Reasons of differences between distribution and density of mackerel icefish (Champsocephalus gunnari) aggregations in
the South Georgia area during summer and autumn periods in different years from the bottom trawl survey data

During the feeding period icefish aggregations are confined to a frontal zone between opposite flows (coastal circumfluent current and ACC) or formed inside quasistationary circulations, where the largest aggregations of food organisms are concentrated at the beginning of the spring period. Icefish concentrations were detected at the periphery of a cyclonic meander or in the centre of the anticyclonic circulation formed by the Weddell waters. Confinement of the fish to the boundary of water masses between the shelf waters and ACC was also traced. Favourable conditions for formation of dense aggregations resulted from the availability of clear-cut frontal zone caused by interaction of warm deep waters and the coastal waters. Such a confinement of fish aggregations to dynamically active zones arises rather from concentration of food organisms in these areas than as a result of favourable conditions for the fish.
The presence of cold intermediate layer shall be considered to be a negative hydrological factor for formation of icefish aggregations as it impedes descending food objects to the horizons inhabited by icefish and migration of fish to the upper 100m layer. Very high water temperature (above 1.8-2.0°C) for this area in the places of food organism aggregation is another obstacle for performing vertical migrations by foraging fish. All physiological processes of icefish begin to recede at such a temperature, and at a higher temperature the fish evidently falls into a condition close to anabiosis. In such locations the fish are distributed deeper than this temperature layer, most often near the ground.
As a rule, transition of icefish to pre-spawning condition is conditioned by visceral fat content (over 2 points). The spawning begins when near-bottom temperature on the spawning ground increases to 1.6°C. Therefore, the beginning of the spawning period is determined in the first place by oceanological factors.