Interannual changes in body fat condition index of minke whales in the Antarctic

To study whether or not wide-ranging pelagic predators should be affected by localized changes in prey availability, interannual variability in body fat condition index (assessed from girth measurements) of minke whales Balaenoptera acutorostrata was analyzed in relation to their distribution, stomach-content mass and sea-ice extent during the austral summer in the Antarctic Ocean between 130°E and 170°W. The research area comprised offshore, ice-edge and Ross Sea areas. Of the 3 years (1990/91, 1992/93 and 1994/95) included in the study, 1994/95 was a year of significantly poor body fat condition compared with the other 2 years. The 1994/95 year was characterized by extensive sea-ice conditions, covering the usually krill-rich slope region throughout the season. Since minke whales were scarce and their stomach-content mass small in the ice-edge area during 1994/95, food availability in the area during the season was considered to be poor as a result of the high sea-ice extent. Antarctic krill Euphausia superba was regularly the dominant prey species throughout the survey area, although on the Ross Sea shelf E. crystallorophias was the codominant prey with E. superba. The spatial distribution of body size and maturity of E. superba indicated apparent unsuitability of the Ross Sea area as a nursery and spawning ground for that species. The Ross Sea area was considered to be an area of low food supply throughout the study period, considering the lower level of stomach contents. Paradoxically, this area usually contained numerous minke whales, especially pregnant females. In conclusion, minke whales appear to have been affected by considerable interannual variability in food availability in the ice-edge area, i.e. potentially the most krill-rich area along their migratory route.