Reasons of differences in icefish distribution and biomass assessment during two surveys made by STM “Atlantida” are revealed. Availability of large icefish concentrations in 2000 in the northwest shelf area in 10-100m layer resulted in formation of adult fish aggregations in water column. The year 2002 was characterized by krill distribution in near-bottom layer, causing icefish presence near the bottom even in dark hours of the day. During the two surveys, fry and immature fish occurred in numbers in pelagic zone. At present, the population mainly consists of immature and first maturing fish, which stay in water column for considerable part of the day. Earlier beginning of pre-spawning period, as distinct from mean annual, was noted in 2002 in the northeastern part of the area.
It is shown that pelagic part of the stock permanently existing during the day is not taken into account by bottom net survey, which, consequently, is disregarded when estimating the TAC. Diverse size groups, mainly juvenile and immature fish, form pelagic and near-bottom components of the stock. Under certain oceanological conditions, length-age composition of pelagic component may be formed by all size classes.
Results of the fishery and investigations in Ch. gunnari habitat show that methodology of census surveys does not conform to their distribution pattern. Conduction of net and acoustic survey will enable a quantitative assessment to be made for bottom (bottom net survey) and pelagic (hydroacoustic survey) components of Ch. gunnari stock. Taking Ch. gunnari pelagic component into account will allow computing the recruitment that will enter the fishery the year to follow. A need to change the survey season to avoid underestimation of fish biomass related to its distribution is demonstrated.