On selectivity of commercial and research trawls when krill catching

In the paper the attempt was made to compare selective features of several midwater krill trawls within the frame of krill density precise estimates provision problem in echo-surveys. The results of fishing one krill aggregation with two types of trawls (PT 72/308 commercial trawl and Isaacs-Kidd research trawl IKTM) are presented. Selectivity differences of those trawls resulted in significant variability of krill size composition in catches. Average krill length differed more than in 6 mm which caused deviations of TS values, estimated with TS/length relationship (SC-CAMLR, 1991) in ΔTS=2.1 dB. Associated variation in density estimates for the same MSBS value amounted to 1.65 .
Theoretical study of trawl selectivity features was carried out using statistical model. It was shown that selectivity of midwater trawl depended both on its design, trawling velocity, and catch value. Trawl selectivity features will especially affect the estimates of composition, abundance and biomass by krill size groups, and primarily in young krill assessment.
Analysis of abundance biomass estimates on the basis of various echo-surveys data requires to take into consideration selective features of trawl used.