Mixed-species fish shoals: any port in a storm?

Mixed-species fish shoals are common, and form for a variety of reasons. We suggest that short term mixed-species shoals, that persist for minutes or hours, might form because lone individuals (or small groups) of one species might benefit from joining larger groups of heterospecifics to reduce predation risk. We carried out a literature survey, which revealed that mixed-species groups seldom contain equal numbers of species; rather one or two dominate, with others present as small minorities. Furthermore, we found that the proportion of minority species decreases as absolute shoal size increases. We suggest that although minority members of mixed-species groups might suffer costs associated with being odd, they might often still do better by grouping than if they remained alone. We term this ‘any port in a storm’. This explanation makes several predictions and assumptions, and we suggest approaches for testing these. For example, minority members should attempt to form single-species groups when the opportunity arises, and mixed species-groups should coincide with periods of heightened predation risk. Mixed-species groups may be influenced by the distribution of shelter, and further work is needed to disentangle the relative importance of aggregation versus social attraction in the formation of mixed-species-shoals. This account of mixed-species groups does not exclude other ecological functions, such as forager-guild formation, but may explain cases of short-term associations of minority species with numerically dominant species in fish shoals. Our predictions are readily testable, and we hope they spur further research in this area.