Frolkina, J.A. and V.I. Shlibanov (1991), Vertical migrations of mackerel icefish (Champsocephalus gunnari) on the South Georgia shelf

High values of coefficient of variation resulting from the calculation of mackerel icefish biomass (Champsocephalus gunnari) from trawl survey data on the South Georgia shelf (Parkes et al., 1989) challenge the rationale for round-the-clock trawling and call for clarification of the techniques used. The objective of the present study is to analyse data on daily vertical migrations of mackerel icefish. The study used data from research vessels which had been fishing for commercial concentrations of C. gunnari in years of high abundance of the species (1982/83, 1983/84 and 1986/87). Catch-per-hour of fishing was analysed by the time of day. Catches made by both bottom trawls and midwater trawls were taken into account. Information from the round-the-clock hydroacoustic observations on vertical migrations of C. gunnari as well as from observations on fish feeding is also considered in the paper. A distinct pattern of daily vertical migrations of C. gunnari was observed on the South Georgia shelf. During the day, fish concentrate near the bottom in concentrations 5 to 7 m thick. At night, fish disperse and migrate to the upper layers 50 to 60 m beneath the surface. The main peak of fish feeding activity was also observed at night. Fishing during this period with bottom trawls had yielded almost no catches at all. The information obtained on vertical migrations of C. gunnari may be used for clarification of survey techniques used in stock assessment. For a number of years scientists of AtlantNIRO have been conducting trawl surveys for C. gunnari during daylight hours only.

Kock, K.-H., I. Everson, S. Campbell, G. Parkes, Z. Cielniaszek and J. Szlakowski (1991), Food and feeding of the mackerel icefish (Champsocephalus gunnari) around South Georgia in January/February 1991

The diet composition and feeding intensity of mackerel icefish (Champsocephalus gunnari) around South Georgia in January/February 1991 were analysed in 3 877 stomachs collected at 44 stations. Both the proportion of krill (Euphausia superba) in the stomachs and the feeding intensity were uncommonly low in a period when energy rich food is needed for the final maturation of the gonads.

Everson, I., G. Parkes, K.-H. Kock, S. Campbell, Z. Cielniaszek and J. Szlakowski (1991), Fish stock assessment survey in Subarea 48.3

A demersal fish survey in the vicinity of South Georgia, CCAMLR Subarea 48.3, is described. Details are given of positions of hauls, catch rates and size composition of catches. The standing stock of one of the most important commercial species in the area, Champsocephalus gunnari, is shown to have undergone a major decline by comparison with results from the 1989/90 season. Other species do not show a similar dramatic reduction. No commercial fishing on Champsocephalus gunnari has been reported from the area. It is suggested that this decline may be due to either a massed migration or high mortality.

Gasyukov, P.S. and R.S. Dorovskikh (1991), A simulation study of the method of deriving natural mortality rate using data for Champsocephalus gunnari in Subarea 48.3

A method of determining the rate of natural mortality, based on the construction of a regression between the rate of annual fishing mortality (determined by VPA) and fishing effort is described. The intersect of this regression must be equal to zero if the natural mortality rate has been correctly identified and something other than zero if it has not been correctly chosen. Data for Champsocephalus gunnari in Subarea 48.3 are used to show that the described method is sound. When a mistake is made in the choice of natural mortality coefficient, the estimate of the intercept of the regression is other than zero. A bias in the intercept will be positive if the assumed natural mortality coefficient is less than the “true” value and close to zero or slightly greater than zero if the assumed value is greater than the “true” one.

Duhamel, G. (1991), Biology and harvesting of Dissostichus eleginoides around the Kerguelen Islands (Division 58.5.1)

Dissostichus eleginoides is becoming an increasingly important species in the fisheries of the Southern Ocean. In Division 58.5.1, a directed trawl fishery has been in operation since 1984/85 and has expanded in the deep western sector of the Kerguelen Shelf. This fishery harvests the sub-adult/adult part of the stock. Fishing effort and corresponding catches are very variable from year to year and highly dependent on the harvestable resources of Champsocephalus gunnari. Vital biological information on population dynamics, as well as demographic structures and trends observed in an abundance index, is provided. Prospects for exploitation are discussed with respect to the development of fishing methods.

Moreno, C.A. (1991), Hook selectivity in the longline fishery of Dissostichus eleginoides (Nototheniidae) off the Chilean coast

Existing information from technical publications on Chilean fisheries of the Patagonian toothfish, Dissostichus eleginoides is reviewed in order to study selectivity of different types and sizes of hooks used in longlines. Original data obtained during 1991 off the coast of Valdivia, South East Pacific, are also taken into consideration. The data show that circle hooks are far more efficient than straight hooks. The length frequency distributions of fish caught with straight hooks Nos 3 and 4 are not significantly different, but hooks No. 3 are nearly 31 % more efficient than hooks No. 4. Using different bait does not result in significant differences. It is worth noting that the ratio of females increases significantly in catches, when the size of straight hooks is increased (e.g., from 4 to 3) or when circle hooks are used instead of straight hooks.

Ichii, T., H. Ishii and M. Naganobu (1991), Krill (Euphausia superba) distribution in relation to water movement and phytoplankton distribution off the northern South Shetland Islands

In December/February 1990/91 a survey was conducted by RV Kaiyo Maru in the vicinity of the South Shetland Islands and Elephant Island. The main objective of the survey was to investigate the mechanisms responsible for the concentration of krill and to estimate krill biomass in the area surveyed. Krill biomass estimation was done by means of hydroacoustics. Phytoplankton distribution was assessed by measuring surface concentrations of chlorophyll a. Water circulation was studied with satellite-tracked buoys equipped with curtain drogues deployed at a level of 30 m from the surface because at this level krill concentrations are most frequently observed. Main krill concentrations were observed in shelf waters north of the islands as usual. Tracking of buoys demonstrated the existence of convergent complex eddies in shelf waters of these islands. A high concentration of chlorophyll a was also recorded in shelf waters. A spatial correlation between distributions of krill and chlorophyll a was observed. It is therefore considered that both hydrodynamic and food availability factors may be responsible for krill concentration. On a traditional fishing ground north of Livingston Island, krill density had increased 3.4 times over a period of 40 days from late December 1990 to early February 1991. This increased krill density in early February 1991 (157 g/m²) was 54% lower than that observed in late January 1988. Information from studies of krill predators by US scientists also indicated that krill abundance during the first half of the 1990/91 summer was lower than that in previous seasons. Total biomass of krill in shelf waters of the area surveyed was estimated at 1.78 million tonnes (56 g/m²) in late January 1991. The obtained estimate of krill biomass should be regarded as the lower level for this time of the season. From mid-February and onwards it was reported that krill abundance had increased to the normal level.

Kasatkina, S.M. (1991), Target strengths of krill at 136 and 20 kHz

Estimation of krill target strength was made by ensonifying encaged live krill and using trawl hydroacoustic method. Ensonification of encaged live krill. For aggregations with mean lengths from 45 to 50 mm the mean single-krill target strengths range from -68 to -69 dB at 136 kHz. For aggregations with mean lengths from 43 to 47 mm the mean single-krill target strengths range from -71 to -77 dB at 20 kHz. Trawl hydroacoustic method. For aggregations with mean lengths from 47 to 50 mm the mean target strengths ranged from -71 to -75 dB. At the same frequency for aggregations with mean lengths in the range 41 to 47 mm the mean single-krill target strength is in the range -76 to -81 dB.

Siegel, V. (1991), Estimation of krill (Euphausia superba) mortality and production rate in the Antarctic Peninsula region

A net sampling survey for krill was carried out by RV Meteor along the Antarctic Peninsula from 26 December 1989 to 14 January 1990. Results were compared with data of an identical survey with RV Polarstern in 1987/88. Seasonal variation between summer and winter abundance/biomass data exceeded a factor of 1:35, while inter-annual variability between available surveys reached only a factor of 1:1.6. Krill mortality was calculated by linearized catch curve analysis and resulted a rate of Z = 0.88 (1989/90) and Z = 0.96 (1987/88). Production was estimated and resulted a P/B ratio of 0.88 to 0.94 for different years. The exchange rate of water masses and krill biomass within one summer season was roughly estimated as two times from which follows a maximum total effective biomass (including production) of 4.3 and 5.0 x 10⁶ tonnes passing through the survey area during the respective summer.

Agnew, D.J. and I.R. Higginbottom (1991), Characteristics of krill swarms from Prydz Bay

The CCAMLR.AUS dataset available to the Subgroup on Survey Design was derived from a survey of Prydz Bay, Antarctica, in December 1990. Swarm length, depth, thickness, Mean Volume Backscattering Strength (MVBS) and inter-swarm length were calculated for swarms of euphausiids encountered along a 78 km transect over the continental shelf close to the CEMP study site at Magnetic Island. The euphausiid species was probably Euphausia crystallorophias. An increase in swarm length was seen with increasing distance from the coast.

Butterworth, D.S., D.L. Borchers and D.G.M. Miller (1991), Some comments on the procedure for testing estimators of krill abundance which utilise survey data

Since krill distributional data do show evidence of spatial correlation, estimators of abundance which attempt to model such effects (such as those based on Kriging techniques) may provide improved abundance estimates from survey data. However, computer simulation studies are first required to test whether such estimators, and alternative possible survey designs, are indeed likely to provide improved performance in practice. Such studies require a simple method for computer generation of krill distribution patterns, which are compatible with existing information on the distributions from surveys. ‘Two-level’ models of krill distribution are considered. These achieve overall spatial correlation by placing krill swarms at random within larger aggregation features termed concentrations; these concentrations are then located at random within the survey area. These ‘two-level’ models provide an encouraging improvement in fits to the distribution of inter-swarm distances observed on the 1981 FIBEX survey by MV SA Agulhas. However, evidence of model misspecification remains. Further work is needed before such models can be used as the basis for the simulation studies required - some suggestions are made in this regard.

Butterworth, D.S., A.E. Punt and M. Basson (1991), A simple approach for calculating the potential yield of krill from biomass survey results

Estimates are calculated for the parameter λ in the yield equation Y = λMB₀ for a krill fishery in which both the fishery and krill growth are seasonal. The criterion used for the calculations is that the probability that the krill spawning biomass drops below 20% of its median pre-exploitation level over a 20 year period of harvesting should equal 10%. The value of, λ depends strongly on the value of M (natural mortality) and σ_R (recruitment variability), and also on the relative values of the ages at maturity and first-capture. Seasonal effects seem to be of lesser importance.

Hampton, I. and D.G.M. Miller (1991), Note on estimating krill abundance from acoustic data on individual aggregations

The fundamental equations and assumptions underlying estimation of krill abundance using echo-integration information and aggregation parameters are briefly described. The chief differences of the two approaches are highlighted. It is concluded that the echo-integration approach is superior for the estimation of regional krill abundance and its variance since it is easier to apply, requires less data analysis and does not require any assumptions concerning aggregation distribution or conformation.

Murphy, E., I. Everson and A. Murray (1991), Analyses of acoustic line-transect data from the waters around South Georgia: estimation of krill (Euphausia superba Dana) – biomass

A large-scale acoustic survey of zooplankton stocks in the waters around South Georgia was carried out during November and December 1981. The data have been analysed using various definitions of strata. The total biomass in the area of 155 786 km² was estimated to be 1.8225 x 10⁶ tonnes (Coefficient of Variation = 9.5%). This gives an estimate of mean density throughout the region of 11.7 tonnes km^-2. Although the estimate of the mean is not sensitive to the definition of strata, the variance of the estimate is sensitive. The analyses highlight the importance of careful planning of acoustic surveys.

Agnew, D.J. and S. Nicol (1991), Small scale krill surveys: simulations based on observed euphausiid distributions

An observed distribution of aggregations of the euphausiid, Meganyctiphanes norvegica is used to model the effects of krill swarm orientation and shape on the success of two proposed survey designs. The simulation involves a number of random paths of parallel or radially arranged transects passing through three distributions each of which is given four rotations. The results indicate that the coefficient of variation (CV) of mean krill density varies inversely with mean density, and is lowest for the survey design utilising parallel transects set at right angles to the long axis of the aggregations. Calculations based on the power of surveys to reliably detect changes in mean density indicate that with probability of Type I and Type II errors 0.1 and 0.2 respectively, about 100 transects would be required to detect changes of 40% in the mean if CVs are as high as those obtained in the simulations.

Kasatkina, S.M. (1991), Midwater trawl catchability as an aspect of a quantitative assessment of krill biomass conducted using a trawl census survey

Midwater trawl catchability is a variable which depends on both the behavioural and distributional characteristics of the target species and trawl parameters. Reliability of biomass assessments will improve with better knowledge of trawl catchability and krill distribution patterns. A comparative analysis of trawl catchability for commercial midwater trawls and small research trawls, in this case using an Isaacs-Kidd trawl, demonstrated the futility of using the latter for trawl census survey biomass assessments due to their highly variable catchability over the survey area and also because the biomass estimates thus obtained were characterised by high relative error. A standard method for conducting krill trawl census surveys needs to be developed. Mathematical modelling fishing trawls as well as hydroacoustic research on catchability properties of trawls can facilitate selecting the optimal construction of commercial midwater trawls. Assessments of the total amount of krill entering the trawl should be carried out when selecting trawl construction.

Marín, V.H., A. Mujica and P. Eberhard (1991), Chilean krill fishery: analysis of the 1991 season

This paper summarises Chilean catch data for 1991. Special emphasis is given to the analysis of haul-by-haul data and its use in studying small-scale spatial distribution of catches. The results show that krill fishing in Subarea 48.1 is particularly intense near CEMP sites. Other ways of using of haul-by-haul data are suggested.

Croll, D.A., J.L. Bengtson, P.L. Boveng, M.E. Goebel and J.K. Jansen (1991), Foraging behaviour and reproductive success in chinstrap penguins: the effects of transmitter attachment

The use of radio telemetry has been accepted as a standard method with which to monitor foraging trip duration in penguins by the CCAMLR (Commission for the Conservation of the Antarctic Marine Living Resources) Ecosystem Monitoring Program (CEMP). This paper reports on: (i) the effects of radio transmitter attachment on nest attendance, foraging trip duration, nest failure, and reproductive success in chinstrap penguins; (ii) differences in these parameters when transmitters are applied to one or both members of a breeding pair; and (iii) variability in foraging patterns of penguins unencumbered by transmitters within the brood period. This study was conducted from December 1990 to January 1991 in a discrete chinstrap penguin colony composed of 666 nests on Seal Island, South Shetland Islands. In total, one member of each pair at 20 nests and both members of each pair at 10 nests were equipped with a radio transmitter, 120 nests were used as a control group. The results of this study demonstrate that, for the size of transmitter used (1.4 cm² front section area); there may be no difference between trip durations obtained by applying the transmitters to one or both members of a breeding pair. However, when all parameters of reproductive success were measured, it was revealed that nests with both members equipped with a transmitter fared worse than both the control group and the group with only one member equipped with a transmitter. Several of the differences in those parameters were shown to be statistically significant. It is therefore recommended to use instruments on only one member of each breeding pair in studies utilising CEMP Standard Methods.

Pakhomov, Y.A., V.A. Bibik and V.V. Gerasimchuk (1991), Data on krill (Euphausia superba Dana) consumption by coastal fishes in Division 58.4.2 (Kosmonavtov and Sodruzhestva Seas)

Current estimates of the level of krill consumption (Euphausia superba) are based on data obtained from the Atlantic sector which have then been extrapolated over all of Antarctica. According to these calculations the entire amount of krill consumed is between 166 and 450 million tonnes; of this amount fish consume from 30 to 32 million tonnes (Lubimova, Shust, 1980 and Shust, 1987). However these extrapolated values of krill consumption by fish and other species may differ significantly from those for specific areas. Localised assessments of krill consumption have thus far only been carried out for the Scotia and Lazarev Seas (Kock, 1985; Miller et al., 1985).Assessments contained in this paper are based on results of research conducted in the Sodruzhestva and Kosmonavtov Seas over many years by the YugNIRO institute. Data obtained from trawl and acoustic surveys as well as on fish feeding patterns and krill production were used in the calculations. Assessments of krill consumption are based on calculations made for individual fish species taking into account their distribution in the particular study area of Antarctica. Total fish stock was calculated for a depth range of 100 to 500 metres in waters between 30 and 80°E. Krill consumption was estimated for summer only (December to April). The assessments obtained indicate that if total fish biomass is 551 300 tonnes, fish will consume 960 000 tonnes of krill. Given the total krill biomass in the study area it is concluded that fish consume from 9 to 18% of annual krill production.

Pakhomov, Y.A. and V.B. Tseitlin (1991), Feeding patterns of nine species of Antarctic fish and assessment of their daily food consumption

Little is known about the feeding patterns of non-commercial fish species in Antarctic waters. The aim of this study is to collect information on the feeding patterns of several commercially non-targeted species in the Indian Ocean sector of the Southern Ocean: Nototheniidae (four species), Channichthyidae (three species) and Bathydraconidae (two species). The study concentrated on analysing the food composition of these species to find out their daily diet. One 36-hour station was carried out in the Kosmonavtov Sea (67°50'S; 41°24'E) in March 1988. All samples were taken from the depth range 300 to 350 m and a total of 1 165 stomachs were collected and analysed. The species studied were classified into the four following categories: (i) Chaenodraco wilsoni and Trematomus eulepidotus - planktivorous species with euphausiids, mainly Euphausia superba, being the staple food; (ii) Trematomus centronotus - a benthophage feeding mainly on amphipods, mobile and sedentary forms of polychaetes, with Euphausia superba also comprising a considerable portion of its diet; (iii) Trematomus hansoni - a euryphage; (iv) Dissostichus mawsoni, Cygnodraco mawsoni, Gymnodraco acuticeps, Chionodraco hamatus and Cryodraco antarcticus - predators feeding mainly on fish. The daily diet of these species was from 1 to 4% of their body weight. Annual food consumption by these species was calculated to be about five times their body weight.

Sabourenkov, E. (1991), Myctophids in the diet of Antarctic predators

The aim of the present study is to review published data on the diet of various top predators (birds, seals, whales, fish and squids) in order to help evaluate the importance of myctophids in their diets. This review covers publications for the period since 1984-85 to the present. In total, over 50 publications contained some information on the myctophid component of the diet of Antarctic predators. Myctophids were found in the diet of one species of squid, 18 species of fish (Nototheniidae - eight, Channichthyidae - seven, Bathydraconidae - two and Rajidae - one), 18 species of birds (penguins - seven, albatrosses - four, petrels - five, prions - one and cormorants - one) and two species of seals. Myctophids in the diet of predatory species are represented by 16 species of genera Krefftichthys (one species), Protomyctophum (four species), Electrona (five species), Metelectrona (one species) and Gymnoscopelus (four species). However, the total number of species may be underestimated because identification of myctophids in predator food samples is still difficult. Many species of fish, birds and seals take myctophids opportunistically in addition to their staple diet. Penguins have the largest proportion of myctophids in their diet. The king penguin in the Antarctic Polar Front area of the Indian Ocean is the only one specialist predator on myctophids. The Bathydraconidae fish, Gynmodraco acuticeps, from the Kosmonavtov Sea (Indian Ocean sector) may be the second most important predator of myctophids. The occurrence of myctophids in the diet of predatory species appears to be highly variable though there are insufficient data to evaluate this. It is obvious that myctophids are widespread in diets of many Antarctic predators. More quantitative studies on as many as possible species of predators are required in order to assess the role of myctophids in the Antarctic ecosystem. Information on the diet of myctophids themselves is also important. These studies should use standardised methods, be carried out in different seasons and have a wide geographical coverage.

Williams, R. (1991), Potential nursery areas for fish in the Prydz Bay region

A survey with a fine-meshed midwater trawl in Prydz Bay in January/February 1991 indicated the potential for the shelf area of the Bay to be a nursery ground for nototheniid and channichthyid fish. Nearly all juvenile fish were caught in the top 50 m over the shelf, and more than 95% were Pleuragramma antarcticum. This species was abundant (up to 9 100 were caught in a 30 minute haul) especially near the shelf break. The dominant size class was of 45 mm standard length, which corresponds to an age of 1⁺. Nototheniids and channichthyid juveniles were much less common, and were found in the central and western parts of the shelf. Although no specific nursery grounds were identified, the whole of the Prydz Bay shelf area could be considered a nursery area.

Everson, I., A. Neyelov and Y.B. Permitin (1991), By-catch of fish in the krill fishery

Observations are reported of icefish being taken as by-catch during krill fishing operations from a research vessel. The results indicate that the by-catch of fish in the commercial krill fishery may be significant in some areas of the South Georgia shelf. The problem is thought to be least in open ocean krill fishing.

Constable, A.J. (1991), Potential impacts of bottom trawling on benthic communities in Prydz Bay, Antarctica

This paper presents data on benthos retrieved in bottom trawls during a survey of demersal finfish species in Prydz Bay by Australia in the 1990/91 season. The data collected are used to examine a potential impact of bottom trawling on benthic communities in this region. The benthos was examined in trawls at 14 of the 22 random stations providing representative samples from different areas in the Prydz Bay. All benthos in the codend was sorted into major taxa and wet-weighed. The composition of the benthos retained was primarily sponges and ascidians. Holothurians and crinoids were the next most abundant taxa in terms of biomass. The wet-weighed ratio of benthos to finfish in the codend was greater than 1 at 12 out of the 14 stations and was mostly greater than 10. The results of this study demonstrate that the biomass of benthos that could be disturbed by commercial bottom trawling could be high in Prydz Bay. Also, the results indicate that there is a need to examine the nature of benthic communities in areas in which commercial bottom trawling may occur so that measures can be taken to ensure the integrity of these communities are maintained.