Fisheries management works: Indian Ocean yellowfin tuna considered healthy in latest stock assessment
The Indian Ocean Tuna Commission (IOTC) Working Party on Tropical Tunas concluded that yellowfin stocks in the Indian Ocean are under sustainable exploitation with an 89% probability
Contributing over 22% global tuna catches, the Indian Ocean contains some of the most productive stocks in the world. While skipjack tuna (Katsuwonus pelamis) is the most caught species in this ocean, yellowfin tuna (Thunnus albacares) is not far behind, making up over 30% of the total catch volumes. Given the stock’s critical importance to the economy of the region, its sustainable exploitation has been a concern of managers since 2015, when it was determined to be overfished, leading to the design of a strict recovery plan. Now it seems that years of regulation may have given results, as the latest yellowfin tuna stock assessment, presented during the 26th session of the IOTC Working Party on Tropical Tunas last November, showed that the stock is being fished at sustainable levels, and no longer overfished.
Traditionally, the stock assessment of yellowfin tuna in the Indian Ocean has been particularly challenging due to a lack of accurate and representative data with which to develop models. However, the inclusion of new data from other fleets, such as longline fisheries, and more knowledge on the biological characteristics of the species, such as growth, mortality and maturity, have provided scientists with a new model, named Stock Synthesis III (SS3) for providing scientific advice for the main tropical tuna species in this ocean.
The inclusion of this new data and more advanced models has led to a more optimistic outlook on the overall status of the stock compared to previous assessments. Notably, a significant increase in spawning stock biomass was observed since 2021, resulting in strong recruitment for the species and, subsequently, an increase in the projected total available biomass. The analysis further indicated that fishing mortality from 2021 to 2023 remained below the Maximum Sustainable Yield (MSY) with a reasonable margin. Overall, the new assessment concluded that yellowfin tuna stock in the Indian Ocean is under sustainable exploitation with an 89% probability.
However, challenges remain in regard to data collection, and researchers emphasized a significant uncertainty regarding catch data reported by some fisheries, particularly certain artisanal ones. This underlines the importance of improving data reporting across all fleets, but also the need to integrate fisheries independent data sources. Recently, innovative tools, such as abundance indexes derived from echo sounder buoys, are emerging as a promising strategy to improve the accuracy and representativeness of assessments. Data from these technologies allow scientists to estimate the relative biomass of tropical tunas across the entire ocean, independent of fishing effort. Integrating these data into assessment models could improve their accuracy by reflecting more comprehensive population dynamics, particularly for juveniles. This would not only help identify priority management areas, but also enable more informed decision-making to ensure the sustainability of tuna stocks.
In addition to these advancements in the yellowfin tuna stock assessment in the Indian Ocean, recent global evaluations have also yielded encouraging findings. The International Seafood Sustainability Foundation (ISSF) released its "Status of the World Fisheries for Tuna" report in November 2024, indicating that 88% of the world’s tuna catches came from stocks with a healthy level of abundance. These positive trends underscore the effectiveness of enhanced data collection, innovative assessment methodologies, and the adoption of sustainable fishing practices in advancing the long-term security of tuna fisheries worldwide.