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Skipjack (Katsuwonuspelamis) fishery improvement project: From satellite and 3D oceanographic models to acoustics, towards predator-prey landscapes

Tuna are an important source of food and protein. Among this group of fishes, seven species represent 90% of the global catches and the main species is the skipjack (Katsuwonus pelamis) with a total production of over three million ton. We worked on a Skipjack Fishery Improvement Project in the Brazilian Coast in order to increase efficiency of the pole and line catch method, considered a sustainable mode to catch these fishes. We started with satellite images and 3D models to identity more productive fishing grounds, associated to enrichment oceanographic processes. At the same time, we recorded environmental conditions at the position of 3,410 catch records and learned more about skipjack preferences in order to improve further our searching skills. After a long series of at lab observation, we participated on a fishing cruise when it was possible to observe skipjack catches and echo records. Due to the strength of these echo records and because skipjack is a bladderless fish we investigated stomach content of fresh fish, full of fresh prey. Skipjack stomachs contained large amounts of small lantern fish and euphausiids. Lantern fishes have swimbladers and euphausiids have oil droplets as energy reserves. We then suggest that skipjack target strength may be different depending on the stage of stomach fullness. Our previous experience in the skipjack distribution area, when we could use more sophisticated echosounders than the one we had on board the fishing vessel, showed that lantern fish and euphausiids can be quite easily detected. Modern multi-frequencies echosounders like SIMRAD EK 80 generate digital water column echograms as well as virtual echograms. If we can identify predator and/or prey acoustically it is possible to generate predator-prey landscapes along the surveyed areas. Virtual echograms with ideal frequencies to each type of target may also generate virtual predator prey landscapes. Monitoring more efficiently predator and prey patches may improve catch efficiency and save fossil fuel reducing CO2 emissions.