Phenotypic variability within and between fish shoals

Theory predicts that individuals within social groups should be more similar (less variable) phenotypically than individuals between groups. We tested this hypothesis by comparing the variability of certain phenotypic characters (body size, parasite load) of individual fish within and between sampled shoals with the phenotypic variability of randomly constituted shoals of the same size. We caught 34 entire fish shoals in a lake, and for each shoal member, we identified the species, measured its body length, and counted the number of black spots (indicating the presence of a parasitic trematode worm) visible through their skin. We also recorded two habitat characteristics, water depth and distance to cover, at the location where each of the 34 shoals was caught to ascertain whether (and to which degree) the spatial distributions of different fish species and fish size classes were determined by certain habitat characteristics. A comparison between the observed and the randomly assorted shoals (obtained through computer simulations) revealed that fish at our study site exhibited a significant association with conspecifics and with individuals of similar body size (within and across species); that is, shoals were assorted by species and by individual body size. Parasite-assortative shoaling was not observed. Different species (and to a lesser degree different size classes within species) showed a tendency to occur in different microhabitats. Nevertheless, despite some degree of habitat segregation, there remained considerable spatial overlap between the different species and fish body size classes. Furthermore, given the spatial proximity of different habitats and the relatively weak habitat association of most species, we therefore suggest that active shoal choice by individual fish was the most important determinant of shoal composition.