Phenology of pelagic seabird abundance relative to marine climate change in the Alaska Gyre

Abstract

In relation to climate change, a common finding from long-term studies on marine ecosystems is earlier annual peaks of abundance for many organisms. Here, we test the hypothe- sis of unidirectional change in the seasonal abundance of pelagic seabirds in association with change in marine climate and primary productivity in the Alaska Gyre. To test this hypothesis, we analyzed data on hydrographic conditions and seasonal at-sea abundance of seabirds along a 1425 km transect (Line P) over 11 yr, 1996 to 2006. Long-term sea surface temperature (SST) data show warming in the study region, advanced and delayed temperature peaks and northwestward isotherm displacement. Using negative binomial regression, we tested seasonal trends in the rel- ative abundance of 15 seabird species and compared abundances to a multivariate ocean climate index we developed with principal component analysis from in situ measurements of water temperature, salinity, density and nitrate concentrations. Overall, 5 species showed no change, 1 declined, and 9 species as well as all species combined showed increasing abundance. By season, 3 species increased in winter, 7 in late spring, and 6 in late summer. Eight of 15 species showed relationships with our environmental index. Increases in seasonal seabird abundance may be related to lengthening of the 'growing season', as demonstrated by temporal temperature shifts and expansion of peak chlorophyll concentrations. Seabirds of the Alaska Gyre are probably responding to changes in forage nekton that are related to this extended growing season by shifting their migration to later dates.