CCAMLR

The impact of midwater trawl selectivity and catchability upon TS and TS 1 kg estimates in echosurveys is discussed. It is known that krill abundance and biomass estimation should be considered as two independent tasks with different requirements to midwater trawl utilization in echosurveys. Application of the proposed method of mean weighted TS 1 kg estimation provides the biomass estimation determined actually only by MSBS (or MVBSS) acoustic parameter distribution and independent on the midwater trawl selectivity and catchability. Krill abundance estimation related to TS target strength values requires another approach to echosurveys, since two actually independent task should be solved in that case: assessment of MSBS (MVBS) acoustic parameter distribution and krill length (weight) distribution in the study area. The latter requires to settle a complex of problems which allow to consider the trawl as a measuring system. However, the information obtained in that case provides not only total krill abundance value but also krill biomass and abundance by length classes as well as other demographic researches of krill.

The relation between mid-water trawl catchability and its construction, trawling regime, krill aggregations distribution in the study area is discussed. It is shown that while the trawl selectivity affects krill length composition in each trawl sample, the trawl catchability affects the estimation of total krill lengths distribution, obtained on the basis of trawl samples from different catched in the study area. Several approaches to trawl samples processing are discussed. It is shown that the total distribution of krill length, obtained using weighting of trawl samples (usually 200 ind.) against the catch, may considerably differ (statistically significantly) from that calculated on the basis of trawl samples weighting against the catch corrected taking in account the trawl catchability for each trawling. The example of trawl samples processing for the study area off Elephant Island is presented. Difference of total krill lengths distribution, calculated using different methods, results in respective difference of density estimates by 48%.

Comparative analysis of krill length distribution in the echosurvey area, obtained with trawl and acoustic methods is presented. Besides, the acoustic estimate of krill length composition was obtained on the basis of the empiric model by Greene et al. (1991) using estimates of target strength in situ at the frequency of 120 kHz as measured with echosounder EK-500 (split-beam) principle). Krill was caught with the commercial trawl RT 741448. It was shown that differences between krill length distribution predicted from the acoustic model by Greene et al. (1991) and those from trawl catches were considerable (statistically significant). Besides, krill length estimates, obtained with acoustic method, were significantly lower (20-25%) those obtained from catch analysis. Utilization of trawl and acoustic methods of krill length estimation in echosurveys implemented in practice as target strength (TS) in situ measurement with EK-500 and calculation of TS talc. on the basis of krill length composition in catches stipulates significant difference of krill density estimates obtained for the same population. In some cases such difference reached 200%. Significance of trawl samples in assessment of krill length composition during echosurveys was shown.

The influences of abiotic and biotic parameters on the occurrence of Antarctic krill (Euphausia superba) concentrations were studied in the waters north of the South Shetland Islands, a major krill fishing ground in the Antarctic, during the 1990/1991 austral summer. From early to mid-summer krill density increased and showed distinct offshore-inshore differences in abundance and maturity stages. In mid-summer, krill density was low in the oceanic zone (8 g m –2, and higher in the slope frontal zone (36 g m –2), being highest along the shelf break (131 gm–2) in the inshore zone. Krill were in the reproductive stage in the oceanic and frontal zones, but non-reproductive in the inshore zone. Water circulation, food and frontal features were considered important environmental factors determining the occurrence of high krill concentrations in these waters. Drifting buoys indicated that the surface water circulation in areas of krill concentrations was characterized by a sluggish current with eddies along the shelf break in the inshore zone and a shear current in the frontal zone, thus enhancing the residence time for krill. Diatom abundance in both the frontal and inshore zones increased from early to mid-summer, improving krill feeding conditions. Gravid females were closely associated with the frontal zone, showing that the latter may also be a favored spawning area because of the higher probability of survival for embryos and larvae, in addition to enhanced opportunities for their transport to nursery grounds by prevailing currents. In the waters north of the South Shetland Islands, krill appeared to be highly adapted for exploitation of the rich food resources of the frontal/inshore zone, taking advantage of oceanographic features that led to their retention in these zones.