Top predator distribution and abundance across the eastern Gulf of Alaska: Temporal variability and ocean habitat associations

Abstract

Abstract We studied interannual variation in marine bird and mammal distribution and abundance in the eastern Gulf of Alaska (GOA) over 8 years, 1996–2003. We identified and enumerated seabirds, pinnipeds and cetaceans along a replicated 1500 km survey path, representing 450 km 2 of coastal and 2000 km 2 of oceanic habitat. Near-surface temperature (5 m depth) fluctuated considerably from year to year, in part due to the timing of the survey, with an early survey in 1996 and a late survey in 2002. Many species were observed across the entire gradient, particularly procellariiform (tubenose) seabirds and Dall's porpoise ( Phocoenoides dalli ). We observed peaks in abundance in the oceanic zone in 1998, 2001, and 2002, owing primarily to influxes of dark shearwaters ( Puffinus spp.) and Leach's storm-petrels ( Oceanodroma leuchora ). Rank correlations indicated similar year-to-year changes in density between species, and species-specific responses to temperature and ocean productivity as indexed by nitrate and chlorophyll a concentrations. We developed topographic/bathymetric models of habitat selection for the coastal zone. Although we found some distinct habitat preferences in this zone, overall we observed a continuum in the marine bird and mammal community across the entire eastern GOA. The strength of the coupling between coastal and oceanic environments as provided by variation in top predator dispersion appears related to large-scale variations in oceanography, though we have yet to fully investigate causal mechanisms.