The social cost of shoaling covaries with predation risk in nine-spined stickleback, Pungitius pungitius, population

The main benefit of grouping is reduced predation risk, while the main costs are thought to derive from competition and increased frequency of social interactions. While the benefits of grouping are well known, its costs have rarely been studied. We studied growth of nine-spined sticklebacks from two marine (high-predation) and two pond (low-predation) populations by rearing them either individually or in groups from hatching until they reached adult size. We found that living in groups had a strong ( up to 14%) negative effect on growth in fish from low-predation populations, despite the lack of constraints originating from resource limitation, predation, reproduction or parasites. Group living had no effect on the growth of fish from high-predation populations. We also studied willingness to shoal: fish from all populations showed strong shoaling behaviour. Our results suggest that the social cost of shoaling can be high, but individuals from high-predation populations seem to have adapted to minimize these costs better than individuals from low-predation populations. (c) 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.