Planktivorous Auklets (Aethia pusilla and A. cristatella) nesting on St. Lawrence Island, Alaska as indicators of marine conditions in the northern Bering Sea

Abstract

Monitoring reproductive success, prey species composition, and colony size of marine birds has been proposed as a method of assessing changes in marine systems that are otherwise difficult to sample (Cairns 1987). I measured inter-annual and intra-seasonal variability in reproductive parameters, taxonomic composition of the diet, and adult body condition of Crested Auklets (Aethia cristatella) and Least Auklets (A. pusilla) at 2 colonies near the village of Savoonga, St. Lawrence Island, Alaska during the 2000-2002 breeding seasons to evaluate how reproductive success of planktivorous seabirds is related to diet. I also assessed the utility of two methods of population monitoring (surface counts and mark-resighting) for detecting annual changes in breeding populations of Crested and Least auklets during the 2001 and 2002 breeding seasons on the Kitnik colony. Average reproductive success was generally high (>60% of nests) for both auklet species during the 3 years of the study, but differed among years. Median hatching dates for both species were 2 weeks earlier in the year of highest reproductive success (2002), compared to the previous 2 years. In all 3 years, the diet of Crested Auklets was predominantly euphausiids, while the diet of Least Auklets consisted primarily of calanoid copepods, but species composition of the diet differed among years for both species. Crested and Least auklets consumed more of the large, lipid-rich copepod Neocalanus cristatus in 2002 than in the other 2 years of the study. The year of lowest reproductive success (2001) was associated with low prevalence of euphausiids in Crested Auklet diets late in the chick-rearing period and high prevalence of the small, low-lipid copepod Calanus marshallae in Least Auklet diets. I observed an increase in total body mass of Crested Auklets during the 2002 breeding season, whereas total body mass declined through the breeding season in the other 2 years. Seasonal changes in adult body mass of Crested Auklets may, therefore, be a useful indicator of food availability. Average body mass of Least Auklets declined in all 3 years, but was lowest in 2001, suggesting that low adult body mass of Least Auklets may reflect poor foraging conditions. Fat reserves of breeding auklets during egg-laying were not highly variable among or within breeding seasons and therefore were not a sensitive predictor of subsequent breeding success. Counts of Crested Auklets in plots on the colony surface were highest in areas of large average boulder size; Least Auklet surface counts were not as variable among plots. Maximum counts of both species of auklets in plots did not differ between years. Patterns of colony surface attendance during the breeding season, however, did differ between years. The colony surface attendance of both auklet species after hatching was higher in the year of high reproductive success. Preventing nest initiation by covering plots with tarps did not reduce subsequent colony surface attendance during chick-rearing (after the tarps were removed) for either species, suggesting that reproductive success, independent of differences in food availability, did not cause a difference in colony surface attendance. I estimated abundance of Least Auklets nesting in two 100-m² plots using mark-resight methods. I concluded that surface counts may provide an indication of among-year differences in colony attendance, but underestimate the number of breeding individuals by a factor of 10. Mark-resighting techniques show more promise for detecting changes in the number of breeding pairs. Reproductive success, adult body mass, and post-hatch colony attendance of Crested and Least auklets appear positively associated with zooplankton availability, particularly the prevalence of N. cristatus in the diet. Annual monitoring of these 3 parameters, together with diet composition, are important for understanding how both natural and anthropogenic climate change may affect trophic structure of the northern Bering Sea ecosystem.