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|Title:||Preferred habitat of tropical tuna species in the Eastern Atlantic and Western Indian Oceans: a comparative analysis between FAD-associated and free-swimming schools|
|Authors:||DRUON Jean-Noel; CHASSOT Emmanuel; FLOCH Laurent; MAUFROY Alexandra|
|Publisher:||Indian Ocean Tuna Commission (IOTC)|
|Type:||Articles in periodicals and books|
|Abstract:||An ecological niche modelling (ENM) approach was developed to describe the suitable habitat of skipjack (SKJ) and juvenile yellowfin (YFT) tuna in the Tropical Atlantic and West Indian Oceans. The environmental envelop of the potential habitat in each ocean was defined using occurrence data independently of the fishing mode and derived from purse seine fishing sets of the French fleet during 1997-2014. Daily satellite-derived chlorophyll-a content (CHL) and fronts (CHL gradient) were used as a proxy for food availability while circulation model derived-sea surface temperature, salinity, height anomaly, current and oxygen as well as the mixed layer depth contributed to identify the physical suitable conditions of each species. Only the cluster that showed no CHL front was excluded for the parameterization in order to enhance the favourable feeding habitat. In a second step, the distances of both the free swimming schools (FSC) and schools associated with drifting Fishing Aggregating Devices (FADs) to the closest potential habitat were computed and compared. The results highlighted (i) high spatial seasonality of both the simulated feeding habitat and tuna populations in the Indian Ocean compared to the tropical Atlantic, (ii) major differences between both oceans regarding the distance of FAD catches to the potential habitat with median values above 200 km in the Atlantic and below 16 km in the Indian Ocean, while equivalent distances for FSC were observed for both species and areas (below 2 km and 43 km respectively) in agreement with stomach content analysis, (iii) an increased rate of FAD fishing operations in the decade from 2003 to 2013 (up to about 70% in the Atlantic and 96% in the Indian Ocean) occurring mostly in poor environments in the tropical Atlantic while frequently in relatively productive waters in the Indian Ocean (except east of 58°N) as well as an overall 300% increase of juvenile YFT presence in both ocean sets and (iv) a recent intensification of fishing effort from March to May in the Mozambique Channel in agreement with an increase of favourable habitat, while no effort of that fleet occurred in the open waters off the Gabon upwelling (from 1°N to 5°S and East of 17°W) from May to September where favourable habitat was enhanced by the model. In all cases the seasonal maximum number of fishing sets corresponded to the minimum extent of potential habitat, which commonly varied by 30% from year to year. Overall, this comparative analysis emphasizes the strong attraction of tropical tuna species to floating objects although feeding opportunities may vary considerably depending on hydrographic regimes and on the dynamics of productive habitats.|
|JRC Directorate:||Space, Security and Migration|
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