CCAMLR

The waters in the vicinity of the South Shetland Islands and Elephant Island were surveyed to investigate mechanisms for the formation of krill concentrations and to estimate acoustic biomass of krill in the 1990/91 austral summer. Main krill concentrations occurred in the shelf waters north of the islands, as usual. Tracks of drifting buoys (with curtain drogues at 30 m depth where krill frequently occur), have elucidated the occurrence of convergent complex eddies in these shelf waters. (Note is that one buoy traveled all the way to South Georgia and has been being trapped in a winter fishing ground for krill.) High concentrations of chlorophyll a were also measured in the shelf waters, showing a spatial correlation between krill and chlorophyll a distributions. It is therefore considered that both hydrodynamic and food environments may accumulate krill. a regular fishing ground on the northern shelf of Livingston In Island, krill abundance increased 3.4-fold over the 40-day period during late December 1990 - early February 1991. This increased abundance (157 g/m2) in early February 1991 was 54 % lower than the estimate obtained there in late January 1988. Information from krill predators’ studies and krill fishery have also indicated that krill abundance was lower during the first half of 1990/91 summer than expected from other season. Therefore, the estimate of total krill biomass throughout the shelf waters in late January 1991, i.e. 1.78 million t (56 g/m2), should be regarded as the lower level for the shelf waters at this time of the season. Krill abundance is reported to have increased to the normal level from mid-February onward.

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) 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 1981FIBEX 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.

The theoretical basis underlying estimation of krill abundance using echo-integration and aggregation information 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 necessitate any assumptions concerning aggregation distribution or conformation.

An observed distribution of Meganyctiphanes norvegica aggregations is used to examine 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 4 rotations. The results indicate that the coefficient of variation (c.v.) 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% if c.v.’s are as high as those obtained in the simulations.

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 35 times, while interannual variability between available surveys reached only a factor of 1.6 times. 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 2 times from which follows a maximum total effective biomass (including production) of 4.3 and 5.0*106 tonnes passing through the survey area during the respective summer.

To delineate zooplankton distribution and abundance in Bransfield Strait, in relation to hydrographic condition, a total of 29 sampling stations was occupied for the CTD casts and plankton towings in a systematic grid from December 29, 1989 to January 7, 1990. Comparisons of temperature and salinity distributions and T-S diagrams suggest that the cold and saline Weddell Sea water entered from the Antarctic Sound flows southwest along the Antarctic Peninsula coast, turns to the north at the east of Trinity Island, then joins the Bellingshausen Sea water (warm and less saline water) near the south of Deception Island, and meanders northeastward in the surface layer of the northern strait. Bellingshausen Sea water enters near Low Island, then, mainly flows toward Deception Island. A total of 39 taxa of zooplankton were identified from samples. Several species of copepoda were the most abundant (57.7%) in terms of number, and followed by salpa (27.7%), chaetognatha (6.7%), and euphausia (1.3%). However, noticeably, macrozooplankton salpa (Ihlea sp. and Salpa sp.) predominated over the entire Bransfield Strait occupying 20 stations, and showed the highest biomass in zooplankton community. The highest number of salpa was shown in the northeastern entrance of Bransfield Strait, but low numbers were found along the stations near the Antarctic Peninsula coast. Fish larvae and euphausia were rarely found in the entire strait

In-situ measurements of individual krill target strength are presented (modal value = -69db). Net samples obtained immediately before the experiment indicate that the animals were reproductively mature and averaged 47.44mm in length. Results agree well with recent predictions of acoustic target strength based on extrapolations of other field experiments using the bent cylinder model of sound scattering by elongated zooplankters. The modal value of the measurements is 6.7dB lower than that predicted by the BIOMASS equation for 47.44mm krill.