Foraging habitat and prey taken by least auklets at King Island, Alaska

Abstract

MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Vol. 65: 141-150, 1990 Published August 2 Foraging habitat and prey taken by least auklets at King Island, Alaska George L. Hunt, Jr, Nancy M. Harrison* Department of Ecology a n d Evolutionary Biology, University of California, Irvine, California 92717, USA ABSTRACT: We examined the foraging distribution a n d prey use of least auklets Aethia pusilla breeding on King Island in the northern Bering Sea. The locat~on of the King Island colony, in Alaska Coastal Water, but within auklet flight range of the oceanic Anadyr Current and the Anadyr-influenced Bering Shelf Water, provided an opportunity to ident~fy the preferred foraging h a b ~ t a t of the least auklet. Least auklets fed on Neocalanus plumchrus, N. cnstatus, Eucalanus bungii, a n d shrimp larvae, all present in Anadyr Current and in Bering Shelf Water. In early J u n e 1984 a n d July 1985, auklet foraging was concentrated at and just beyond a front ca 30 km west of Klng Island, separating Alaska Coastal Water from Bering Shelf Water to the west, large copepods of oceanic ongin occurred throughout Bering Shelf Water. Data from 1985 indicated varlabllity In the path of the Anadyr Current; rapid changes also took place in the distribution of the least auklets' planktonic prey. In late July 1985, we documented an intrusion of Anadyr Water from 45 km to withln 25 km of King Island. Least auklets often overflew the Alaska Coastal Water and foraged in stratified Benng Shelf Water, a n d in Anadyr Water beyond the front. In 1986, Alaska Coastal Current Water overlaid Bering Shelf Water in much of the area to the west of King Island and a majority of auklets foraged where Bering Shelf Water w a s closest to the surface. Throughout the study, foraglng least auklets were absent from the Alaska Coastal Current near King Island, even though alternative prey, Calanus rnarshallae, taken elsewhere by breeding least auklets, w e r e present. We conclude that at King Island least auklets sought specific preferred prey in stratified water and near fronts a n d shifted their foraging over large distances in response to the mobile marine habitat in which their prey were abundant. INTRODUCTION Marine birds forage in habitats that are patchy and dynamic. Within habitats, prey patches are ephemeral, with large prey patches generally lasting longer than small patches (Steele 1976, Haury et al. 1978). Addi- tionally, the habitats themselves are mobile, with ani- mal communities carried by currents. Thus, breeding marine birds foraging from a central place not only have to locate patches of prey within a given habitat, but also must respond to movements of the water mass within which the patches of preferred prey occur. We report here the foraging distribution of least auklets Aethia pusilla breeding on King Island, northern Be- ring Sea, Alaska, in relation to water masses supporting different communities of zooplankton on which these auklets feed. Factors determining the distribution of foraging sea- Present address: Nature Conservancy Council, l? Rub~slaw Terrace, Aberdeen AB1 IXE, Scotland 0 Inter-Research/Printed in F. R. Germany birds in the vicinity of their breedlng colonies have long been of interest. Initial emphasis focused on the possi- bility of density-dependent limitation of seabird num- bers due to intraspecific competition for prey near colonies (e.g. Ashmole 1963, Gaston et al. 1983, Furness &Birkhead 1984, Hunt et al. 1986), and themechanisms for resource partitioning by which interspecific com- petition might be reduced (e.g. Ashmole & Ashmole 1967, Bedard 1969). Cody (1973) suggested that the principal means of resource partitioning in coexisting species of alcids (e.g. auks, puffins) was the use of foraging zones at different distances from their col- onies. Bedard (1976) showed that much of Cody's interpretation was flawed. Bedard suggested that the foraging distribution of alcids near their colonies was more likely to be determined by ' . . . the general water circulation and oceanographic conditions. . . ' in the vi- cinity of the colony. Recently, Kinder et al. (1983) found that frontal systems associated with shoaling of water near the Pribilof Islands (Alaska) supported higher numbers of foraglng birds than expected by chance.