Behaviour of tuna and non-tuna species at fish aggregating devices (fads), ascertained through fishers' echo-sounder buoysimplications for conservation and management

Résumé

Thousands of drifting fish aggregating devices (FADs) are built, deployed, and actively monitored by tropical tuna purse seiners globally, most of them equipped with echo-sounder buoys, which provide fishers with accurate geolocation information and remote estimates of fish biomass. Despite FADs attract tuna species, and thus facilitate their catch, they also serve as aggregation sites for non-tuna species. However, the reasons behind this associative behaviour remain largely unknown. Do environmental factors determine the presence and biomass density of species at FADs? Are fish fine and mesoscale behaviours species-specific? Works using electronic tags have investigated the individual behaviour of fish at FADs, providing insights on the homing behaviour of species and information on their residence-absence times and excursion rates, among others. However, these studies do not provide information on the behaviour of the entire aggregation as they are based on the tagging of a limited number of individuals, hindering inference on collective behaviour patterns and dynamics of the aggregations. In addition to providing interesting information for fishing purposes, buoys can also collect large quantities of data that could be used in scientific studies. However, up to date, the data generated by these buoys has not yet been used by scientists. This thesis presents for the first time scientific research results derived from fishers¿ echo-sounder buoys, in particular on the key factors driving the dynamics and behaviour of fish aggregations at FADs at different spatial and temporal scales. The present works also developed a behaviour based post-processing method to be applied on the echo-sounder buoy data and provides information on the effect of the new technologies, and particularly echo-sounder buoys, on fleets¿ behaviour and fishing strategy.