Seabirds are high trophic predators in marine ecosystems and are sensitive to change in food supply and thus seabirds can be used as monitors of the marine environment. In order to study the foraging responses of Japanese cormorants Phalacrocorax filamentosus breeding at Teuri Island, Hokkaido to changes in fish availability, the diet was assessed from the regurgitations of parents and chicks, and diving behavior was measured by using time‐depth recorders. Breeding performance (brood size, chick growth, breeding success) was monitored using conventional methods to study their breeding responses. Japanese cormorants changed the diet and foraging behavior over four summers. The birds fed mainly on epipelagic schooling fish when they were available and on demersal fish when pelagic fish availability was low. They tended to dive deeper and longer in a year when they fed mainly on demersal fish than the other years, reflecting the change in the depth distribution of prey fish. Chick growth rate did not differ among years, but fledging success was lower in the years of demersal fish as their meal delivery rate was low. When epipelagic schooling fish were considered scare, parents maintained chick growth by reducing brood size. High variability and unpredictability in pelagic fish abundance are key factors affecting the foraging and breeding performance of Japanese cormorants, which could potentially be used to monitor fish resources.

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