Assessment Of Ecological Carrying Capacity Of Silver And Bighead Carp Based On Ecopath Model And Its Relation To The Resources Of Coilia Nasus In Taihu Lake

As the third largest freshwater lake in China,Taihu Lake is rich in fishery resources.In recent years,pollution has entered Taihu Lake through various forms,and the problems of unreasonable structure of aquatic resources,decreasing biodiversity and increasingly fragile ecosystem in Taihu Lake have become increasingly prominent,and the fish in Taihu Lake have become mono-and miniaturized.In this study,we studied the utilization status of Coilia nasus resources through survey data of previous years,and conducted Ecopath modeling of Taihu Lake using data from 2017-2018 and reviewing relevant literature,assessed the nutrient structure and energy characteristics of Taihu Lake ecosystem,evaluated the ecological capacity of the released species of Silver and bighead carp and the impact on the ecosystem when the ecological capacity was reached,and analyzed the impact of the scale of Silver and bighead carp on the ecological capacity of Silver and bighead carp.This study aims to understand the ecosystem characteristics of Taihu Lake and the ecological capacity of Silver and bighead carp at the ecosystem level,and to provide a theoretical basis for the sustainable development of Taihu Lake fisheries.The main results of this study are as follows:1.Analyzing the data on the body length of C.nasus in seven years,the average catch length of C.nasus in each year was lower than the optimal one.In terms of fishing pressure,F/M values in other years were greater than the optimal fishing pressure except in 2016 when F=M was close to the optimal fishing pressure.In terms of relative resource quantity,the relative resource quantity B/B₀of C.nasus in Taihu Lake is higher than the optimal relative resource quantity in other years except 2012 and 2020,which is far higher than the warning line of 0.5B_msy.Therefore,the resource quantity of C.nasus in Taihu Lake is relatively rich.Fisat II software was used to estimate the resource quantity of C.nasus was 33,737.44 t,with a large resource quantity,and the total mortality coefficient Z was 1.48.2.The trophic level of the ecosystem of Taihu Lake ranged from 1～3.973,with a high trophic level for carnivorous fish and a trophic level of 2.051 for Silver and bighead carp.The changes of Sliver and bighead carp biomass were more sensitive to the EE value of phytoplankton（-8.84%～8.84%）and also affected the EE value of zooplankton（-0.80%～0.80%）,and the changes of the biomass of C.nasus were the most sensitive to the EE value of zooplankton（-35.52%～35.52%）.The current sum of all respiration is799.961 t/（km²·a）;the sum of all consumption is 2320.373 t/（km²·a）;the sum of all exports is 1238.119 t/（km²·a）and sum of all flows to detritus is 3029.265 t/（km²·a）,and the total ecosystem throughput is 7387.718 t/（km²·a）.The mean trophic level of catch in Taihu Lake is 2.603,CI is 0.210,SOI is 0.067,the FCI is 21.65,the average Finn path length reaches 3.625,and TPP/TR is 2.548.The energy flow in Taihu Lake ecosystem.There are two main pathways:one is the detritus food chain:detritus benthic and zooplankton-phytoplankton and benthic feeding organisms-large carnivorous fish and fishing;the other is the grazing food chain:phytoplankton-zooplankton and zooplankton feeding fish-small fish carnivorous fish and fishing;the aquatic plant nutrient flow is an important branch of the Taihu Lake ecosystem.The transfer efficiency of trophic levels II-III-IV-V-VI are 10.19%,9.76%,12.91%,18.81%respectively;the efficiency from primary producer stream to each trophic level is 11.24%,9.78%,12.99%,18.83%;the transfer efficiency from detritus stream to each trophic level is 10.03%,9.75%,12.92%,18.81%.12.92%,and 18.81%.The total transport efficiency of the system was 10.87%.The total flows of detritus into the system was 3029 t/（km²·a）,accounting for 41%of the total system flow,which exceeded the production of primary producers（2401 t/（km²·a））,the amount of detritus group re-entering the system was 1820 t/（km²·a）,and the rest of the debris was removed from the system due to mineralization deposition.3.The existing stock of Sliver and bighead carp in Taihu Lake is 10.21 t/km²,the biomass of Sliver and bighead carp can increase to 5.054 times to 51.6 t/km².The trophic level of Taihu fishery catch decreased gradually with the increase of Sliver and bighead carp biomass from 2.603 to 2.23 when Sliver and bighead carp reached its ecological capacity,and the value was 2.23 when the maturity of Taihu Lake ecosystem reached its ecological capacity,CI and SOI reflected the degree of energy recycling,and both values remained stable throughout,FCI and FMPL remained stable during the increase of Sliver and bighead carp,and when Sliver and bighead carp reached its ecological capacity,the inhibition of phytoplankton increased further.The inhibition of phytoplankton increased after the ecological capacity was reached.The EE value of zooplankton increased to a critical value of 1,the EE value of phytoplankton increased from 0.13 to 0.23,and the EE value of organic detritus increased from 0.60 to 0.63.When the ecological capacity of Silver and bighead carp was reached,there was a trend of increasing transfer efficiency,from 11.24%to 14.18%for trophic level II from producers;and from 9.78%to 10.15%for trophic level III.The total transport efficiency from primary producers also increased from 11.26%to 12.32%;the transport efficiency of trophic level II from detritus increased from 10.03%to 10.51%;the transport efficiency of trophic level III increased from 9.75%to 10.14%.The total transport efficiency from primary producers increased from 10.81%to 11.12%;the total transport efficiency of Taihu Lake increased from 10.87%to 11.38%.4.In this model,when the biomass of C.nasus is reduced to 9.49 t/km² and the biomass of Silver and bighead reaches the maximum ecological capacity 206.4 t/km²,the sum of all respiration of the ecosystem is 1317.291 t/（km²·a）;The sum of all consumption is 3205.641 t/（km²·a）;The sum of all exports is 720.79 t/（km²·a）;Flows to debritus is 2349.016 t/（km²·a）;the total ecosystem throughput is 7592.737 t/（km²·a）.The average trophic level of Taihu Lake catch was 2.115,CI and SOI values always remained stable,and FCI and FMPL maintained a small increase during the increase the biomass of Sliver and bighead carp.