Diet is a crucial element of seabird ecology, playing a vital role in understanding their trophic niches, which explains their role in the ecosystem. Dietary choice and its composition reveal the availability and distribution of prey species within the foraging range. However, the dietary changes of many seabird species on a long-term temporal scale were not revealed well. Our study examined the inter and intra-annual changes of the diet of streaked shearwaters (Calonectris leucomelas) rearing chicks on Awashima island situated in the central part of the Sea of Japan off the Niigata coast. This is the first comprehensive analysis of the shearwater diet in the Sea of Japan in the less-productive Tsushima current region. We investigated a total of 622 stomach content samples collected over nine years.

Japanese anchovy is the preferred prey of Awashima shearwaters. Results revealed prominent intra-annual changes in the dietary composition. Anchovy showed a steady decrease towards the end of the chick-rearing period, whereas benthic fish increased significantly, indicating a high dependency in the latter half. Moreover, shearwaters on Awashima also show gender-specific differences in their diet, which could be related to the foraging strategy led by sexual size-dimorphism.

Keywords: Calonectris leucomelas, Sea of Japan, intra-annual, Awashima, sexual size-dimorphism