TEMPORAL AND SPATIAL PATTERNS IN THE DIET OF THE COMMON MURRE IN CALIFORNIA WATERS

We investigated temporal and spatial variation in the diet of the Comm on Murre Uria aalge, the most abundant, locally breeding seabird of th e central California continental shelf. We collected murres in coastal , mid-shelf, and outer-shelf waters of the Gulf of the Farallones duri ng the murres' pre-breeding (March-April), breeding (May-August), and wintering (September-February) periods, 1985-1988, Diet samples formed persistent groups as a function of these six spatio-temporal combinat ions of murre foraging habitat and life-history periods. Temporally, d iets varied on a seasonal and interannual basis, with diets during win ter and El Nino periods being the most disparate. Spatially, diets dif fered among the three habitats, independent of time. During the pre-br eeding season after the onset of upwelling, euphausiids and juvenile r ockfish (Sebastes spp.) became prevalent in the diet. Diets were least diverse during the breeding season because of the dominance of rockfi sh, especially among murres foraging in mid- and outer-shelf habitats. Other important prey were also significant for commercial or sport fi shing: Engraulis mordax and Clupea harengus in coastal waters, Merlucc ius productus and Loligo opalescens in mid- and outer-shelf waters, an d surfperch Cymatogaster aggregata and Brachyistius frenatus in coasta l and outer-shelf waters (near reefs), respectively. Results support a hypothesis that diet varies as a function of where murres forage. Hen ce, if the most characteristic prey of one habitat disappears, murres switch foraging areas, bringing a switch in diet. Owing to environment al changes in the region, murres may be losing the option of prey swit ching as a strategy to maintain an adequate intake of food.