Line Transect Estimates of Short-tailed Shearwater Puffinus Tenuirostris Mortality in the South-eastern Bering

Ornithologists survey marine birds to determine patterns of distribution and abundance, and to estimate the size of their populations (Spear et al. 1995, Hunt et al. 1996a). Additionally, wildlife biologists survey beach-cast seabirds systematically, and whenever large die-offs occur (Stenzel et al. 1988, Piatt et al. 1990, Piatt & Van Pelt 1997). Estimating baseline mortality rates is particularly useful because it places unusually large seabird dieoffs in perspective, and facilitates the comparison of natural and anthropogenic impacts on seabird populations (Piatt et al. 1990, Bodkin & Jameson 1991, Piatt & Van Pelt 1997). During the fall of 1997, an estimated 600 000 Short-tailed Shearwaters Puffinus tenuirostris starved to death in the Bering Sea. Carcasses were sighted across a vast geographical area spanning from Cape Anadyr, Russia and the Aleutian Islands as far west as Adak, eastwards to Bristol Bay (Mendenhall 1997, Vance et al. 1998, Baduini et al. 2001). We used standard population sampling techniques to estimate the number of floating carcasses within three survey grids during and after the 1997 die-off. In this paper, we compare the extent and magnitude of the die-off with background mortality levels during subsequent years (1998 and 1999) when no die-off was detected (Baduini et al. 2001, V.M. Mendenhall pers. comm.).