Abstract
During summer 1997, hundreds of thousands of emaciated short‐tailed shearwaters (Puffinus tenuirostris) died in the south‐eastern Bering Sea. Using strip transect methodology, we documented the distribution and abundance of short‐tailed shearwaters during cruises conducted prior to, during, and after the die‐off, as well as the distributions and abundances of floating carcasses. The distributions and abundances of short‐tailed shearwaters in 1997 were similar to those found during the 1970s and early 1980s. In August–September 1997, we observed 163 floating shearwater carcasses, most of which were between St Paul Island and Nunivak Island. We estimated ≈ 190 000 carcasses were afloat in the study area, about 11% of the surveyed population. Between spring (June) and autumn (August/September), mean net body mass of shearwaters decreased by 19%, mean pectoral muscle mass decreased by 14%, and mean percentage body lipid content decreased by 46%, from 15.6% in spring to 8.4% in autumn. Compared with spring, short‐tailed shearwater diets broadened in autumn 1997, to include, in addition to adult euphausiids Thysanoessa raschii, juveniles of T. inermis, T. raschii and T. spinifera, crab megalops, fish and squid. We discuss how the ecosystem anomalies in the south‐eastern Bering Sea during spring and summer 1997 relate to the mortality event and suggest possible implications of long‐term climate change for populations of apex predators in the south‐eastern Bering Sea.
Highlights
Short-tailed shearwaters (Puf®nus tenuirostris) migrate annually from breeding grounds in south-eastern Australia and Tasmania to forage in the northern Paci®c Ocean and Bering Sea during the austral winter
Short-tailed shearwaters arrive in the south-eastern Bering Sea in May and June, ®nish their moult by August and depart in September and October (Shuntov, 1961; Schneider and Shuntov, 1993)
These shearwaters are the most numerous seabirds in the Bering Sea, with estimates for the entire region ranging from 8.7 million (Shuntov, 1972) to 20 million birds (Hunt et al, 1981). They feed on a variety of ®sh, squid, and euphausiids throughout their range (Ogi et al, 1980; Krasnow and Sanger, 1986; Montague et al, 1986; Sanger, 1986; Skira, 1986; Weimerskirch and 2 Cherel, 1998), the diet of short-tailed shearwaters over the south-eastern Bering Sea shelf in the past was apparently dominated by the euphausiid Thysanoessa raschii (96% of diet; Ogi et al, 1980; Schneider et al, 1986; Hunt et al, 1996)
Summary
Short-tailed shearwaters (Puf®nus tenuirostris) migrate annually from breeding grounds in south-eastern Australia and Tasmania to forage in the northern Paci®c Ocean and Bering Sea during the austral winter. Short-tailed shearwaters arrive in the south-eastern Bering Sea in May and June, ®nish their moult by August and depart in September and October (Shuntov, 1961; Schneider and Shuntov, 1993) During their summer dispersal, these shearwaters are the most numerous seabirds in the Bering Sea, with estimates for the entire region ranging from 8.7 million (Shuntov, 1972) to 20 million birds (Hunt et al, 1981). We present information on the status of shearwater populations within our study area, estimate the number of dead birds and report on body condition, diet trends, and the availability of prey to these birds during spring and autumn. 4 Results are discussed in Hunt et al (1999a,b)
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