VARIATION IN THE DIET OF CASSINS AUKLET REVEALS SPATIAL, SEASONAL, AND DECADAL OCCURRENCE PATTERNS OF EUPHAUSIIDS OFF CALIFORNIA, USA

We investigated spatial and temporal variation in the diet of a plankt ivore, the Cassin's auklet Ptychoramphus aleuticus, in the Gulf of the Farallones, California, USA, where its population has decreased ca 50 % in size coincident with the general decline of zooplankton in the Ca lifornia Current. Stomach samples were collected and compared among bi rds foraging at sea during the non-upwelling, post-spring transition ( to upwelling), and upwelling periods each year from 1985 to 1988. Upwe lling-period samples also were compared with samples regurgitated to c hicks on the Farallon Islands during that period; such samples had bee n collected during 1971 and 1977-1991. During the non-upwelling period , auklets fed on the small, energetically inferior, oceanic euphausiid Euphausia pacifica, but following the spring transition, the auklets switched to small, juvenile fish (mostly Sebastes spp.) until these ha d grown too large for capture. Thereafter, the auklets switched to the larger and energetically more valuable euphausiid Thysanoessa spinife ra, which had become available upon the onset of upwelling. This gener al pattern was repeated each year, although some between-year variabil ity among ancillary prey was evident. Auklet foraging locality, howeve r, varied only slightly throughout the year; auklets were most dispers ed during El Nino. It was apparent that the availability of T. spinife ra was Linked to upwelling. The decrease in the prevalence of T. spini fera in the diet fed to chicks from 1971 to 1991 is consistent with de creased intensity of upwelling in the region, as is the reduction in n umbers of auklets in the region over the same period. Our results indi cate the value of studies of easily accessible marine predators, which can temporally and spatially integrate patterns in the prevalence of prey species, thereby increasing our understanding of the important pr ocesses that structure food webs.