Impact Of Tuna Fisheries On Indian Ocean Pelagic Ecosystem Based On LeMaRns Model

The concept of Ecosystem based fisheries management（EBFM）is increasingly accepted by researchers and international organizations as the role of marine ecosystem studies in fisheries management is becoming more and more recognized.The Indian Ocean is one of the important fishing areas for Chinese tuna fisheries.The Indian Ocean is one of the important fishing areas for Chinese tuna fisheries.At the 14th meeting of the Working Party on Ecosystems and Bycatch（WPEB）of the Indian Ocean Tuna Commission（IOTC）,information was provided on the preliminary methodology of EBFM,which specifically divides the study of ecosystems into several parts.However,due to the short time of implementation and the impact of the new crown epidemic in the last two years,no practical application of EBFM has been carried out for studying the Indian Ocean ecosystem.Based on the application case of LeMaRns（A Length-based Multi-species analysis by numerical simulation in R）model in the North Sea,this paper is the first attempt to construct LeMaRns,a multi-species ecosystem model based on the length of fish body structure of 19 major species in the Indian Ocean tuna fishery,to study the effects of different fisheries on the Indian Ocean ecosystem at different fishing levels were investigated,with a view to providing a reference for the management of tuna fisheries in the Indian Ocean and providing ideas for further research on the assessment of multi-species fisheries.This paper assumed that there were four types of fisheries in the Indian Ocean:longline（LL）,gillnet（GN）,purse seine（PS）and other（OT）.The effects of fishing on the ecosystem were studied in terms of different fishing scenarios and dynamic fishing,respectively.Firstly,a full factorial experimental design was used,assumed four fisheries at 10fishing effort levels(0,0.2,0.4,0.6,0.8,1,1.2,1.4,1.7,2×E_2010-2019)for 50 years of fisheries.The results showed that（1）fishing led to a decline in stock biomass and had a negative impact on ecosystem structure,but different fisheries had different effects on different stocks.In the fisheries management,the impact of different fishing levels on the ecosystem should be considered according to different fisheries;（2）longline has the greatest impact on Large Fish Indicator（LFI）and Mean Maximum Length（MML）,indicated that it has a great impact on the structure of the ecosystem,and in the fisheries;（3）the population collapse risk indicator is 0,which means that no population is in a state of collapse and the ecosystem is relatively stable.Skipjack tuna（Katsuwonus pelamis）,albacore tuna（Thunnus alalunga）,yellowfin tuna（Thunnus albacares）,bigeye tuna（Thunnus obesus）,flatfin sailfish（Istiophorus platypterus）,and Indian marlin（Istiompax indica）had relative stock biomass between 0.6～0.7,0.6～0.7,0.7～0.8,0.6～0.7,0.5～0.6 and 0.5～0.6,respectively,assuming twice the current fishing effort,with skipjack tuna and albacore tuna doubling their stock biomass in the absence of fishing,yellowfin tuna and bigeye tuna had 1.4 and 1.5 times the current stock biomass,respectively,and flatfin sailfish and Indian marlin had 1.9～2 times the current stock biomass,this index can detect the change of stock resource and has some guidance for the recovery of the collapsed stock.Secondly,a time-varying fishing effort was assumed for each fisheries for 20 years of dynamic fishing.The results showed that:（1）the trend of the total catch of each fisheries and the catch of single species over time were similar,the catch of longline reached the highest when the fishing effort increased,indicating that the fishing intensity had exceeded the regeneration capacity of the stock resource at this time,the total catch of gillnet and the catch and biomass of flatfish and Indian marlin did not change much before and after fishing,indicating that the ststus of stock of flatfish and Indian marlin are stable.The total catch of purse seine and the catches of skipjack tuna and yellowfin tuna have been increasing with time,but the slope of the catch curve is slowly decreasing,indicating that the fishing pressure is increasing;（2）the change trend of LFI and MML is the same,both are decreasing and then increasing with time;LFI decreases from 0.27～0.28 to 0.23～0.24,and then increases to 0.27～0.28.to between0.27～0.28,overall,the final value is slightly smaller than the initial value;MML decreased from 121～122 cm to between112～113 cm,and then increased to between120～121 cm,overall,the final value is about 1 cm lower than the initial value.The trend of the two indicators and the trend of fishing effort over time for the longline were opposite,indicating that the longline had the greatest effect on the two indicators.（3）The relative stock biomass of most species was around 1,while the relative stock biomass of Japanese mackerel,skipjack tuna,yellowfin tuna and striped tetrasail sailfish showed greater variation.The decrease in the relative stock biomass of skipjack tuna and yellowfin tuna was due to ongoing fishing by purse seine,while the increase in the relative stock biomass of Japanese mackerel and striped tetras was probably due to the effect of interspecific interactions.The results indicate that fishing has a negative impact on the ecosystem,but different fisheries have different impacts on the ecosystem at different levels of fishing;fishing can alter species interactions,further altering the resources of various stock,and species interactions are very common and complex,suggesting that IOTC should accelerate its research on multi-species reference points or ecosystem indicators research;LeMaRns model is feasible to be applied to pelagic ecosystems,but it needs to rely on certain assumptions.Finally,based on the problems found during the study,some shortcomings of this study are discussed and several suggestions are made for further research.