Dynamic modeling of the pelagic ecosystem in the western and central Pacific Ocean based on a multispecies size-spectrum model

Ecosystem models have been increasingly used for assessing and predicting the impact of fishing activities on marine ecosystems. The western and central Pacific Ocean (WCPO) is the most productive oceanic region for tunas, and its ecosystem stability is crucial for sustaining global tuna production. As a medium-complexity ecosystem model, the size-spectrum model can incorporate more ecological processes with less data, making it suitable for analyzing the dynamics of the WCPO pelagic ecosystem where data are scarce. In this study, we developed and calibrated a multispecies size-spectrum model for the pelagic ecosystem of the WCPO based on publicly available fisheries statistics and Chinese tuna longline observer data. Six ecological indicators were used to reflect the dynamic changes in ecosystem structure, including total biomass, large fish index (LFI), mean maximum weight, maximum weight, mean trophic level (MTL), and slope of community size-spectrum. Additionally, we conducted a projection of species biomass and ecosystem structure over the next decade under different fishing scenarios: 0, 0.2 F, 0.5 F, F (average fishing mortality coefficient from 2018 to 2021), 2 F, and 5 F. The validation results indicated that for most species, there was a strong consistency between the catches predicted by the calibrated model and the reported catches. Projections indicated that even if fishing intensity remains at recent levels (F) or is reduced (0.2 F and 0.5 F), the biomass of tuna species(albacore, yellowfin and bigeye tuna) other than skipjack tuna will continue to decline to varying degrees. With regard to ecological indicators, a clear inverse correlation was observed between the increase in fishing intensity and six ecological indicators, especially the slope of community size-spectrum. Our results suggested that conservation and management measures will need to be continually adjusted in future fisheries management to ensure ecosystem stability and sustainable development of fishery resources. The results of our study can serve as a scientific reference for conducting ecological impact assessments of tuna fisheries in the WCPO, as well as for promoting the sustainable exploitation of tuna stocks.