Foods, trophic relationships, and migration of Sooty and Short-tailed Shearwaters associated with squid and large-mesh driftnet fisheries in the North Pacific Ocean

We salvaged dead birds from high seas driftnets in the Central North Pacifi c Ocean during a study of the impact of high seas driftnet fishing on marin e ecosystems. Digestive tract contents and stable isotopes of nitrogen and carbon in breast muscles of these birds were analyzed to assess the effect of driftnets on the trophic relationships of marine birds. The diets of Soo ty (Puffinus griseus) and Short-tailed (P. tenuirostris) Shearwaters associ ated with high seas driftnets in the transitional region of the North Pacif ic Ocean over lap broadly. The principal differences between them include 1 ) Sooties prey more heavily on immature stages of invertebrates, especially the barnacle Lepas fascicularis than do Short-tails, 2) Pacific saury (Col olabis saira) are the primary fish taken by Sooties while lanternfish (Myct ophidae) are the primary fish taken by Short-tails, and 3) Sooties lake a w ider variety of prey than do Short-tails. During the last years of extensiv e high seas driftnet fishing (early 1990's), both shearwaters augmented the ir diets with about 15% offal and discards from fishing vessels. Values for stable isotopes of nitrogen and carbon in breast muscle tissues (delta(15) N and delta(15)C) indicate the extent and timing of movements of Sooty Shea rwaters: including arrival of migrants from the south Pacific (April-May), east to west movement from the eastern Pacific (May-June), and west to east movement from Japan (August-September). Fall migrant Sooties were not dete cted. The only movements of Short-tails that ive detected were a few south- bound migrants in September-November.