Size‐assortativeness in multi‐species fish shoals

Abstract

Theory predicts that fish should assort in shoals on the basis of similar phenotypic traits to minimize predation risk and to maximize foraging efficiency. A single phenotypic character, body size, was considered and the hypothesis tested that free‐ranging fish shoals are sizeassorted. Furthermore, a second test investigated whether fish within multi‐species shoals are more strongly size‐assorted with conspecifics than with heterospecifics. Twelve fish shoals, each comprising two different species (golden shinerNotemigonus crysoleucas, and banded killifishFundulus diaphanus) were caught in the littoral zone of a north temperate lake using a beach seine. Shoal membership size ranged from 36 to 776 fish, and mean standard body length of members ranged from 18 to 34 mm. Fish were assorted by body size at two different levels, namely, between shoals and at the level of species within shoals. Body sizes of shiners and killifish within shoals were significantly different in seven out of 12 shoals, with killifish being the larger species in five cases and shiners being the larger one in two cases. Because there is considerable overlap in body size between the two species in the population, the observed species‐related size‐assortativeness within shoals was not just the by‐product of a directional size difference (between species) in the population. These findings provide strong quantitative evidence for size‐sorting in free‐ranging fish shoals and raise questions concerning the formation of multi‐species fish shoals.