Variables of prey availability between inshore and slope/offshore foraging areas were compared to evaluate important factors for the formation of foraging areas of Chinstrap penguins (Pygoscelis antarctica) and Antarctic fur seals (Arctocephalus gazella) at Seal Island. Advantages of foraging inshore are: 1) proximity to breeding colonies and 2) high krill (Euphausia superba) abundance, whereas those of foraging slope/offshore are: 3) less patchy krill distribution, 4) shallow krill distribution, 5) larger krill size and 6) occurrence of energy-rich bioluminescing myctophid fish at night. Chinstrap penguins foraged in two modes during chick-brooding period: daytime foraging chinstrap penguins foraged in the inshore region, while overnight foraging penguins foraged in the slope region. It was suggested that important factors for the formation of daytime foraging areas may be mainly 1), while those of overnight foragers may be primarily 3) and secondarily 6) considering chinstrap penguins may have difficulty in perceiving prey at night. Antarctic fur seals foraged mostly in the slope/offshore region during pup-rearing period. It was suggested that important factors may be mainly 4) and 6). In conclusion, feeding success may not be necessarily associated with dense aggregation of krill, but with distribution pattern of krill which enhance prey capture, and availability of myctophid fish.