Study On The Dynamics Of Pelagic Ecosystem In Western And Central Tropical Pacific Based On LeMaRns Model

With the increasing globalization and exploitation of marine fish stocks,marine biodiversity has been severely damaged.The sustainable development of fisheries has become a hot central issue in the exploitation of fishery resources and marine environmental protection.The shift from the traditional single-species management model to the ecosystem-based fisheries management（EBFM）is a new trend in the development of fisheries management.Among them,ecological indicators are measurable characteristics of ecosystem composition,structure and function.They are the basis for monitoring changes in the state of ecosystems and evaluating related ecological and environmental policies,as well as an important link between ecological science and decision-making management.Fishery resource assessment models is powerful tools in ecosystem-based fisheries management.By applying ecological indicators to the assessment model,information can be conveyed to the outside world.We can better understand the state of the ecosystem or predict future changes,and realize the coordination of resource development and ecological conservation.The western and central tropical Pacific ocean supports many productive commercial fisheries,and is also the main tuna fishing area in the world.However,fishery management in the region is currently in traditional single-species models,targeting maximum sustainable yield（MSY）.Compared with the offshore,there is still very little research on the western and central tropical Pacific ocean.Based on the samples collected by China’s tuna longline observers in the tropical waters of the central and western Pacific Ocean from June to November,2017,previous authors selected key species in this area based on topology.according to the catch,biomass,fishing mortality and the corresponding biological parameters of species,LeMaRns,an ecosystem model based on body length structure,was established to simulate and analyze the effects of tuna fishery on common species and ecosystem indicators.The specific study contents and results are as follows:Five fishing strategies,including F=0（i.e.no fishing activity）,F=F_msy（i.e.fishing mortality at maximum sustainable yield）,F=F_recent（i.e.average fishing mortality in recent years）,F=0.8F_msy,and F=1.2F_msy,were designed to simulate fishing for 10species and make 10-year projections.The results showed that the biomass of the species basically tended to be in equilibrium at F=0.Under fishing pressure,the biomass of bigeye tuna,striped marlin,oceanic whitetip shark,silky shark,swordfish,yellowfin tuna,blue marlin,skipjack tuna and albacore decreased by 0.26%-99.74%,and the spawning stock biomass decreased by 0.26%-99.75%;the biomass of shortfin mako shark,Indo-Pacific sailfish,and black marlin increased by 0.01%-12.51%,and the spawning stock biomass increased by 0.01%-10.23%;the biomass of blue shark increased by 36.74%,and the spawning stock biomass decreased by 60%.The resource status of the remaining species remained almost unchanged.The biomass of species varied significantly under different fishing strategies.Under the fishing scenario based on Frecent,species resources within the ecosystem remain largely stable over the next10 years.The results of the study indicate that the current fishing intensity of the main fish species in this ecosystem is at a sustainable level.Meanwhile,the effects of five fishing strategies on four ecosystem indicators of Large fish indicators（LFI）,Mean maximum length（MML）,typical length（Ty L）and length quantile（LQ）were analyzed.In the first six years of simulated fishing,MML,Ty L,and LQ increased with increase of fishing effort,and then decreased;LFI always increased with increase of fishing effort.The results shows that fishing fundamentally affects the size and composition of fish community.The overall abundance of the ecosystem is not only affected by fishing intensity,but also affected by factors such as interspecific competitive predation or natural mortality.By comparing the time series of four indicators under fishing,the fluctuation trends of LFI and Ty L are the most similar.The results confirmed that Ty L can provide information equivalent to LFI under the fishing strategies,thus Ty L can completely replace LFI as the optimal indicator for evaluating the simulated community.The study showed that the LeMaRns model can be used to analyze large ecosystems such as the western and central tropical Pacific Ocean,providing a potential ecosystem-level approach for fisheries management.The results of this study also have implications for the current fisheries management,and provide a basis for further research on the ecosystem-based tuna fishery management in this area.