Balancing fishing effort along the tropical tuna abundance-size spectrum

The ecosystem approach to fisheries is widely recognised as a key management goal, yet its definition and implementation remain debated. Most fisheries management relies on single-species strategies with technical measures to reduce bycatch. However, selective removals disrupt species composition, affecting ecosystem dynamics and resilience. We present a proof-of-concept model based on balanced harvesting that allocates fishing pressure proportionally across three tuna stocks—yellowfin (Thunnus albacares), skipjack (Katsuwonus pelamis), and bigeye tuna (Thunnus obesus)—in the Indian Ocean according to their size-biomass ratios. The model optimises fishing effort by gear using a predefined objective function based on length-based population dynamics, ensuring a balanced harvest while maintaining each tuna species' biomass within its maximum sustainable yield (BMSY) limit. By assigning fishing mortality (F-multiplier) to each fleet, the model aims to maintain, within the bounds of BMSY for each stock, the ecosystem structure (based on size-abundance relationships) over a 20-year simulation. Results indicate significant reductions in fishing mortality across gears relative to 2020 levels. While some gears, such as purse seine free-school, show increased catches and revenues (146%), others, like purse seine log-school, experience declines (-22%). Overall, fishing at BMSY improves total revenues and catches by 51% and 34%, respectively, compared to 2020. This work demonstrates that it is possible to maintain each tuna stock within BMSY bounds by managing fishing fleets while preserving ecosystem structure, a significant goal of the ecosystem approach to fisheries.