A general theory on fish aggregation to floating objects: An alternative to the meeting point hypothesis

An immense variety of fish may, on occasions, aggregate around or be associated with floating structures such as drifting algae, jellied zooplankton, whales, floats or anchored fish aggregating devices (in effect, there are over 333 fish species belonging to 96 families recorded in the literature). Several hypotheses have been advanced to explain this behaviour of pelagic fish, although the most widely accepted theory is that fish use floating materials, to some extent, to protect themselves from predators. However, we think that aggregation under floats may be the result of behaviour that has evolved to safeguard the survival of eggs, larvae and juvenile stages, during dispersion to other areas. Natural floating structures (e.g., algae, branches of trees) drift in sea currents that originate in places where the floating objects are frequently found (e.g., river estuaries, coastal areas). These same sea currents also introduce some of the planktonic production generated in these areas into the oligotrophic pelagic environment. Fish associated with drifting floating structures probably feed on invertebrates associated with the structures. However, they may also benefit from the accumulated plankton in the converging waters. Adult fish of some migratory species (tuna, dolphinfish, etc.) have also developed similar associative behaviour around drifting objects for other reasons (e.g., resting places, presence of bait fish, geographical references and school recomposition). In this context, the meeting point hypothesis is only applicable to one specific case, the tuna and tuna-like species. Aggregative and associative behaviour, under and around floating devices, may be the result of convergent behaviors that result from different motivations. However, generally this behaviour can be explained by the fact that drifting floating objects represent a means of reaching relatively rich areas, where larvae and juvenile fish have an increased chance of survival.