ABSTRACT: Migratory dynamics and dietary change of sooty shearwaters Puffinus griseus sampled in the North Pacific from May to September in 1990 and 1991 were investigated using the method of d¹³C and d¹⁵N analyses. d¹³C and d¹⁵N values of muscle tissues of shearwaters ranged from -21.9 to -15.1o/oo and from 8.1 to 16.6o/oo, respectively. The d¹⁵N values of immature shearwaters collected east of longitude 170° W differed from those west of this longitude. In the northwestern Pacific, d¹³C and d¹⁵N of both mature and immature birds were lowest in May (both mature and immature: d¹³C -20.7 ± 0.9o/oo, d¹⁵N 8.8 ± 1.0o/oo; mean ± SD) and then gradually increased until early summer. In contrast, no seasonal variations in d¹³C and d¹⁵N of mature birds were observed in the northeastern Pacific and d¹³C and d¹⁵N of immature birds in the northeastern area were the highest in May (d¹³C -15.9 ± 0.5o/oo, d¹⁵N 15.9 ± 0.8o/oo; mean ± SD), and then gradually declined. We attribute these isotopic trends primarily to differences in the isotopic signatures of food-web components due to oceanographic factors, and the influence of sooty shearwater migrations. Low or high d¹³C and d¹⁵N values of immature and mature shearwaters in the western and eastern North Pacific in May may be caused by the transient effects of food consumed in the Southern Hemisphere before or during the northward migration. In particular, the high d¹⁵N of the immature birds in the northeastern area seems to be caused by feeding on ¹⁵N rich diets in the eastern tropical Pacific, off Peru and California, during the northward migration, where occurrence of denitrification with a high nitrogen isotope fractionation factor of up to 1.04 is well known. The pattern of the migratory behaviour of sooty shearwaters in the Pacific Ocean was successfully clarified by their isotopic signature.

KEY WORDS: Puffinus griseus · Stable isotope · Carbon 13 · Nitrogen 15 · Migration

Published in MEPS Vol. 158 (1997) on November 17
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997