Fuel consumption of free-swimming school versus FAD strategies in tropical tuna purse seine fishing

Different fishing strategies have been adopted in the last decades by tropical tuna purse seiners fleet, including fish aggregating device (FAD) and free-swimming school (FSC) fishing strategies, which has raised issues about the different carbon footprint of those fishing modes. Here we show the activity and energy patterns of a Spanish tuna purse seiner operating in the tropical Indian Ocean, based on the monitoring of energy consumption over 10 consecutive fishing trips; and we also assess the fuel use intensity of FAD versus FSC fishing by analysing 14 further trips of different tropical tuna purse seiners. The average time of a fishing trip lasted 33.1±11 days. The dominant activity during the fishing trip was cruising (with 68.5% of the time), followed by inactive period at sea (15.6%), fishing (7.7%), and in port (8.1%). The vessel consumed 381±113 t fuel/trip, of which 90.4% was spent in cruising, 4.3% in fishing, 3.7% during the inactive period at night and 1.6% while staying in port. The main engine consumed 75% of the total fuel, while the auxiliary engines the remaining 25%. Furthermore, our results demonstrated that FAD fishing (543.6 L/t) is more fuel intensive, than FSC fishing (439.4 L/t). However, FADs fishing successful rates are higher, around 95.7±3.8%, than FSC rates (around 80.6%±5.8). It is worth noting that the differences may be driven by seasonality and FSC availability, number of FADs in an area, vessel characteristics and equipment, and skipper skills rather than the adopted fishing strategy. Nonetheless both FAD and FSC fishing are more energy efficient than longline (1069 L/t), trolling (1107 L/t), or pole and line (1490 L/t) fisheries for Atlantic tuna, but similar or slightly less efficient than Maldivian pole and liners.