Results of acoustic assessment of krill biomass in Subarea 48.2 during summer 1996

In the period from 24 February to 7 March 1996 in the cruise of RV "Atlantida" acoustic assessment of krill biomass and distribution was carried out in study area restricted with 59° 10' S - 60°30' S and 42 °00' W - 50°00 'w within Subarea 48.2. The study area of 19.200 sq.mi1es was located in the zone of two circulation systems of climatic scale interaction: Antarctic Circumpolar Current and Weddell Sea water circulation. Researches in the study area included a broad spectrum of hydrobiological and oceanographic observations and studies.Onboard hydroacoustic complex was represented by the measuring system EK-5001BI 500. Calibration of echo sounder EK-500 was performed by specialists of Simrad company in Horten, Norway. Echosurvey was carried out using uniform square grid of tacks with between-tacks distance of 20 miles. The echosurvey was carried out diurnally. Krill target strength was estimated by means of TS/length relationship, recommended by WG-krill (SC-CAMLR, 1991). Recorded range of MSBS values variations was (-77÷-43) dB. Kri11 biomass estimation in the study area amounted to 1.12 mln. t at 95% confidence limit CL=93.1 thous.t. Variation coefficients of krill density and biomass within strata speciefied exceeded no 8%. Previous data obtained by AtlantNIRO showed that biomass estimated was below that expected. One of the reasons is diurnal regime of echo survey execution. According to Demer and Hewitt (1995) exclusion of kri11 vertical diurnal migrations from consideration results in underestimated acoustic biomass by 1.9 dB at s.d. = 0.6 dB. Besides, the study area selected included no the southern part (southwards of 60 ° 30'S) of Subarea 48.2 where southwards off Coronation Island considerable kri11 fishing grounds occure. According to AtlantNIRO data krill biomass southwards of 60 ° 30'S amounts about (883±264.9) thous. t.
Therefore, total krill biomass in Subarea 48.2 may be assessed as 2.6 mln.t at 95(% confidence limit CL=298.2 thous.t