Inter-Annual, Seasonal, and Sex Differences in the Diet of a Surface Feeding Seabird, Streaked Shearwater Calonectris leucomelas, Breeding in the Sea of Japan

Seasonal and inter-annual changes in the diets of seabirds are expected to relate to the abundance and distribution of their prey species. We analyzed the stomach contents of 227 male and female Streaked Shearwater Calonectris leucomelas during the chick-rearing period on Awashima in the Sea of Japan between 2008 and 2013. They fed predominantly on Japanese Anchovy Engraulis japonicus early in the season but changed their diet, consuming increasing numbers of benthic fish species (mainly Gadidae, Sebastidae, and Zoarcidae) later in the season. Being surface feeders, they possibly fed on these benthic species discarded by the trawl fishing boats in intense fishing grounds of both the Pacific and the Sea of Japan. Moreover, the shearwaters also showed sex-specific differences in their diet. Females fed more on anchovies, while males tended to feed more on benthic species, presumably when they fed in the Pacific and the northern part of the Sea of Japan. They fed more on benthic fish in years with high sea surface temperatures (SST), probably due to low availability of epipelagic fish. With the trend of global sea warming, our results indicate the importance of fisheries-related food sources far from the colony for this generalist surface-feeding seabird.