SIZE ASSORTATIVE SHOALING

Size Assortative Shoaling

Abstract

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 aculeatus, the 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.

Table of Content

Introduction4

Results4

Discussion5

Conclusion7

References8

Size Assortative Shoaling

Introduction

Much research has been carried out concerning the adaptive significance of group living, especially the effect of group size on the costs and benefits involved with group membership. Numerous studies have shown that an increase in group size may enhance food finding and predator avoidance (Curio, 1996). Yet, acknowledging the fact that individuals differ in their characteristics, it is conceivable that the payoff to an individual of attending a particular group is affected, not only by the size of the group, but also by its composition. In other words, an individual's relative safety in a group is potentially influenced by its appearance in relation to that of its group mates (Bertram, 1998).

Results

Neither the foraging time nor the feeding rate of small sticklebacks varied significantly with their appearance in the shoal (Table I). ...