Towards of a firmer explanation of large shoal formation, maintenance and collective reactions in marine fish

Avoiding predation is generally seen as the most common explanation for why animals aggregate. However, it remains questionable whether the existing theory provides a complete explanation of the functions of large shoals formation in marine fishes. Here, we consider how well the mechanisms commonly proposed to explain enhanced safety of group living prey explain fish shoals reaching very large sizes. By conceptually re-examining these mechanisms for large marine shoals, we find little support from either empirical studies or classical models. We address first the importance of reassessing the functional theory with predator-dependent models and the need to consider factors other than predation to explain massive fish shoals. Second, we argue that taking into account the interplay between ultimate benefits and proximate perspectives is a key step in understanding large fish shoals in marine ecosystems. Third, we present the growing body of evidence from field studies that identify shoal internal structure as an important feature for how large shoals can form, maintain and react as a coordinated unit to external stimuli. In particular, we consider a mechanistic basis of local rules of interaction for group formation and collective dynamic properties that can account for groups reaching very large sizes. Recent research in collective animal behaviour has shifted focus from the importance of global properties (group size) to local properties (local density and information transfer). In contrast to studies of fish shoals in the laboratory, the difficulty in measuring behaviour in large shoals in marine systems remains a major constraint to further work. Advances in acoustical observation have shown the greatest potential to provide data that can link proximate mechanisms in, and ultimate functions of, large marine fish shoals.