Association of predators and prey at frontal features in the California Current: competition, facilitation, and co-occurrence

We investigated variations in the small- to meso-scale abundance and distribution of the 6 most abundant seabird species in the northern California Current-black-footed albatross Phoebastria nigripes, sooty shearwater Puffinus griseus, pink-footed shearwater Puffinus creatopus, fork-tailed storm-petrel Oceanodroma furcata, common murre Uria aalge, and Cassin's auklet Ptychoramphus aleuticus-during the upwelling season of 2000 and 2002. Covariates (21 total), with importance assessed using logistic and generalized linear modeling and an information theoretic approach, included physical features such as sea surface temperature, dynamic height (apparent water-column pressure), and pycnocline depth; biological factors such as chlorophyll maximum; and food-web factors such as the density of 3 size classes of zooplankton, the density of potential piscine competitors, i.e. Pacific salmon Oncorhynchus spp., and abundance of fish prey such as zooplankton, northern anchovy Engraulis mordax, and the juveniles of salmon and demersal fishes. The most important factors explaining seabird occurrence changed from mesoscale physical features during a food-rich year (2002; exhibited over 15 to 30 km) to smaller-scale occurrence of actual prey patches during a food-poor year (2000; <1 km). Spatial overlap in occurrence of murres and shearwaters with adult salmon was interpreted as co-occurrence and, perhaps, competition for prey species; a negative spatial overlap between shearwaters and abundance of forage fish was interpreted as evidence for prey depletion (or predator-induced alteration of availability) by the birds and other co-occurring predators (salmon). Overall, results and other information indicated the value of adding spatially explicit data on predator and prey species abundance and predator-prey behavior to improve food-web modeling.