed in northern Australia) and described the river as “a significant ‘stronghold’

for the species”.

Himantura chaophraya (freshwater whipray)

It was not until 1989 that H. chaophraya was identified from Australia, based on a

specimen from the Daly River (Taniuchi et al. 1991). Himantura chaophraya is the

only Australian stingray to live entirely in fresh and estuarine waters (Last and

Stevens 1994) and it has not been recorded from euhaline (30-40 ppt) marine waters

anywhere within its known range (Pogonoski et al. 2002). The maximum size of this

species recorded from Australia is over 1.2 m disc width (Thorburn et al. 2003),

however, elsewhere it is reported to reach a disc width of almost 2 m and about 600

kg (Last and Stevens 1994). This species has previously been recorded from

Telegraph Pool and below Camballin Weir (barrage) in the Fitzroy River (Thorburn et

al. 2003) (Figure 1), and anecdotal evidence places this species upstream of Geikie

Gorge.

Carcharhinus leucas (bull shark)

This species has a world wide distribution in tropical and warm temperate seas with

an Australian range extending as far south as Sydney on the east coast, and Perth on

the west coast. It occurs in a wide range of habitats and although considered a marine

species, it is often described as a ‘freshwater’ species as a significant part of their

juvenile life is spent in freshwater. Additionally, this species is known to penetrate

freshwater systems for extended periods of time. Carcharhinus leucas are sometimes

confused with other coastal whaler sharks, and in particular with the very similar

pigeye shark (Carcharhinus amboinensis). The maximum known length of the bull

shark in Australia is about 3.4 m (Last and Stevens 1994). This species has been

encountered by Thorburn et al. (2003) as far inland as Camballin Weir in the Fitzroy

river, but anectdotal evidence has also placed this species further upstream near

Fitzroy Crossing (Figure 1).

Glyphis sp. C (northern river shark)

Glyphis sp. C is a medium sized whaler shark belonging to the Carcharhinidae. While

little is known regarding their biology, this species may be distinguished from Glyphis

4

sp. A, the other species occurring in Australia, by vertebral count (147-148 total

vertebral centra in sp. C cf. 217 of sp. A) (Campagno and Niem 1998). Although

known from several localities in the Northern Territory, this species is currently only

known in Western Australia from King Sound, and as yet has not been encountered in

the freshwater reaches of the Fitzroy River (Morgan et al. 2002, Thorburn et al. 2003,

Thorburn et al. 2004) (Figure 1).

Pristis clavata (dwarf sawfish)

Pristis clavata is a tropical species distributed in Australia from Cairns (Qld) to the

Kimberley (WA), in coastal and estuarine habitats. Although not included in the

‘freshwater’ category, it has been reported well inland, almost into freshwater, and is

common on mudflats in the Gulf of Carpentaria. In the Fitzroy River this species has

been encountered in the lower Telegraph Pool, and outside the river mouth in the

macrotidal mangrove systems and mudflats of King Sound (Last and Stevens 1994,

Morgan et al. 2002, Thorburn et al. 2003) (Figure 1).

1.3 Current threats to elasmobranchs in the Fitzroy River

Compared to teleosts, which may produce high numbers of eggs and develop rapidly,

sharks and rays are generally slow growing animals that mature after an extended

period and produce low numbers of well-developed offspring, thus resulting in them

being easily threatened. Furthermore, estuarine and freshwater elasmobranchs are

even more prone to population decline as they combine the above life history

strategies with all the problems associated with restricted habitats (eg. water quality,

habitat availability, dams and land uses). The popularity of the Fitzroy River by

recreational fishers, and the fact that fishes (including elasmobranchs) continue to

form part of the diets of people in the area, means that additional pressure is placed

upon these species. Thorburn et al. (2003) documented the threat to these species, as

a bycatch to the popular recreational barramundi fishery, in which they encountered

numerous dead sawfish on the banks of Telegraph Pool on two occasions. Many of

these carcasses still had fishing line attached, and several had their rostrums removed

(see Figure 12), probably taken as souvenirs.

5

1.4 Objectives

This project primarily aimed to determine the seasonal abundance of freshwater

elasmobranchs in the lower, middle and upper sections of the Fitzroy River. By

comparing these abundances, inferences relating to the movements of these species

can be made. Sampling over two periods also presented the opportunity to initiate a

sawfish tagging program, and although dependent upon the recapture of tagged

individuals, is additionally useful in monitoring movements over time.

6

2 METHODS

2.1 Sampling strategy

Originally, sampling was to be conducted to compare the seasonal abundance of

elasmobranchs in the lower, middle and upper reaches of the Fitzroy River, during the

wet and dry season. However, a small wet season over the summer of 2002/2003

meant that water levels encountered in May/June were much lower than expected, and

thus unrepresentative of wet season conditions (i.e. minimal flow rates and low water

levels). The subsequent sampling period (which was to be an exclusive dry season

sample) was therefore deferred, and provided a comparison between early dry

(May/June), and late dry conditions in November.

2.2 Sample sites

A total of 24 gill net sets, at 22 individual locations, were used to sample for

elasmobranchs during the early dry and late dry season in 2003 (Figure 2 and 3,

Appendix 1). Sampling in the Fitzroy River was conducted in the lower reach (upper

estuarine), Snag Pool and Telegraph Pool, the middle reach, below Camballin Weir,

and from the upper reach, upstream of Geikie Gorge. Longitude and latitude were

recorded at each sample site using a Global Positioning System (GPS). A sampling

site and distribution map was created using the aforementioned GPS data and

MapInfo (Mapinfo Corporation 1998). Sample site locations are provided in

Appendix 1.

Figure 4 illustrates the types of locations present at the time of sampling in all three

reaches. Figures 4a and b show Telegraph Pool in the lower reach of the Fitzroy

River in the early and late dry season, respectively. Waters in the early dry were near

fresh (0.4 ppt). Although water remains in this large pool of the lower estuary

throughout the late dry, water is not flowing from upstream into the pool, rather it is

now only supplied by incoming marine waters on large high tides. In the middle

reaches, the late dry sees a large drop in the water levels, and flow rates become close

to zero as shown by the lack of flow over Camballin Weir (Figures 4c and d). Pools

in the middle reaches have also become heavily contracted (Figures 4e and f). In

contrast, at Geikie Gorge, in the upper reach of the Fitzroy River, a large volume of

water is present in both the early and late dry season (Figures 4g and h). However,

7

Figure 2 Sites sampled for elasmobranchs in the Fitzroy River during J

WesternAustralia

Add

Rep

1

23

46

57

108

Derby

Fitzroy River

F

Middle

Lower

Upper

une 2003.

itional sample sites

licate sample sites

119

itzroy Crossing

8

Figure 3 Sites sampled for elasmobranchs in the Fitzroy River duri

2003.

WesternAustralia

Addi

Repl

Derby

Fitzroy River

Fi

12, 131415

1617

1819

222021

Middle

Lower

Upper

ng November

tional sample sites

icate sample sites

tzroy Crossing

23,24

9

a b

c d

fe

g h

Figure 4 Typical sampling sites and river morphology of the Fitzroy River during

June (left column) and November (right column) 2003. (a and b) Lower reach

(Telegraph Pool), (c and d) Camballin Weir (barrage), (e and f) middle reach (pools

below Camballin Weir) and (g and h) upper reach (Geikie Gorge).

10

although there is still a large volume of water at this site in the late dry, it no longer

flows out of Geikie Gorge (there is now a large sandbar at the lower end of the gorge

through which some water percolates), and the river above the gorge has become

heavily contracted and isolated. The size of Geikie Gorge may therefore provide a

significant refuge to these species through the dry season.

2.3 Elasmobranch migration

In an attempt to standardise sampling, 18 of the 24 gill net sets (i.e. excluding gill net

sets conducted at night) were used to compare abundances of elasmobranchs, with

three replicate samples conducted in each of the lower, middle and upper reaches of

the Fitzroy River, respectively, in both the early dry (June) and again in the late dry

(November) (Figure 2 and 3, Appendix 1). Sampling was conducted during daylight

hours using a combination of 20 m (2 m drop) monofilament gill net panels of 5, 7.5,

10, 15 and 20 cm stretched mesh, respectively. During sampling the 5 and 7.5 cm

stretched mesh were joined, as were the 10, 15 and 20 cm stretched mesh. Gill nets

were primarily anchored to run at right angles to the river bank or to block as much of

the channel as possible. Upon capture, individuals were removed from the net,

identified and released. The numbers caught and the length of the netting period was

also recorded. Additionally, all the sawfish were tagged (see below).

2.4 Interpretation of abundance data

The number of individuals captured by each 100 m replicate gill net set (Appendix 1)

was divided by the number of hours the net was fishing, thus providing a catch per

hour for the combined 100 m of net used at each sample site.

2.5 Tagging

The use of tags, such as that shown in Figure 5, is an effective way to monitor the

movements of fishes over time. Sampling in the Fitzroy River therefore presented the

opportunity to utilise tags from the Department of Fisheries Western Australia, which

are already used in elasmobranch tagging in offshore areas of the Kimberley Region.

Although dependent upon the recapture of the same individual at a later date,

information such as changes in growth and migration patterns can be collected. While

the data provided by recaptures is beyond the scope of this project, tagged individuals

may be captured and the information relayed back to the Department of Fisheries

11

Western Australia for years to come. Along with a unique number, the compact,

highly visible tags also include contact details for the aforementioned authorities. In

the case of sawfish, individuals were tagged on the pectoral fin or second dorsal fin.

Figure 5 The cattle-style tags used for tagging sawfish in the Fitzroy River.

.

12

RESULTS

3.1 Overall catch composition

A total of 68 elasmobranchs from three species were caught while sampling in the

Fitzroy River in the early and late dry season of 2003 (Appendix 1). Only two

‘freshwater’ elasmobranch species were encountered, these being P. microdon (26

individuls) and C. leucas (18 individuals). In addition, P. clavata (24 individuals)

was also captured in the lower reaches of the Fitzroy River. Of these, 39 of the

aforementioned elasmobranchs were captured during replicate sampling conducted

during daylight hours in the lower, middle and upper reaches of the Fitzroy River

(Table 2), 21 of these being P. microdon, four C. leucas and 14 P. clavata. The

remainder of the specimens (as listed in Appendix 1) were captured during night sets.

3.2 Catch composition of replicate sampling and abundance

Pristis microdon was the most abundant species encountered (21 individuals) during

replicate sampling, despite not being recorded in the upper Fitzroy River during

daylight sampling. This species was captured in both the lower and middle reaches,

during both sampling periods (Table 2). The mean number caught per net per hour, in

the lower and middle reaches during the early dry was 0.45 and 0.88 individuals,

respectively. In comparison, sampling during the late dry saw a reduction in the

abundance of this species in both the lower and middle reaches (0.20 and 0.40

individuals, respectively) being less than half that caught in the previous sample

(Figure 6).

During the early dry season, C. leucas was only encountered during daylight sampling

in the middle reach of the Fitzroy River, with an average of 0.18 individuals being

caught per net per hour (Table 2, Figure 7). While also caught in the middle reaches

during the late dry season sample, at an average rate of 0.61 individuals per net per

hour, it was additionally encountered in the lower reaches (at an average rate of 0.10

individuals per net per hour). Similar to P. microdon, most individuals were collected

from the middle reaches of Fitzroy River, in pools downstream of Camballin Weir

(see Discussion for the effect of the Camballin Weir on migration).

13

Table 2 Combined total number of elasmobranchs caught, and mean number of elasmobranchs caught per 100 m of net per hour, during

sampling from the lower, middle and upper reaches of the Fitzroy River in June (early dry) and November (late dry).

Pristis microdon Pristis clavata Carcharhinus leucas Month Season Reach Total # Mean catch/hr Total # Mean catch/hr Total # Mean catch/hr

Jun Early dry Lower 4 0.45 0 0 0 0

Jun

Early dry Middle 9 0.88 0 0 2 0.18

Jun Early dry Upper 0 0.00 0 0 0 0

Nov Late dry Lower 2 0.20 14 1.42 1 0.10

Nov Late dry Middle 6 0.40 0 0 1 0.06

Nov Late dry Upper 0 0.00 0 0 0 0

Total 21 14 4

14

Reach of the Fitzroy River

Num

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Cau

ght

Per H

our

June SampleNovember Sample

Lower Middle Upper

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Figure 6 The mean number of Pristis microdon captured during daylight sampling in

the early dry season (June) and late dry season (November).


Num

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ght

Per H

our


Lower Middle Upper

0.2

0.4

0.6

0.8

1.0

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Figure 7 The mean number of Carcharhinus leucas captured during daylight

sampling in the early dry season (June) and late dry season (November).

15


Num

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Cau

ght

Per H

our


Lower Middle Upper

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Figure 8 The mean number of Pristis clavata captured during daylight sampling in

the early dry season (June) and late dry season (November).

Pristis clavata was the second most abundant species encountered during this study.

As can be seen in Table 1 and Figure 8, this species was only caught in the lower

reach (upper estuary) of the Fitzroy River in the late dry season. During this period,

this species had the highest capture rate, with an average of 1.42 individuals caught

per net per hour.

3.3 Tagging Data

A total of 44 sawfish were tagged during sampling in June and November. Of these

25 were P. microdon, ranging in length from 815 to 2770 mm TL, and 19 were P.

clavata, which ranged in length from 1295 to 2255 mm TL. Of the 12 P. microdon

tagged in the early dry season, only one sawfish was recaptured, near the point of first

capture in the middle reach of the Fitzroy River in the late dry. In the 4 months

between captures, this individual had grown 30 mm, from 2150 to 2180 mm TL.

16

4 DISCUSSION

4.1 The current survey

This project aimed to identify movements of freshwater elasmobranchs in the Fitzroy

River by comparing the seasonal change in relative abundances of each species from

sampling sites in the lower, middle and upper reaches of the river. Replicate sampling

conducted during daylight hours in each reach of the Fitzroy River saw a large degree

of variation in the number of individuals caught. The naturally low abundance of

elasmobranch species thus meant catches were generally low, however inferences in

relation to the movements of elasmobranchs in the Fitzroy River can still be made

from the data collected, and observations made in the field. Notwithstanding this, 68

elasmobranchs were captured in the river.

4.2 Elasmobranch migration in the Fitzroy River

Although the contrast in river morphology between the wet and dry season is extreme,

a large degree of variation also exists between early dry and late dry conditions. In

contrast to the late dry, flow rates remain quite high in the early dry, and large pools

throughout the river are interconnected allowing migration between pools.

Additionally, freshwaters flow into the lower reaches with the salt water interface

being closer to the sea. Salt waters are only encountered in the lower reaches during

this period on very large tides experienced in King Sound (up to 12 m). Waters at

sites such as Telegraph Pool therefore have low salinities during the early dry,

however in the late dry, negligible freshwater input means that these sites become

more saline. In comparison, Geikie Gorge has the least variation in conditions

between the early and late dry.

More P. microdon were caught in both the lower and middle reaches of the Fitzroy

River during the early dry than in the late dry season sampling period. In reference to

the middle reach, the comparatively high abundance of this species may be attributed

to the presence of Camballin Weir. As water levels fall after the wet, the weir

presents a significant obstacle to individuals moving upstream, thus concentrating

sawfish below this point. This is highlighted by the recapture of one sawfish that

remained below the weir from June to November. During the early dry season

sample, gill netting was conducted a few kilometres below the weir in a series of large

interconnected pools. These sites therefore provide a refuge during the dry season,

17

but allowed individuals to move freely between these pools. Although it could be

expected that abundances may be higher in these pools during the late dry, as water

levels drop and individuals become concentrated, this was not found to be the case, as

abundances were seen to be reduced in the late dry. It is possible that the higher catch

rates encountered while netting in the early dry season were due to higher levels of

activity, and unrestricted movements between pools. In contrast, during the late dry

water levels are reduced, pools become contracted and thus the connections existing

between these pools became fewer. The lower abundances observed during sampling

in the late dry season may therefore have been ‘pool’ dependent, as the sawfish

become ‘locked in’ and are not as active in this reach. It may however also be

possible that fishers in the area reduce the numbers of animals trapped in these pools

over the months of the dry season, when sites such as the Camballin Weir are

accessible.

The contraction of the river may also be used to explain the reduction in P. microdon

numbers in the lower reaches between the early and late dry season. In the early dry,

sampling sites were of low salinity and individuals were able to move upstream from

Telegraph Pool. In the late dry however, the minimal freshwater input and

contraction of the river at this point resulted in saline conditions at these sites. As the

river fell, sawfish may therefore have moved upstream to waters of lower salinity,

prior to the late dry season sample. Additionally, sampling during this period was

conducted soon after a large +12 metre tide, thus connecting Telegraph Pool to

freshwater pools upstream for a short period. Unlike sampling in the lower and

middle reaches, where nets were often set in waters <2 m deep (thus blocking a

significant amount of the rivers channel) and the channel is comparatively narrow,

sampling in the upper reaches (Geikie Gorge) was often conducted in deeper water in

pools that stretched for >20 km, with the nets only blocking a relatively small

proportion of the channel. Although two P. microdon were captured in Geikie Gorge

at night, none were caught during daylight (replicate) sampling, this possibly being

attributed to the sampling effort conducted in the large deep water body.

Similar to P. microdon, C. leucas was also encountered in higher numbers in the

middle reaches of the Fitzroy River, below Camballin Weir. While few specimens

were collected during replicate sampling (four individuals) in the pools below

18

Camballin Weir, 10 individuals were caught at night immediately below the weir

during the early dry season, and another two in the late dry season. The concentration

of elasmobranchs below the weir strongly suggests that C. leucas and P. microdon are

restricted for up to 10 months below this impassable barrier, and subsequently become

‘locked’ in pools over the late dry. Here, predation by other sharks may be high, and

the capture of these species by fishers likely, as Camballin Weir is popular with

recreational fishers and campers.

Pristis clavata was only encountered during this study in the lower reaches of the

Fitzroy River during the late dry season sample. It is likely that the cessation of

freshwater entering the lower pools, and subsequent increase in salinity due to tidal

inflow favours this marine species.

Although H. chaophraya was not captured during this study, one imprint (often

created by rays while feeding) was observed in the sand in shallow waters

immediately below Camallin Weir during the early dry season. Similarly, Thorburn

et al. (2003) also observed (and subsequently captured) this species moving upstream

in shallow water (<0.5 m deep) at this location. Another specimen was also captured

at Telegraph Pool, in the lower Fitzroy River, with the use of a baited longline. Gill

nets are however essentially ineffective at capturing this species. Anectdotal evidence

also places this species above Geikie Gorge.

In reference to Glyphis sp. C, this species has yet to be recorded within the Fitzroy

River, and although often referred to as a ‘freshwater’ species, may favour the

macrotidal marine feeder creeks entering King Sound (see Thorburn et al. 2004).

4.3 Freshwater sawfish of the west Kimberley

Like many of the elasmobranch species occurring in northern Australia, distribution

data for P. microdon is lacking. Although anecdotal records elude to the fact that this

species was occurring in waters in the west Kimberley, it was not until a

comprehensive survey of the Fitzroy River by Morgan et al. (2002), and another study

by Thorburn et al. (2003) with the specific aim of capturing elasmobranch species,

that the significance of the Fitzroy River population was revealed. The remoteness

and difficulties in accessing many of the rivers in the west Kimberley, have therefore

19

meant that inshore areas, and rivers have received limited attention, with several

species (such as Glyphis sp. C) remaining unreported until recently.

Although some sampling by the authors has been conducted throughout King Sound,

and other rivers entering it (such as the May and Robinson Rivers), thus far P.

microdon has only been captured by the authors within the rivers, and have only been

a maximum of 2.8 m in length, despite this species being reputed to reach 7 m in

length (Figure 9 illustrates the length frequency-histograms for female and male P.

microdon caught in the west Kimberley by the authors and other researchers).

Furthermore, all the male specimens have been immature, as indicated by small non-

calcified claspers, such as that seen in Figure 10a. Similarly, dissection of several

female specimens of ~2.5 metres in length revealed they were also immature. Thus, it

is possible that the freshwaters of the Fitzroy River act as a nursery for juvenile

animals of this species, similar to the strategy used by C. leucas, where large mature

individuals remain offshore, and juveniles penetrate freshwater riverine reaches far

inland.

While there are currently no published records of P. microdon occurring offshore

(although some anectdotal evidence has been collected), this species was recently

captured during another project undertaken by Murdoch University researchers, at two

sites a significant distance south of King Sound (80 Mile Beach (Wallal) and near

Cape Keraudren). The latter two specimens were large, with one being a ~3.5 m

female, and the other a >3 m mature male (see Figure 10b illustrating the large

calcified claspers). Thus, these captures provide some support for the notion that

upon maturation, individuals move out of the rivers, and remain offshore, and may

move inshore near the mouths of rivers to pup (Figure 11).

The capture of P. microdon at Cape Keraudren is also interesting as this species has

not been recorded in the nearby De Grey River, or in any other river throughout the

Pilbara region. Futhermore, there are no large rivers between King Sound and Cape

Keraudren, as between these points the Great Sandy Dessert extends to the coast.

Furthermore, to the north of King Sound, much of the land is topographically high,

such as the Mitchell Plateau, and thus numerous rivers end in waterfalls. A short

estuary often exists, however the steepness of the land provides a barrier to upper

20

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Length-frequency histograms of female and male P. microdon captured

itzroy River during this study, from those specimens collected from the

ver (and Robinson River) and during a survey conducted by Thorburn et al.

large female collected from Cape Keraudren, and large male from 80 Mile

llal).

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Figure 10 The male reproductive appendage (claspers) of P. microdon from a (a) 2.2

metre immature male, and (b) mature male ~ 3 m in length. Note the large calcified

claspers on the mature male.

a) b)

WesternAustralia

Fitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy River

80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach


BroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroomeBroome

De Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey River

Fitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy Crossing

Figure 11 The sites at which P. microdon has been recorded by the authors.

22

riverine reaches. While conducting surveys of the Drysdale and King Edward rivers

(north Kimberley), and working out of Kalumburu, the authors were unable to find

any anecdotal accounts of sawfish occurring in fresh inland waters. Thus the waters

of King Sound may provide the most significant habitat for this species in this region.

Indeed, the collection of more data may demonstrate that King Sound (and rivers that

flow into it) may act as the only nursery areas for this species in the region with

mature animals radiating out from this area following the coastline in inshore marine

waters, and perhaps returning to this locality to pup (and/or breed). The likelihood

that King Sound and its rivers are the most important nursery ground for this species

in the region may be due to the following:

• The Fitzroy River has the highest run-off of any river in Western Australia,

and largest catchment (~90 000 km2) of any river in the Kimberley, with the

river comprised of a series of large often unconnected pools and billabongs

during the dry.

• Tropical rivers are often more productive than temperate systems. This is

highlighted by the high diversity of fishes in the large deep pools of the

Fitzroy River (the richest fluvifaunulae in Western Australia (Morgan et al.

2002, in press)). These species often occur in extremely high abundances and

are potentially an important food source for many of the larger predatory

species, such as the freshwater sawfish.

• The topography of the land surrounding King Sound and the Fitzroy River is

comparatively flat (especially below Fitzroy Crossing where the floodplain

extends some 300 km to the coast), thus the entrance to this river is accessible

throughout the year. Even during the dry season, large tides connect the upper

estuary with freshwater pools, and thus individuals can move upstream into

relatively ‘safe’ waters.

• Large potential predators of sawfish, such as adult sharks and estuarine

crocodiles, rarely penetrate farther upstream than the estuary.

The length-frequency histograms (Figure 9) indicate that more females than males

were caught in the Fitzroy River, with a sex ratio of females to males of ~1.5:1 (i.e. 29

females and 19 males). The collection of more length-frequency data may produce a

more accurate sex ratio, however these discrepancies, together with the larger size

23

classes of the females, may indicate that the males mature at a smaller size, and/or

leave the river earlier, possibly after attaining maturity. For example there appears to

be three distinct modal (year) classes of females (modal lengths of 800-900, 2000 and

2400 mm TL) and males (modal lengths 800, 1450 and 2000-2150 mm TL), and that

the modal lengths of these ‘year’ classes are considerably larger for the females.

4.4 Further research

In terms of elasmobranch migration in the Fitzroy River, a more comprehensive

understanding could be achieved by conducting multiple samples in more locations in

each reach, throughout all seasons, in conjunction with utilising recapture data from

tagged individuals. Although covering a large catchment, access to the river is

limited, however the expansion of the current project should additionally include sites

further upstream of Geikie Gorge, those sites less accessible to the general public, and

during the wet when the river can be negotiated by boat. The continuation and

expansion of a tagging program will also provided data on the movement of

individuals over an extended period of time. In light of the current project, the

authors, in conjunction with the Kimberley Land Council, traditional owners and

World Wildlife Fund (WWF) Australia, have instigated a tagging program that will

involve training indigenous persons from various communities along the Fitzroy

River to tag and record movements of P. microdon. While elasmobranchs are

important as a food source to many people living within the Fitzroy catchment,

including traditional owners, anecdotal evidence suggests their numbers and

occurrences are in decline in the upper reaches of the Fitzroy River. Thus by

providing education, the management of this resource and longevity of the occurrence

P. microdon throughout the Fitzroy River will be maintained.

Satellite tag tracking is another way to monitor the movement of individuals over a

short period. This would be especially useful in tracking individuals in the wet season

when there are no barriers, such as Camballin Weir, to their movements. Data

collected from such tags also enable researchers to understand daily movements

within rivers and patterns of activity.

The continuation of fish surveys in rivers throughout the Kimberley region is essential

if accurate estimates of species distribution and population status are to be

24

determined; data that are essential for the management of these species. Furthermore,

an understanding of life-histories, and utilisation of different habitats throughout their

life, means that we will be better able to manage them, and their environment. In the

case of P. microdon, an extensive survey of inshore embayments and offshore areas of

King Sound will determine the true significance of this area, and demonstrate if in

fact, it provides an essential breeding and birthing ground.

Although only two freshwater elasmobranch species were encountered in the Fitzroy

River during this study (and an additional marine sawfish species), H. chaophraya

and Glyphis sp. C were not. While Glyphis sp. C was the focus of another study by

the authors, H. chaophraya has received little attention. Few records of this cryptic

species exist throughout Australia, however anecdotal evidence suggests the

occurrence of large numbers of this species in isolated upstream areas of the Fitzroy

River. It is possible therefore, that as is the case for P. microdon, the Fitzroy River

represents a significant habitat for this species.

4.5 Recommendations

Of immediate concern in relation to the conservation of P. microdon, P. clavata, H.

chaophraya and Glyphis sp. C is the lack of protection of these species in inshore

waters (between the low water mark and three nautical miles offshore) and in the

Fitzroy River itself. As each of these rare species is predominantly found in State

waters, immediate consideration should be given for their protection. Penalties would

subsequently exist for acts such as that witnessed by Thorburn et al. (2003), in which

numerous P. microdon were slaughtered by recreational fishers in Telegraph Pool

(one H. chaophraya and two C. leucas were also found dead).

At this point in time, and while our knowledge is still lacking, public education

regarding the rarity of freshwater sawfish, sharks and rays, and the significance of the

environment in which they exist is likely to offer the greatest benefit to the survival of

these elasmobranch species in the Fitzroy River. Signs at popular fishing spots and

access points are an effective way to illustrate to those who utilise the Fitzroy River

the uniqueness of these species, and additionally creates a sense of stewardship to

these iconic species for communities of the west Kimberley. Awareness can also be

achieved by promoting these rare species to tourists and local wildlife and fishing

25

guides. Additionally, efforts should be made to encourage people not to discard

rubbish, such as fishing line, in or near the river, as, for example, the entanglement of

debris with the rostrum of a sawfish can cause serious or fatal injury, as seen in Figure

12.

Throughout this project the effects of a barrier such as Camballin Weir on the

migration of fishes was graphically illustrated, as animals accumulated directly below

the weir and in nearby downstream pools whilst presumably heading upstream. While

the weir itself is inundated during the peak floods of the wet season, for much of the

year it is exposed. In light of this, a comparison of the fish community and

abundances of pools immediately above and below the weir would provide valuable

insight into the effects the weir has on migrating species, such as elasmobranchs,

barramundi and mullet, and thus managers would be better informed as to what action

(such as the construction of a fishway, or removal of the weir) should be taken in

regards to the dilapidated irrigation dam.

Figure 12 The effects of discarded fishing line on the rostrum of a freshwater

sawfish. NB There is no hook attached

26

5 REFERENCES

Boulter, S.L. (2002). Coast Law in Western Australia. Environmental Defenders

Office WA, Perth, Australia.

Compagno, L.J.V. and Niem, V.H. (1998). Order Carcharhinidae. pp 1312-1360. In:

Carpenter, K.E. and Niem, V.H. (eds.), FAO Species Identification Guide for

Fisheries Purposes. The living marine resources of the western central Pacific.

Volume 2. Cephalopods, crustaceans, holothurians and sharks. FAO, Rome

Herre, A.W.C.T. (1955). Sharks in freshwater. Science 122: 417.

IUCN (2003). The Conservation Status of Australian Chondrichthyans: Report to the

IUCN Shark Specialist Group Australia and Oceania Regional Red List

Workshop, Cavanagh, R.D., Kyne, P.M., Fowler, S.L., Musick, J.A., and

Bennett, M.B. (eds). The University of Queensland, School of Biomedical

Sciences, Brisbane, Australia: 170 pp.

Last, P.R and Stevens, J.D. (1994). Sharks and Rays of Australia. CSIRO Division of

Fisheries, CSIRO, Australia.

Merrick, J.R. and Schmida, G.E. (1984). Australian Freshwater Fishes. Biology and

Management. Griffin Press, Netley, South Australia.

Morgan, D., Allen, M., Bedford, P. and Horstman, M. (2002). Inland Fish Fauna of

the Fitzroy River Western Australia, including the Bunuba, Gooniyandi,

Ngarinyin, Nyikina and Walmajarri Aboriginal names. Report to the Natural

Heritage Trust: 56 pp.

Morgan, D.L., Allen, M.G., Bedford, P. and Horstman, M. (in press). Fish fauna of

the Fitzroy River in the Kimberley region of Western Australia – including the

Bunuba, Gooniyandi, Ngarinyin, Nyikina and Walmajarri Aboriginal names.

Records of the Western Australian Museum.

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Pogonoski J.J., Pollard, D.A. and Paxton, J.R. (2002). Conservation Overview and

Action Plan for Australian Threatened and Potentially Threatened Marine and

Estuarine Fishes. Report to Environment Australia.

Thorburn, D.C., Peverell, S., Stevens, J.D., Last, P.R. and Rowland, A.J. (2003).

Status of Freshwater and Estuarine Elasmobranchs in Northern Australia.

Report to the Natural Heritage Trust: 79 pp.

Thorburn, D.C., Morgan, D. and Rowland, A.J. (2004). Search for the Northern River

Shark (Glyphis sp. C). Report to the Natural Heritage Trust: 22 pp.

Whitley, G.P. (1940). The fishes of Australia. Part 1. The sharks, rays, devilfish and

other primitive fishes of Australia and New Zealand. Australian Zoological

Handbook. Royal Zoological Society of New South Wales, Sydney.

28

APPENDIX 1 Sites sampled for elasmobranchs in the Fitzroy River, the number of hours gill nets were set (small and large mesh refers to the

combination of the 5 and 7.5 cm, and 10, 12.5 and 15cm stretched meshes, respectively), and species captured at each site.

Set # Sample Location Reach Set Latitude Longitude Small mesh (hrs set)

Large mesh (hrs set) C. leucas P. clavata P. microdon

June sample 1 Telegraph Pool Lower rep 1 17.638 123.567 2.33 3 1 2 Snag Pool Lower rep 2 17.598 123.574 1.5 2 2 3 Snag Pool Lower rep 3 17.596 123.565 2.75 2.92 1 4 Camballin Weir Middle night set 18.188 124.492 - 12 10 5 Camballin Pools Middle rep 1 18.177 124.464 3.58 3.33 5 6 Camballin Pools Middle rep 2 18.168 124.461 3.75 4.42 1 1 7 Camballin Pools Middle rep 3 18.183 124.472 3 3.17 1 3 8 Geikie Gorge Upper rep 1 18.034 125.744 5.33 5.08 9 Geikie Gorge Upper rep 2 18.021 125.759 2 2 10 Geikie Gorge Upper rep 3 18.044 125.740 2.5 2.5 11 Geikie Gorge Upper

night set

18.111 125.698 2 (2) 2 (2) November sample 812 Snag Pool Lower rep 1 17.586 123.568 4 4.25 1 13 Snag Pool Lower night set 17.586 123.568 12.5 12.75 2 10 1 14 Telegraph Pool Lower rep 2 17.636 123.567 3 3.25 1 11 15 Telegraph Pool Lower rep 3 17.638 123.567 3 3.25 2 2 16 Camballin Weir Middle night set 18.177 124.464 12 (6,8) 2 2 17 Camballin Pools Middle rep 1 18.183 124.471 5.67 5.5 1 3 18 Camballin Pools Middle rep 2 18.176 124.464 4.67 4.5 3 19 Camballin Pools Middle rep 3 18.168 124.461 4 4 20 Geikie Gorge Upper rep 1 18.024 125.758 5.5 6 21 Geikie Gorge Upper night set 18.038 125.744 - 9 2 22 Geikie Gorge Upper rep 2 18.016 125.762 5.5 6 23 Geikie Gorge Upper rep 3 18.044 125.740 3.75 7.25 24 Geikie Gorge Upper night set 18.044 125.740 1.17 13

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elasmobranchs in the fitzroy river, western australia · fitzroy river, have identified the fitzroy...

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