Altered search speed and growth: social versus independent foraging in two pelagic juvenile fishes

Prior studies have demonstrated that juvenile walleye pollock Theragra chalcogramma forage socially in schools for spatially and temporally clumped food, but forage more independently for spatially and temporally dispersed food. One advantage of social foraging is that fish in schools may be able to locate more food clumps than fish foraging individually. However, data also indicate that walleye pollock swim faster when foraging socially. We conducted laboratory experiments to evaluate the effect of food distribution upon the energetic foraging costs incurred by juvenile walleye pollock and sablefish Anaplopoma fimbria. We predicted that when given identical rations, fish receiving clumped food would swim faster, expending more energy, and therefore grow more slowly than fish receiving dispersed food. After 2 wk under these 2 foraging regimes, juvenile walleye pollock receiving clumped food swam 50 % faster, but experienced 19 % lower growth, than walleye pollock receiving dispersed food. Sablefish demonstrated only a weak swim speed response, with no difference in growth between food distributions. Our results demonstrate that although social foraging may increase encounter rates with food, in some species there may also be an energetic cost for this behavior, which will have an influence upon energetic efficiency, potentially affecting growth and survival.