Mackerel icefish biomass and distribution on the results of acoustic survey carried out in February–March 2002

Results of the acoustic survey of icefish carried out by Russian vessel STM-8390 Atlantida in February-March 2002 in Subarea 48.3 are presented. Biomass and distribution of icefish were obtained using multi-frequency echosounder EK-500 and Sonar Data EchoView software. Target strength estimates of icefish were made by comparison an observed TS in situ histograms with the size composition of the ensonified fish, obtained by trawling. Results of acoustic estimates of icefish biomass and abundance values by size class in pelagic zone and in the near-bottom 8m layer (covered by bottom trawling survey) are discussed. The major part of pelagic biomass was concentrated within 50m above the bottom 8m layer and amounted to 23,037.4 tonnes. This estimate can be considered as the minimum value of the pelagic component of the icefish stock, since it does not take into account the biomass of the fish distributed within higher depth ranges where, according to the data of trawling and acoustic sampling, icefish are also available. The bottom biomass resulted in the estimate of 62,328.0 tonnes. The sum acoustic estimate of the icefish biomass in the subarea of South Georgia amounted to 92.6 thous. tonnes. Nearly 30% of this biomass was concentrated in the pelagic zone. If acoustic estimates of pelagic and bottom components of the icefish biomass are compared it will become evident that the pelagic part of biomass makes up nearly 42% of the bottom biomass. Our acoustic estimate of the pelagic biomass constitutes about 60% of biomass obtained from bottom trawling survey. In additional to the above-states estimates the abundance of juvenile fishes (speciments less than 6cm) was obtained in the sub-surface 50-m layer where, according to trawl samples, the latter predominated. Sum abundance of young fish constituted 7344.6 million pcs. The horizontal and vertical distributions of krill with reference to investigation of regularities of icefish behavior and distribution are discussed.