Some comments on the procedure of krill target strength assessment in echosurveys

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.