Differences in distribution and population structure of krill (Euphausia superba) between penguin and fur seal foraging areas near Seal Island
Shipboard tracking studies of krill-eating predators (penguins and female fur seals) near Seal Island were conducted to identify and evaluate their foraging areas during early January 1990/91. Penguin foraging areas were found in inshore regions where krill frequently occurred but higher density areas of krill (~250 g/m2) were rather limited. In contrast, fur seal foraging areas were found in offshore regions where krill occurred only occasionally but in large aggregations (surface length about 2 to 3 km) of higher densities (~250 g/m2). In the inshore foraging areas krill undertake diurnal vertical migrations, tending to be at a deeper range from 50 to 100 m in the day while at a shallower range from 20 to 50 m at night. In the offshore foraging areas krill do not undertake any diurnal vertical migrations, staying close to the surface throughout the day. With regard to body size and maturity of krill in the inshore foraging areas, middle-sized krill (modal length 43 mm), which consisted mainly of non-gravid krill, were dominant with occasional occurrences of juveniles (modal length 21 mm). In contrast, in the offshore foraging areas large krill (modal length 47 mm) were dominant, the majority of which were gravid females. Thus, horizontal and vertical distributions and population structure of krill were totally different in the foraging areas of penguins and fur seals. The reasons why fur seals chose offshore foraging areas instead of inshore foraging areas are discussed.