Functions of Shoaling Behaviour in Teleosts

Abstract

Predators and food are the keys to understanding fish shoals; synchronised cooperation defeats predators, and optimal food gathering in shoals reflects a shifting balance between joining, competing in, or leaving the group. In the wild, predators may arrive while shoaling fish are feeding, and so vigilance is a crucial behaviour. Once detected, predator defence takes precedence over feeding, since an animal's life is worth more than today's dinner. Travelling fish schools display impressive coordination and were once viewed as egalitarian leaderless societies (Breder 1954; Shaw 1962; Radakov 1973) in which cooperation preserved the species. In contrast to such classical group-selectionist views, contemporary ethology reveals social behaviour to be nothing more than animals cooperating only when it pays. Distinct coexisting behavioural strategies of sneaking or scrounging are often evolutionarily stable (Barnard 1984; Parker 1984). In fish shoals, homogeneity and synchrony have been overemphasised; recent work reveals that individuals constantly reappraise the costs and benefits of being social. Reappraisal is reflected in decisions to join, stay with or leave groups, and observed behaviour allows us a glimpse of these underlying tensions. In teleosts, a major constraint is swimming; fish physiologically and morphologically adapted to cruise fast, such as mackerel, break ranks less often to avoid the alternative of rapid dispersal. Under some circumstances , however, the underlying tensions between individuals may be uncovered in even the most phalanx-like of cruising fish shoals. Shoaling behaviour has attracted much speculation about function (e.g. Shaw 1978; Partridge 1982a), but until recently few critical experiments have been performed. The aim of this chapter is to review in the light of current theory the areas where such evidence of function has been gathered. The arguments presented in this chapter do not support the views of Hamilton (1971) or Williams (1964) that shoaling is primarily a matter of cover-seeking, or Breder's (1976) view that hydrodynamics is the major factor. Furthermore, I will present arguments that, for fish shoals, simple attack avoidance and attack dilution have been incorrectly associated with selection for grouping behaviour. Definitions of Shoaling A clarification of terms will facilitate discussion of function, since shoaling behaviour continues to suffer from the semantic confusion of the 1960s 294 T. J. Pitcher (ed.), The Behaviour of Teleost Fishes