Seabird nestling diets reflect latitudinal temperature-dependent variation in availability of key zooplankton prey populations

We report on historical (1978 to 1982) and more recent (1996 to 2000) variation in the nestling diet of Cassin's auklet Ptychoramphus aleuticus breeding at Triangle Island (southern) and Frederick Island (northern), British Columbia, Canada; these islands are influenced by the California and the Alaska Current ecosystems, respectively. Ocean climate conditions off the British Columbia coast varied tremendously between 1978 and 2000. At both colonies, the nestling diet was composed largely of copepods and euphausiids, with fish contributing substantially in some of the warmer years at Triangle Island. The copepod Neocalanus cristatus was the single most important prey item at both colonies, and Stage V copepodites dominated in all sampling periods. We used a recently published temperature-dependent phenology equation to estimate the timing of peak biomass of Neocalanus near Triangle and Frederick Islands. During warm water years (such as 1996 and the El Nino of 1998), the timing and duration of N. cristatus availability in surface waters near Triangle Island was early and limited (mismatched) in contrast to cooler years (such as 1999 and 2000), when this prey was available to birds throughout the breeding season (matched). We argue that Cassin's auklet nestling diet data reflect the temperature-related timing of Neocalanus prey availability to seabirds in surface waters. Our results support the argument that inadequate overlap of prey availability and predator breeding (i.e. temporal trophic mismatch) is more likely on Triangle Island, where zooplankton peaks often occur earlier and are narrower, than on Frederick Island, where prey peaks are later and more protracted. Poor reproductive performance is the biological consequence of such trophic mismatch for Cassin's auklet. If the frequency of El Nino-like events increases and if ocean temperatures rise in the future, we predict an increase in the frequency of trophic mismatch events in the northeast Pacific Ocean.