Influence of prey behaviour and other predators on the foraging activities of a marine avian predator in a Low Arctic ecosystem

Air-breathing marine predators can expend considerable time and energy searching for patchily distributed prey and, thus, likely employ varying foraging strategies to minimize these costs. We studied the diurnal foraging patterns of the northern gannet Sula bassana along with the distributional patterns of its main prey, capelin Mallotus villosus, off the NE Newfoundland coast from 1999 to 2005. We explored whether gannets minimize searching costs for near-surface (<30 m) capelin by (1) aggregating in shallow waters (<30 m) where capelin are accessible throughout the clay, (2) aggregating in deeper waters where they concentrate foraging effort at dawn and dusk, when diel vertical migrations make capelin accessible, or (3) associating with other marine predators that increase the accessibility of capelin. We integrated continuous recordings of gannet foraging activities with vessel-based survey data on the density and distributional patterns of capelin, gannets and whales. Capelin typically comprised >70% of gannet diets. Gannets aggregated in shallow areas (<30 m) where spatially and temporally persistent shoals of capelin were accessible through daylight, but were also found in deeper water where capelin were inaccessible. Gannet diving activity was only recorded during daylight, and although activity peaked at the start and end of daylight, most dives occurred when capelin had migrated to inaccessible depths. Gannets were more tightly associated with whales than with near-surface capelin shoals, suggesting that they may successfully track capelin by cueing to the foraging activities of whales. A mixed strategy of memory-based and local enhancement foraging tactics appear to play important roles in minimizing the effort required to access prey.