Analysis of pelagic ecosystem dynamics in the western and central tropical Pacific based on the LeMaRns model

The western and central tropical Pacific Ocean is one of the most productive marine ecosystems and a principal tuna fishing area globally. To investigate the effects of fishing on the marine ecosystem's structure in this region, LeMaRns model, an ecosystem model based on body length structure, was developed. It simulates and analyzes the impacts of tuna fisheries on key species and ecosystem indicators. The model evaluated five fishing strategies, including F=0 (i.e. no fishing activity), F=F_msy (fishing mortality at maximum sustainable yield), F=F_recent (average fishing mortality in recent years), F=0.8F_msy, and F=1.2F_msy, with projections for 10 years on 10 species. The study analyzed the effects of these strategies on two single species indicators of biomass (B), spawning stock biomass (SSB) and four ecosystem indicators of Large fish indicators (LFI), Mean maximum length (MML), typical length (TyL) and length quantile (LQ). Findings indicate that the biomass of the species generally reached equilibrium at F=0. Under fishing pressure, the biomass of tuna species such as bigeye tuna, swordfish, yellowfin tuna, skipjack tuna and albacore decreased by 0.26% to 99.75%, while that of shortfin mako shark, Indo-Pacific sailfish, and black marlin increased by 0.01% to 12.51%. The biomass of blue shark increased by 36.74%, and the spawning stock biomass decreased by 60%. Other species' resource status remained largely unchanged. Over the first six years of simulated fishing, MML, TyL, and LQ initially increased with fishing effort and then decreased; LFI consistently increased. The study suggests that fishing will exert minimal impact on the ocean's ecological structure over the next decade. The LeMaRns model offers a valuable ecosystem-level approach for fisheries management in large marine areas like the western and central tropical Pacific Ocean. with implications for current management practices.