CCAMLR

Physical characteristics of 437 krill aggregations detected acoustically in the area o£ S.0rkney are discribed.All swarm parameters were registered and processed automatically.The aggregations thickness and length (2.5 m and 10.4 m)were considerably smaller, then those published by. other authers. The aggregations were on average 2.2 km apart. It is shown that aggregation parameters depend in great extent from spatial resolution capability of swarm measurements.

Observations are reported of icefish being taken as bycatch during krill fishing operations from a research vessel. The results indicate that the bycatch 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.

Estimates are calculated for the parameter λ in the yield equation Y = λ M B0 for a kri11 fishery in which both the fishery and kri11 growth are seasonal. The criterion used for the calculations is that the probability that the kri11 spawning biomass drops below 20% of its average pre-exploitation level over a 20 year period of harvesting should not exceed l0%. The value of λ depends strongly on the value of M (natural mortality) and σR (recruitment variability), and also on the relative values of the ages at maturity and first-capture. Seasonal effects seem to be of lesser importance.

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.