Assessing the response of Indian Ocean yellowfin tuna (Thunnus albacares) stock to variations in DFAD fishing effort

The Indian Ocean Tuna Commission (IOTC) and the regional Indian Ocean stakeholders have noted concern over the extensive use of DFAD fishing (drifting Fishing Aggregation Devices) by the industrial purse seine fleets. One of the major concerns is the increased fishing mortality of juvenile yellowfin tuna (Thunnus albacares) due to DFAD fishing. In 2020, sustainable biomass levels were exceeded by ~27-32%, propelling long-term declines in the stock. Furthermore, with the current stock status subject to overfishing and overfished there is potentially a threat to the global supply chains and employment. In this paper, we perform medium-term deterministic projections for the yellowfin tuna stock in the Indian Ocean considering four scenarios for the industrial purse seine fishing effort compared to the reference or base case relating to 2020 effort levels (scenario 1): a 50% reduction of DFAD sets without re-allocation of effort on free school sets (scenario 2a), a 50% reduction of instrumented buoys deployed in the water (scenario 2b), a seasonal closure of DFAD fishing during the third quarter of the year (scenario 3), and finally an extreme case called DFAD-free, i.e. zero DFAD sets all year (scenario 4). We indicate the potential impacts and benefits of fishing at the same level as that in 2020, projected over 10 years. We show that the 3^rd quarter temporal DFAD closure (e.g., IOTC Resolution 23/02 - zero sets on FADs in Q3) is the most beneficial of the scenarios (excluding the full cessation of DFADs all year round). The 3^rd quarter temporal DFAD closure performance projected future increases in spawning stock biomass (SSB) of between ~12-14%, catch increases of ~5-7%, and recruitment increases of ~2-4% relative to fishing at the same level as that in 2020. In contrast, the scenarios 2a resulted in ~6-7% loss of SSB, and between ~1-2% loss in recruitment numbers. Further research in the area of input-based measures is necessary given the ever-increasing levels of purse seine efficiency to the stocks biomass targets. A Management Strategy Evaluation (MSE) framework for testing alternative candidate harvest control rules (HCR) would possibly be the best approach to develop harvesting strategies given the uncertainties associated with DFAD year on year fishing efficiency increases, and reaching future desired biomass targets for sustainable Indian Ocean yellowfin stocks.

Therefore, we invite the IOTC to:

• Note the importance and implications of this research to reach or maintain stocks relative to biomass targets.
• Note the importance of gaining a better understanding of DFAD instrumented buoy deployments versus the number of DFAD sets and thus catch rates by acquiring more detailed data on the number of instrumented buoy deployments by all PS fleets.