Size-assortative shoaling in fish: the effect of oddity on foraging behaviour.

Current theory predicts that fish should show size-assortative shoaling in order to avoid increased predation risk by being the odd one out (oddity effect), or in order to minimize competition for food. I investigated with three-spined sticklebacks, Gasterosteus aculeatusthe importance of the oddity effect in promoting size-assortative shoaling. The greater an individual assesses its predation risk the less actively it is likely to forage. Hence, I examined with small and large fish whether an individual's foraging activity depends on its appearance (size) in relation to that of others in a shoal. The shoals were composed of three, six and 12 fish. Either one individual deviated in size from the rest of the shoal members or all the fish in a shoal were of similar size. When a stickleback was larger than others in the shoal its foraging activity was lower than that of large individuals in a shoal dominated by large fish or those in a size-assorted shoal. Small sticklebacks, however, did not change their foraging activity on the basis of their appearance in a shoal. These responses of individuals to their appearance did not depend on shoal size nor on the presence or absence of a predator. The results suggest that the oddity effect is likely to prevent larger sticklebacks from joining shoals of smaller individuals. They also suggest that factors other than the oddity effect, potentially food competition, may be more important in leading individuals to avoid the company of larger ones and prefer shoaling with matching conspecifics.