Distribution and size of Antarctic krill (Euphausia superba Dana) in the Polish commercial catches in the Atlantic sector of Antarctica in 1997–1999

The distribution, density and length composition of krill in commercial concentrations were investigated in the regions of South Shetland, Elephant, South Georgia and South Orkneys in spring and summer 1997-1999. The concentration densities varied with area and season. The densest concentrations were found near South Shetland. Concentrations at night were several times less dense than those during the day. Between February and April the concentration density increased, and in May and June it decreased. Since during the period of February - June day illumination progressively becomes shorter it must be assumed that there are other factors besides illumination which have an impact on the degree of krill concentration. The internal structure of the concentration was very diverse and the mean density varied from 11 to 31,370 specimens * m-3. At night, the krill concentrations were scattered throughout the water, however, no clear evidence of vertical migrations was found. Between February and April, the night concentrations occurred in much shallower waters than during the day, while in May and June they occurred at the same depths as the day concentrations or sometimes even deeper. The day concentrations between February and April, ocuured in shallower waters, while between April and June they were found in deeper waters. The size structure of the krill was diverse during all periods and in all areas, however, a decrease in krill size was observed as the season progressed.