Comparisons in prey distribution between inshore and offshore foraging areas of chinstrap penguins and Antarctic fur seals at Seal Island

At Seal Island breeding predators which incorporated an overnight period into their foraging trip such as Antarctic fur seals (Arctocephalus gazella) and overnight foraging chinstrap penguins (Pygoscelis antarctica) went beyond the inshore region and foraged in the slope/offshore region, even though the slope/offshore region was less abundant with krill and more remote from their breeding sites than the inshore region. Only diurnal chinstrap penguins foraged in the inshore region. The following advantages are considered for visual predators to feed in the slope/offshore region at night:(l) krill showed more uniformal distribution in the slope/offshore region in contrast to its very patchy distribution in the inshore region so that krill may be easily found in the slope/offshore region; (2) krill tend to be larger in body size and more matured in the slope/offshore region than those in the inshore region; larger gravid female krill, which may be easily found and captured by predators, were available in the slope/offshore region; (3) Myctophid fish occurred near surface at night in the slope/offshore region; their ventral light organs may become easier target for predators at night. Thus, aggregation pattern of krill, body size/maturity stage of krill and availability of myctophid fish may make the slope/offshore region favorable foraging areas in spite of its lower krill density.