Małgorzata Godlewska Department of Antarctic Biology Polish Academy of Sciences Warsaw, Poland

XX Polar Symposium Lublin, 1993

NATURAL AND MAN INDUCED VARIATIONS IN KRILL BIOMASS (WESTERN ANTARCTIC, 1981-1989)

INTRODUCTION

Krill, Euphausia superba Dana, are a major component of the Southern Ocean food chain. Knowledge of their distribution and biomass is fundamental not only for assesing the importance of krill to the Antarctic marine ecosystem, but also for managing krill fishery (BIOMASS 1979).

Investigations of the biology and distribution of Antarctic krill had its origins in the pioneering studies of the Discovery Committee between the First and Second World Wars. In the early 1970's, growing pressure on many traditional fisheries prompted an increased interest in the commercial potential of Antarctic living resources, and particularly krill. The krill fishery began in the late 1960's and expanded during the 1970's to a maximum annual catch of just over half amillion tonnes (Fig. 1). Growing need for the management of the resources and protection of the Antarctic marine ecosystem was widely recognized and resulted in the formulation in 1976 of an international research program, BIOMASS (Biological Investigations of Marine Antarctic Systems and Stocks), in which Poland actively participated taking part in two international experiments FIBEX and SIBEX. To asses the impact of fisheries on the resource, the variations in krill abundance resulting from either natural variations in the krill population dynamics or induced by environmental factors have to be known.

The first attempts to estimate total krill standing stock were based on indirect methods, estimates of predator consumption, primary production and the apparent surplus of krill arising from the decline in large baleen whale stocks (see for review: Everson 1977, Lubimova et al. 1980, Lillo and Guzman 1982). They have shown a wide range of values, from 130 min t (Everson 1977) to 1350 min t (Makarov and Shevtsov 1971) and indicated a very large krill standing stock, substantially exceeding the global fish catch (Gulland 1970, Laws 1985, Anon 1986). The direct, net and acoustical measurements of krill biomass also varied by more than the order of magnitude indicating krill biomass estimates, up to 1200 min t (Doi and Kawakami 1980). According to Polish investigators krill stock in the Antarctic is about 440 min t (Kalinowski and Witek 1981). By

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means of existing methods it is not possible to measure krill biomass in the whole area of its occurrence, about 36 x 106 km2. The short time measurements, often performed in areas of high krill concentrations, expanded over the whole range of krill occurrences lead to very high krill estimates, suggesting no danger in overfishing the resources. From management point of view, not that much total biomass, what the information by regions is of crucial importance. Western Antarctic, the object of Polish investigations during four expeditions, is the area known to be tradicionally rich in krill (Marr 1962, Mackintosh 1973, Kalinowski 1982, Hampton 1983). This region is of particular interest for two reasons; it is the region of highest krill concentrations (Anon 1986) and of the heaviest krill fishery (SC-CAMLR 1991). Thus, this area is especially exposed to man induced disturbations of ecological balance.

The aim of this paper is to look at information on krill biomass variability from acoustic surveys during four Polish expeditions (1981-1989) and compare this with data from the fishery.

MATERIALS AND METHODS

Hydroacoustical data collected on board of „Profesor Siedlecki" during 4 expeditions to the Western Antarctic were used for analysis. The information on dates and places of the expeditions are summarized in Table 1. The SIMRAD echosounder EK 120 (operating frequency 120 kHz) and analog echointegrator QMMKII were used in a 24-hour hydroacoustic watch system. The depth range of echosounder was 250 m and of echointegrator 110m. The echo signal was integrated in 1 Nm intervals and recorded together with time, position and sea depth. In the last expedition additionally computer IBM-PC/XT equipped with 40 MB hard disc and standard A/D converter card was used for data storage and analysis.

The acoustic method converts echo energy to absolute biomass by assuming that the echo return is the sum of individual scatterers and by assuming empirical (Kalinowski et al. 1980, Samovolkin 1980, Foot et al. 1990, Greene et al. 1991, Hewitt et al. 1991) or modelled (Johnson 1977, Godlewska 1982, Stanton 1988) acoustic target strength for individual krill. Detailed analysis of the acoustic data followed the methods recommended by BIOMASS Working Group (Anon 1986). Mean surface density of krill was calculated using formula:

p _ J Q 0.1 (SV 4-10 log R-TS)

where p — is krill density (ind/m2) R — is thickness of integration layer (m)

TS — mean target strength of krill (dB) SV — volume backscattering strength (dB)

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Value of SV is characterising scattering properties of krill swarms and is calculated from echointegrator readings according to equation:

SV = C+ lOlog I

where I — deflection of echointegrator in mm

С — constant, dependent on echosounder performance and settings.

TS is the target strength of krill, whose definition is:

T S = i o i o g ( i A )

where I0 — is incident intensity I r — is reflected intensity measured at distance of 1 m.

The Working Group on Krill on its latest meeting in Jalta (Anon 1991), recommended to use for the target strength the following formula:

TS=-127.45 + 34.85 log L

All the biomass values cited in present work are recalculated according to this new expression. The weight of krill was calculated from the equation (Rakusa-Suszczewski 1981):

W = 0.0018 L 3-3831

where L — length of krill in mm

W — wet weight of krill in mg

and the biomass density from:

B=W in g/m2 or t/Nm2

The area investigated was devided into fine scale rectangles 0.5° latitude x 1° longitude (as required currently by CCAMLR for reporting krill catches) and in each of them mean krill density was calculated (Fig. 2). The total biomass was received as summ of biomasses in separate rectangles.

RESULTS

Data on krill abundances during considered expeditions have been sum-marized in Fig. 2. They show great variability both in space and time, with differences in the mean krill density of 2 or even 3 orders of magnitude between the rectangles frequently encountered. The detailed anylysis of the map leads to conclusion that the largest krill concentrations are on shelf of Antarctic Peninsula and of South Shetlands, in the Bransfield Strait and around the

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Elephant Island. The most poor in krill are open waters of the Drake Passage. The time variability is also well pronounced. During FIBEX (Feb/March 1981) krill concentrations were very high, especially in the southern part of the Bransfield Strait where they exceeded 1000 t/Nm2 (E8, F8, H7,16). Total krill biomass, estimated in the area of 47909 Nm2 investigated, was 22 mln t. During SIBEX (Dec. 1983/Jan. 1984) krill concentrations were the lowest ever met. In comparison to FIBEX results there was a dramatic change in krill biomass in the Bransfield Strait, where for some of the rectangles the difference was as much as two or even three orders of magnitude (E8, H7,16). In the few boxes only, mainly in the south of the Elephant Island krill concentrations excedeed 100 t/Nm2 . The overall biomass during SIBEX, measured on 53757 Nm2 was about 10 times smaller than during FIBEX and accounted to 2.3 mln t. During BIOMASS III expedition measurements were performed twice, in October/November 1986, that is during Antarctic early spring, and again two months later, in January 1987. The average concentrations of krill in spring were 2.4 times smaller than in summer, although in northern Bransfield Strait (H5,15) the difference was as big as nearly 10 times. The total biomass measured during spring on area of 2305 Nm2 accounted to only 250 thousands tons, while during summer on the area of 9736 Nm2 accounted to 2.4 mln tons. The BIOMASS IV (Dec. 1983/Jan. 1984) experiment was performed along the ice edge in the northern Weddell Sea. The overall biomass was low, 1.3 mln tons on 14861 Nm2 . Krill distribution was not even, comparatively high krill concentrations were in the northern Bransfield Strait and near Elephant and South Orkney Islands, and very low in the middle part of the investigated area, between the Elephant and South Orkney Islands. Comparison of krill abundances during all four expeditions is presented in Table 2.

DISCUSSION

It is evident from Fig. 2 and Table 2 that krill biomass undergoes large fluctuations both in space and time. Exist regions traditionally rich in krill, like Elephant Island and Bransfield Strait and regions of low drill abundances as the Drake Passage. The interannual variations for the total area are of the order of magnitude. There could be many factors responsible for observed fluctuations of krill biomass. One of them is seasonal variability. During summer, with development of phytoplankton blooms, also krill grows rapidly. It has been estimated by Godlewska and Rakusa-Suszczewski (1988) that increment of krill biomass due to growth is about two times per month. This partly explains higher biomass observed during FIBEX than during other expeditions performed ealier in season. Another cause for biomass variability in the region could be the interannual demographic changes. Priddle et al. (1988) estimated that with

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a complete recruitment failure in one season, the total population would be reduced to 36.6% of the stable level. From these estimates it is clear that biological factors alone do not explain the range of biomass variations observed. Apart from biological causes there are also physical ones, which affect krill distribution. It has been documented in literature (Marr 1962, Everson 1977, Maslennikov 1980, Lubimova et al. 1980, Amos 1984) distribution is determined by water circulation pattern. Also on local scale hydrodynamical forces may lead to redistribution of that large scale krill biomass and cause its regional fluctuations. Finally, variations may be caused by man due to fishing activity. CCAMLR (Commission for the Conservation of Antarctic Marine Living Resources) devided the Southern Ocean into the three major Statistical Areas, for which information on krill catches has to be reperted. In the Atlantic sector of the Southern Ocean there is Statistical Area 48, where over 90% of the total world catch is currently taken (Everson and Goss 1991). It accounts to 31276 Nm2 and covers three subareas at localities around Antarctic Peninsula, South Orkney Islands and South Georgia, thus it includes areas where polish investigations were performed. To estimate krill biomass in the Area 48 on the basis of polish data we assumed that the mean krill density in subsequent years is equal to the value received from measurements during relevant expeditions (Table 2). The estimated krill biomass together with reported catches (Everson 1988, SC-CMLR 1992) for 4 seasons considered are presented in Table 3. In none of years the total catch exceeded 0.4 mln tons and was much lower than precautionary catch limit on krill in Area 48 set by CCAMLR to 1.5 mln ton. The total catch was also much smaller than natural variations in krill abundance and such small decrease of biomass as was due to fishery could not be detected with the present sampling systems.

However, although the total krill catch is low relative to estimates of total stock in Area 48, high localized catches may be having a detrimental effect on some predators, especially during the months in which they are breeding and having to forage for food in the vicinity of their breeding sities where these are situated on land. The primary areas of commercial fishing in the Western Antarctic are to the north of Livingstone, King George and Elephant Islands, west of South Orkney Islands and in winter around South Georgia (SC-CAMLR 1992). Fishing fleets prefer to operate close to islands because concentrations of krill tend to occur in predictable locations there, associated with shelf and bottom topography. These locations are closer than 100 km from the islands, the range of foraging for most of land based predators. The fishing season starts in November and the largest catches are usually taken from January through to March (SC-CAMLR 1992). It means that fishing is concentrated in the months and locations that are critical to land based predators. Penguins that are rearing chicks have restricted foraging ranges from the end of November until February and lactating for seals have a restricted foraging range from December through

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March. Available evidence does not allow a determination of whether the fishery is having a marked effect on colonies of penguins and seals. For the assesing of real impact of fishery on krill and krill-related population the smaller scale analysis than one available at the moment need to be undertaken.

REFERENCES

Amos A. F., 1984: Distribution of krill (Euphausia superba) and the hydrography of the Southern Ocean: large-scale processes. J. Crust Biol. 4 (spec No 1): 306-329

Anon, 1986: Post-FIBEX A c o u s t i c workshop, Frankfurt (FRG) 3-14 September 1984, BIO-MASS Rep Ser 40

BI OM ASS, 1979: Meeting of the Group of specialists on Living Resources of the Southern Oceans. BIOMASS Report Series 7, 15 pp

Doi Т., Т. Kawakami, 1980: The estimation of krill abundance in the Antarctic by analysis of echogram. Working Party News 2(5): 1-11

Everson I., 1977: The living resources of the Southern Ocean GLO/SO/77/1 Rome, 155 pp Everson /., 1988: Can We Satisfactorily Estimate Variation in Krill Abundance. In: Antarctic

Ocean and Resources Variability, Ed. D. Sahrhage, Berlin, Springer-Verlag, 199-208 Everson I., K. Goss, 1991: Krill fishing activity in the southwest Atlantic, Antarctic Science 3:

351-357 Foot K. G.. Everson /., Watkins J. L., Bone D. G., 1990: Target strength of Antarctic krill

(Euphausia superba) at 38 and 120 kHz. J. Acoust. Soc. Am. 87: 16-24 Godlewska M„ 1982: Acoustic target strenght of krill. Pol. Polar Res. 3: 253-257 Godlewska M., Rakusa-Suszczewski S., 1988: Variability of krill, Euphausia superba, Dana 1852

(Crustacea, Euphausiacea), distribution and biomass in the Western Antarctic (Bransfield Strait, Drake Passage, Elephant Island) during 1976-1987, Inv. Pesq. 52(4): 575-586

Greene C.H., Т. K. Stanton, P. H. Wiebe, S. McClatchie, 1991: Acoustic estimates of Antarctic krill. Nature, 349: 110

Gulland J. A., 1970: Food chain studies and some problems in world fisheries. In: Marine food chains. Ed. J. H. Steele, Oliver and Boyd, Edinburgh, 296-315

Hampton /., 1983: Preliminary report on the FIBEX acoustic work to estimate the abundance of Euphausia superba. Proceedings of the BIOMASS colloquium in 1982. Memoirs of National Institute of Polar Research, Special issue No. 27: 165-175

Hewitt R. P., D. A. Demer, 1991: Krill abundance. Nature, 353:310 Johnson R. K., 1977: Sound scattering from a fluid sphere revisited. J. Acoust. Soc. Am. 62:44-52 Kalinowski J., A. Dyka, L. Kilian, 1980: The target strength of krill. Pol. Polar Res. 1(4): 147-153 Kalinowski J., Witek Z., 1981: Rough estimation of Antarctic krill stock. ICES С. M. L: 18, 6p Laws R. M., 1985: The ecology of the Southern Ocean. Amer. Scient. 73: 26-40 Lillo VS, Guzman O. F., 1982: Study ofthe abundance, distribution and behaviour of krill at the

Bransfield Strait and Drake Passage by means of hydroacoustic techniques. INACH Sci Ser 28:17-45 Lubimova T. G., Shust К. V., 1980: Quantitative Study of the Antarctic krill consumed by

pronciple groups of species. In: Biological Resources ofthe Antarctic Krill, Ed: T. G. Lubimova, VNIRO, Moscow, 203-224

Mackintosh N. A., 1973: Distribution of post-larval krill in the Anntarctic. Discovery Rep. 36: 95-196

Makarov R. R., Shevtsov V. V., 1971: Nekotorye problemy raspredelenija i biologii antarctiches-kogo krila. In: Osnovy biologiczeskoj produktivnosti okeana i ego ispolzovanie, Nauka, Moscow, 81-87

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Marr JWS, 1962: The natural history and geography of Antarctic krill (Euphausia superba). Discovery Rep 32: 33-464

Maslennikov V. V., 1980: Modern concepts on the large scale circulation of the Antarctic water and routes of mass drift of Euphausia suprba. In: Biological Resources of the Antarctic Krill. Moscow, VN1RO: 8-27

PriddleJ., J. P. Croxall, I. Everson, R. B. Heywood, E. J. Murphy, P. A. Prince & С. B. Sear, 1988: Large-Scale Fluctuations in Distribution and Abundance of Krill — A Discussion of Possible Causes. In: Antarctic Ocean and Resources Variability, Ed. D. Sahrhage, Berlin, Springer-Verlag, 169-182

Rakusa-Suszczewski S., 1981: Polish Antarctic Expedition 1975/76. Biological investigations of marine Antarctic systems and stocks (BIOMASS), Scott Polar Research Institute, Cambridge, vol. II: 151-155

Samovolkin V. G., 1980: Backward scattering of ultrasonic waves by shrimps, Oceanology 20: 1015-1020

SC-CAMLR (1991): Report of the third meeting of the Working Group on Krill (SC-CAMLR X/4), CCAMLR, Hobart, Australia, 98 p

AC-CAMLR, 1992: Report of the fourth meeting of the Working Group on Krill (SC--CAMLR-XI/4), CCAMLR, Hobart, Australia, 70 p

Stanton Т. K., 1988: Sound scattering by cylinders of finite length. I. Fluid cylinders. J. Acoust. Soc. Amer. 83: 55-63

Address of the author: dr Małgorzata Godlewska, Department of Antarctic Biology, Polish Academy of Sciences, Żwirki i Wigury 97/99, 02-089 Warsaw, Poland

ZMIENNOŚĆ BIOMASY KRYLA SPOWODOWANA CZYNNIKAMI NATURALNYMI I RYBOŁÓWSTWEM (ZACHODNIA ANTARKTYKA 1981-1989)

Streszczenie

Na podstawie pomiarów hydroakustycznych w czasie 4 polskich ekspedycji w rejon Zachodniej Antarktyki oszacowano biomasę kryla w Rejonie Statystycznym nr 48 (CCAMLR). Stwierdzono, że biomasa w różnych latach zmieniała się o rząd wielkości (od 1.3 mln t. do ponad 14 mln. ton), przy czym jej rozmieszczenie było bardzo nierównomierne. Połowy kryla w tym rejonie w żadnym roku nie przekroczyły 0.4 mln ton i były niższe od dopuszczalnych połowów na tym obszarze ustalonych przez CCAMLR na 1.5 mln ton.

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Table 1. Cruise details

tKpediti on Cruise start Cruise end Regi on

FIBEX 14 Feb. 1981 21 March 1981 Br ans-f i el d Strait

Drake P a s s a g e

SIBEX 21 Dec. 1983 8 Jan.1984 Brans-field Strait

Drake P a s s a g e

BIOMASS III 26 Oct. 1986 20 Jan 1987 Br an s-f i e 1 d Strait

E l e p h a n t Island

BI011ASS IV 25 Dec. 1988 17 J a n . 1989 Bransfield Strait

Elephant Island S . O r k n e y Islands

Table 2. Abundance data statistics

Expedi ti on Average density t/Nm2

Vari ance Standart devi ati on

S t a n d a r t error

Coe-f f i ci ent o-f

variation

FIBEX 471.35 3.04E6 1743.1 214.6 3.70

SIBEX 42.84 6119.3 7 8 . 3 10. 7 1.83

BIOMASS III 206.90 17124.6 130.9 2 7 . 3 0.63

BIOMASS IV 90. 10 4543.8 6 7 . 4 16.9 0.75

Table 3. Comparison o-f krill stock and catch in CCAMLR area 4 8 .

Season Estimated biomass mln tons

Annual catch mln tons

1980/81 14.74 0.368

1983/84 1.30 0 . 100

1986/87 7.80 0 . 3 4 6

1988/89 2.80 0.394

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Years

Fig. 1. Annual krill catch in CCAMLR Statistical Area 48

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Fig. 2. Mean krill densities in Western Antarctic from 4 polish expeditions 1981-1989 + FIBEX Feb./March 1981; A SIBEX Dec. 1983/Jan. 1984; • BIOMASS III Oct/Nov. 1986; О BIOMASS III Jan. 1987; * BIOMASS IV Dec. 1988/Jan. 1989

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