Variation in the diet of Cassin's auklet reveals spatial, seasonal, and decadal occurrence patterns of euphausiids off California, USA

We investigated spatial and temporal variation In the diet of a planktivore, 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 California Current.Stomach samples were collected and compared among birds foraging at sea during the nonupwelling, post-spring transition (to upwelling), and upwelling periods each year from 1985 to 1988.Upwelling-period samples also were compart~ti with samples regurgitated to chicks on the Farallon Islands during that period; such samples had been collected during 1971 and 1977-1991 During the non-upwelling period, auklets fed on the small, cnergetically ~nferior, oceanic euphausiid Euphausia pacifica, but following the sprlng transition, the auklets s\,vltched to small, juvenile fish (nlostly Sebastes spp.) until these had grown too large for capture.Thereafter, the auklets switched to the larger and energetically more valuable euphaus~id Thysanoessa spinifera, which had become available upon the onsct of upwelling.This general pattern was repeated each year, although some betwcenyear variability among ancillary prey was evident.Auklct foraging locality, however, varied only slightly throughout the year; auklets were most dispersed during El Nirio.It was apparent that the availability of 7 spinifera was linked to upwelling.The decrease in the prevalence of T spinifera in the diet fed to chicks from 1971 to 1991 is consistent with decreased intensity of upwelling In the reglon, as is the reduction in numbers of auklets in the region over the same period.Our results indicate 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 processes that structure food webs.