Role of tunas in the Western and Central Pacific Ocean: Evidence from a mass-balanced model

Tunas represent one of the most economically and ecologically significant marine taxa worldwide. The Western and Central Pacific Fisheries Commission (WCPFC) accounts for approximately 58% of global tuna harvests annually. Using data from the 2023 High-Seas Fishery Resources Survey in the Western and Central Pacific Ocean (WCPO), the role of tuna species in ecosystem dynamics was assessed through the Ecopath with Ecosim modeling framework. Eighteen functional groups were delineated based on shared ecological characteristics. Biomass estimates derived from in situ observations were complemented with literature-sourced parameters to maximize model robustness. Monte Carlo simulations were employed to analyze the impact of input uncertainty on key model-derived indices. Modeled trophic levels ranged from 1.00 to 4.50. A comparative analysis with 2007 baseline data revealed a decline in trophic positions for tunas and other apex taxa such as sharks, swordfish, and pelagic stingrays. Tuna species exhibited the highest keystone index and relative total impact, underscoring their central structuring role in pelagic food webs. Mixed trophic impact analysis identified widespread negative interactions exerted by tunas, especially on larval fish populations. The WCPO system demonstrated a total system throughput (TST) of 6 912.24 t·km⁻²·year⁻¹, substantially exceeding the 2002 estimate of 4 846.58 t/km²/year. The low system omnivory index (0.27), these overall ecosystem characteristics suggested that the WCPO ecosystem is currently disturbed to a low extent and lead to a relatively stable system compare with other studies. Sensitivity analysis highlighted strong model dependence on biomass and production-to-biomass (P/B) ratios across functional groups. These findings refine current understanding of pelagic food web architecture in the WCPO and provide a quantitative foundation for implementing ecosystem-based fishery management strategies in high-seas environments.