ABSTRACT: The harp seal Phoca groenlandica, which is numerous and widespread in the Northwest Atlantic, may have significant influences on the structure of this ecosystem. To quantify this influence, we must understand the functional relationship between harp seals and their prey. If seals are discriminating in their choice of prey, then their consumption of a particular species will not necessarily vary in relation to its availability or catchability. By applying Chesson's index of selectivity to stomach contents and research trawl data collected in several near- and offshore locations, we found that harp seals preferentially selected capelin Mallotus villosus relative to other prey species, irrespective of their local abundance, when given the choice. Arctic cod Boreogadus saida were also preferred in nearshore areas, but not in the offshore. In general, these predators were neutrally selective towards Atlantic cod Gadus morhua, American plaice Hippoglossoides platessoides and Greenland halibut Reinhardtiushippoglossoides. These patterns rationalize the dietary patterns reported for harp seals generally. They also explain the harp seals' switch from a reliance on capelin to Arctic cod seen in the mid 1980s, when evidence suggests these cod became more abundant than capelin in nearshore waters (where their respective energy densities are similar).

KEY WORDS: Harp seal · Arctic cod · Capelin · Chesson's selectivity index

Published in MEPS Vol. 163 (1998) on March 12
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998