Does fishing on drifting fish aggregation devices endanger the survival of trop...( Fishermen hold empirical knowledge t...)

Fishermen hold empirical knowledge that tuna aggregate under floating objects, such as lengths of old rope, pieces of wood, or even large marine mammals. There is still no full explanation for this aggregation behaviour, but the past 20 years have seen purse-seine fishery operators take advantage of the associated concentrations of fish. Fishermen cast off floating rafts equipped with buoys which act as FADs. An enormous purse-seine net, deployed in a wide arc on either side of the vessel, encircles the school of tuna that come to shelter under the FAD. The lower part of the net is tightened, enclosing the fish in a hemisphere large enough to entrap a mass of tuna.

A sudden growth in the size of tropical tuna catches taken from under these artificial drifting objects was observed for the early 1990s. This was true especially for juveniles. Between 1996 and 2005 the average annual catch taken on FADs reached 1 115 000 tonnes, nearly a third of the global figure for tuna, all species considered together. In Japan, the fish processing industry furthermore had long reported that the flesh from floating-object associated tuna was less plump than that of specimens caught from free schools. This prompted an IRD research team to investigate whether or not the practice of drifting FAD fishing could set up an ecological trap for the tropical tuna species.This trap concept is a notion from population biology used to describe situations in which the population falls following a sudden change in its environment, most often linked to human activity. An example is give by marine turtles which, after hatching on beaches, use the sparkle of moonlight on the sea surface to guide themselves back to the ocean. However, high light pollution levels on urbanized coastlines in certain regions disturbs their sense of direction. Young turtles therefore set off on a path that leads them to land, where they die from dehydration.

Over the past ten years, over 30% of world
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