Prey-switching by Cassin's auklet Ptychoramphus aleuticus reveals seasonal climate-related cycles of Euphausia pacifica and Thysanoessa spinifera

Euphausia pacifica and Thysanoessa spinifera comprise a substantial proportion of the diet of Cassin's auklet Ptychoramphus aleuticus in the Gulf of the Farallones, California. Ocean climate variability has been linked to fluctuations in euphausiid abundance and life-history parameters. We tested the hypothesis that seasonal patterns of prey use by auklets correspond to within-season variation in upwelling and sea surface temperature (SST), by comparing the proportion (by number) of E. pacifica and T. spinifera in auklet diet samples obtained during the chick-rearing period to SST and an upwelling index (UI) over 11 years. We found that the proportion of E. pacifica adults in auklet diet decreased over the chick-rearing period, and increased with increasing UI and decreasing SST. We detected curvature in the relationship between adult E. pacifica and UI, with E. pacifica increasing to a point and decreasing thereafter. T. spinifera in auklet diet showed an increase over the chick-rearing period, but no relationships were detected with UI or SST. Mass of individual adult E. pacifica increased with increasing UI and decreasing SST. We detected moderate curvature in the relationship between mass of individual adult T. spinifera and SST. The proportion of E. pacifica juveniles in auklet diet did not change seasonally and was not related to UI or SST, while the proportion of T, spinifera juveniles in auklet diet increased seasonally and with increases in UI. Seasonal changes in the ingestion of euphausiids by auklets appeared to be affected by ocean climate variability; however, prey-selection via a shift in foraging location or competition with other marine predators are alternative valid explanations. This study highlighted the utility of using top marine predators to sample prey availability, thereby complementing traditional net sampling procedures for determining the distribution and abundance of euphausiids in marine systems.