FAD: Fish Aggregating Device or Fish Attracting Device? A new analysis of yellowfin tuna movements around floating objects

Tropical tuna are known to associate with floating objects. Because fish are easier to detect and to catch when around these objects, fishermen have extensively deployed a large number of artificial floating objects in the tropical oceans. Although such objects are referred to as Fish Aggregating Devices (FADs), there is yet no strong evidence that fish do show an aggregative behaviour around them. The high probability of finding tuna around FADs may be the result of an aggregation process (high density of fish because fish stay for a long time around FADs) as well as an attraction process (high flow of fish through FADs). We analysed the movements of 14 yellowfin tuna, Thunnus albacares, in relation to FADs moored in the Indian and Pacific Oceans (corresponding to all the published tracking data) to determine whether the observed movement patterns resulted from an aggregation or an attraction process. Tuna appeared to be attracted by FADs. In general, they did not stay for long close to the FADs reached, and so did not aggregate there. Some FADs may nevertheless act as Fish Aggregating Devices. The possible reasons why tuna associate with floating objects are discussed in the light of these results.