CCAMLR

Method of estimation of krill drift velocity in water current,using repeated acoustic transects on rectangular polygon is discribed.Dimensions of polygon and frequency of acoustic observations have to be determined with taking into account distribution patterns of krill,its patchiness, between-swarm distance (BSD) and between-clusters distance (BCD).For ocean current velocity- 20 sm/sec,ship speed-8 knots,mean BSD-5km,mean BCD-6.3 km dimensions of polygon must be 10 12 km and discretness of acoustic transects 4 hours.

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.