Catch and by-catch analyses of tropical tuna fishery in the Gulf of Guinea from 2010 to 2012.

Canned tuna is one of the most widespread and recognizable fish commodities in the world. Over all oceans 80% of the total tuna catches are caught by purse seine fishery and in tropical waters their target species are: yellowfin (Thunnus albacares), bigeye (Thunnus obesus) and skipjack (Katsuwonus pelamis). Even if this fishing gear is claimed to be very selective, there are high levels of by-catch especially when operating under Fish Aggregating Devices (FADs). The main problem is underestimation of by-catch data. In order to solve this problem the scientific community has developed many specific programs (e.g. Observe Program) to collect data about both target species and by-catch with observers onboard. The purposes of this study are to estimate the quantity and composition of target species and by-catch by tuna purse seiner fishery operating in tropical waters and to underline a possible seasonal variability in the by-catch ratio (tunas versus by-catch). Data were collected with the French scientific program ”Observe” on board of the French tuna purse seiner “Via Avenir” during a fishing trip in the Gulf of Guinea (C-E Atlantic) from August to September 2012. Furthermore some by-catch specimens have been sampled to obtain more information about size class composition. In order to achieve those purposes we have shared our data with the French Institute of Research for the Development (IRD), which has data collected by observers onboard in the same study area. Yellowfin tuna results to be the main specie caught in all trips considered (around 71% of the total catches) especially on free swimming schools (FSC) sets. Instead skipjack tuna is the main specie caught under FADs. Different percentages of by-catch with the two fishing modes are observed: the by-catch incidence is higher on FADs sets (96.5% of total by-catch) than on FSC sets (3.5%) and the main category of by-catch is little-tuna (73%). When pooling data for both fishing sets used in purse seine fishery the overall by-catch/catch ratio is 5%, a lower level than in other fishing gears like long-lining and trawling.