Ocean climate, euphausiids and auklet nesting: inter-annual trends and variation in phenology, diet and growth of a planktivorous seabird, Ptychoramphus aleuticus
Variability in ocean climate results in fluctuations in the timing and abundance of zoo- plankton. In turn, fluctuations in zooplankton as prey may have consequences for reproductive deci- sions and performance of planktivorous seabirds. We examined interrelationships between mean sea- sonal oceanographic conditions and prey availability and the timing of breeding, nestling diet, growth and productivity (as number of chicks fledged per breeding pair) of Cassin's auklet Ptychoramphus aleuticus in the central California Current System. Substantial inter-annual variability in seasonal ocean climate parameters as well as in auklet reproductive and diet characteristics was evident. Hatching dates and the proportion of all euphausiids in the diet were positively correlated with sea- sonal sea-surface temperature and negatively correlated with seasonal upwelling values. A significant negative correlation was found between the proportion of Euphausia pacifica in the diet and hatch date, while significant positive correlations were found between Thysanoessa spinifera in the diet and productivity, and between productivity and growth. Based on the California Cooperative Fisheries In- vestigations euphausiid data set, significant positive correlations were found between E. pacifica abundance and auklet meal mass and productivity and between T. spinifera abundance and auklet productivity. As demonstrated by the auklet diet, euphausiids responded to changes in ocean climate on multiple temporal scales. In most years, auklets responded adaptively to variation in oceanographic conditions and prey availability by modifying timing of breeding. However, in years when ocean con- ditions deteriorated after the initiation of breeding, prey availability was severely reduced beyond the auklet's adaptive response, resulting in diminished nestling growth and productivity.