Balancing fishing effort along the tropical tuna abundance-size spectrum

• Ecosystem-based model balances harvest across three Indian Ocean tuna stocks. • Fishing effort is optimised by gear to stay within species-specific B MSY limits. • Model maintains size structure using length-based dynamics and F-multipliers. • Balanced harvest raises total catches (+34 %) and revenues (+51 %) vs. 2020. • Some fleets benefit (free-school PS +146 %), others decline (log-school PS −22 %). 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 (B MSY ) limit. By assigning fishing mortality (F-multiplier) to each fleet, the model aims to maintain, within the bounds of B MSY 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 B MSY 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 B MSY bounds by managing fishing fleets while preserving ecosystem structure, a significant goal of the ecosystem approach to fisheries.