Prey-dependent foraging tactics and prey profitability in a marine mammal

Predators face decisions about which prey to include in their diet in order to maximize fitness. The foraging tactics used to capture prey and the resulting profitability of prey influence these decisions. We present the first evidence of prey-dependent foraging tactics and prey profitability in a free-ranging pinniped. We studied 39 adult male harbour seals Phoca vitulina at Sable Island, Nova Scotia using an animal-borne video system. Each male wore the camera system for 3 d during which 10 min video samples were recorded every 45 min from 06:00 h, resulting in approximately 3 h of videotape per male and a total of 1094 capture attempts of identified prey. Males foraged mainly on sand lance Ammodytes dubius and flounders (Pleuronectids), but salmonid and gadoid fishes were occasionally pursued. Foraging tactics differed among and within prey types based on differences in prey behaviour. Sand lance was both a cryptic prey, when in the bottom substrate, and a conspicuous schooling prey. Seal swimming speed, handling time and capture success differed between cryptic and conspicuous sand lance. The highest capture success and handling time was recorded for flounders. Estimated profitability, i.e. net energy intake per unit time, also differed with prey type and prey size. Our results suggest that diet selection may have important implications on the foraging energetics of pinnipeds.