Differences in distribution and population structure of krill (Euphausia superba) between penguin and fur seal foraging areas near Seal Island

Shipboard tracking study for krill-eating predators (penguins and female fur seals) at Seal Island were conducted to identify and evaluate their foraging areas during early January 1990/91. Penguin foraging areas were formed in inshore region, where krill frequently occurred but higher density areas of krill (≥250g/m2) were rather limited. In contrast, fur sea! foraging areas were formed in offshore region, where krill occurred only occasionally but they tended to form extensive aggregations (ca. 2-3 km in horizontal distance) of higher densities (≥250g/m2). In the inshore foraging areas krill showed a diurnal vertical migration, tending to be at deeper depth range (50-100 m deep) in the day time while at shallower layer (20-50 m) at night. In the offshore foraging areas krill showed no diurnal vertical migration, staying close to the surface throughout the day. As for body size and maturity of krill, in the inshore foraging areas middle-sized krill (modal length 43 mm), which consisted mainly of nongravid krill, were dominant with occasional occurrences of juveniles (modal length 21 mm). In contrast, in the offshore foraging areas large-sized krill (modal length 47 mm), of which the majority were gravid females, dominated. Thus horizontal and vertical distributions and population structure of krill were totally different between the two foraging areas. The reasons why the fur seal chose offshore foraging areas over inshore foraging areas were discussed.