Importance of myctophid fish distributions for formation of foraging areas of chinstrap penguins and Antarctic fur seals at Seal Island

Shipboard tracking study of breeding chinstrap penguins (Pygoscelis antarctica) and Antarctic fur seals (Arctocephalus gazella ) at Seal Island were conducted to identify and evaluate their foraging areas in late December 1994 (Leg I) and late January 1995 (Leg II). Leg I and Leg II corresponded to the incubation and guard period, respectively, for chinstrap penguins ; penguins were divided into diurnal and overnight foragers during Leg II. Highest krill (Euphausia superba) densities regularly occurred on the shelf north of Seal Island. However, all predators except diurnal foraging penguins went northward beyond the shelf, and foraged in offshore region during Leg I and on slope during Leg II. The diurnal foraging penguins alone fed on the shelf. The following advantages are considered for feeding in the offshore/slope regions over on the shelf: (1) krill were present near the surface throughout the day so that only shallow diving was required to catch krill even in the day time; (2) krill tend to be larger in size; and (3) Myctophid fish occurred within the predators' maximum diving range at night. Considering that overnight foraging was always made in the offshore/slope regions, (3) is regarded as the most important factor making the predators except diurnal foragers choose the offshore/slope regions (less abundant areas for krill) over the shelf region (more abundant areas for krill).