Preferred feeding habitat of skipjack tuna in the eastern central Atlantic and western Indian Oceans: relations with carrying capacity and vulnerability to purse seine fishing

A single Ecological Niche model was developed for skipjack tuna (Katsuwonus pelamis) in the eastern central Atlantic Ocean (AO) and western Indian Ocean (IO) using an extensive set of precise spatial occurrence data from the European purse seine fleet during 1998-2014. Productive fronts of chlorophyll-a were used as proxy for food availability while mixed layer depth, sea surface temperature, oxygen concentration, salinity, current velocity and sea surface height anomaly were selected to define skipjack physical oceanographic preferences. The common environmental feeding niche identified for skipjack emphasized highly contrasted oceanographic regimes between oceans with seasonal occurrence of gyre-type productive features at mesoscale in the IO and large scale upwelling systems that seasonally shrink and swell in the AO. About 60% of free-school (FSC) sets and 46% of fishing aggregating device (FAD) sets were found within favourable feeding grounds for skipjack. About 34% of FAD sets in the AO were however found to occur at a distance further than 100 km from favourable feeding conditions, mostly in the poor environment of the Guinea Current, and 10% for the FAD sets observed in the IO, as compared to 8% for all FSC sets. The ecological role of the Guinea Current remains unclear as regards to feeding and spawning since this particularly poor environment is remote from upwelling-rich areas while skipjack is known to spawn nearby feeding grounds (income breeding strategy). The results also emphasized in the IO a higher exposure of schools to purse seiners in months where preferred feeding habitat is reduced which may result in a geographic concentration of skipjack populations at the habitat scale. Finally, the significant positive correlation observed between the annual size of favourable habitat for feeding, the annual nominal catch rates and the total catches of skipjack in the IO i) agrees with the near full exploitation of skipjack in the IO since the 2000s and in the recent years for the AO, and ii) suggests to interpret the size of favourable habitat for feeding as an indicator of carrying capacity of the environment to sustain populations of this fast-reproducing species.JRC.D.2-Water and Marine Resource