Evolution Of Ecosystem Structure And Function In The East China Sea In The Context Of Fisheries Policies And Climate Change

Fisheries policies and climate change are the stressors affecting the structural andfunctional changes of the marine ecosystem.Studying the responses of ecosystems under the two stressors can provide theoretical support for fishery management and resource recovery.The East China Sea（ECS）is one of the most important fishing areas carrying about 40%of the total catch in China.The ECS is shallow and influenced by the Kuroshio Current and the Changjiang diluted water.High primary productivity contributes to many important fishing grounds,such as Mindong,Zhoushan.However,Many traditional commercial fish stocks such as the large yellow croaker were exhausted,and the length of the fishing season has declined in some fishing grounds under the influence of many factors such as fishing and environmental changes.Here,we built two mass-balance models for 1997–2000（M1997）and 2018–2019（M2018）using Ecopath with Ecosim 6.6 to illustrate the interannual changes over the past two decades in the East China Sea（ECS）.We then simulated two dynamic scenarios from 1997 to 2018,SFM(M2018_SFM)and no SFM(M2018_no-SFM),to test the role of the SFM under fishing pressure in the ECS.We also explore the resource variations of important function groups in the ECS under different fishing pressure.Last but not least,we evaluated the impact of climate change and fishing in two future scenarios on the ECS ecosystem.The followings are the main conclusions for this study:1)The flow diagram for the ECS showed similar trophic levels（TLs）during the two periods,from 1.00–4.05 and 1.00–4.24 in M1997 and M2018,respectively.The mean TLs of caught fishes showed a slight increase,from 3.11 in M1997 to 3.33 in M2018.Transfer efficiency was 10.56%in the M2018 model,which was less than the M1997model（12.01%）,closing to adjacent systems such as the Southwest Yellow Sea and the Bohai Sea,but lower than the northern South China Sea,and the pattern probably related to the different energy transfer efficiencies among low trophic levels.The higher the energy utilization between lower trophic levels,the higher the energy transfer efficiency of the ecosystem.2)The ECS ecosystem was more mature and stable in M2018,as reflected by the higher total primary production/total respiration（TPP/TR）and SOI as well as by the lower Finn’s cycling index and mean path length（Table 2）.The TPP/TR declined from4.89 to 2.74 between M1997 and M2018.The Flow to Detritus/TST was 33.33%in M2018 and 40.36%in M1997,indicating that more energy flowed into production rather than detritus.Ascendency decreased from 40.54%to 35.65%from 1997 to 2018,and overhead increased from 59.46%to 64.26%,suggesting that the ECS ecosystem was more robust to resist external disturbance.In addition,the Finn’s cycling index values approximately doubled from 1997 to 2018,and there was a slight increase in mean path length（from 2.31 in M1997 to 2.50 in M2018）.This further implied an increase in the proportion of material recycling.The same performance was also observed in the SOI.Versus M1997,the SOI increased from 0.13 to 0.18,while the connectivity index remained constant between M1997 and M2018.However,the ECS is still a developing ecosystem with a mass of unused energy.3)The mixed trophic impact showed the increasingly negative impact of fisheries in the ECS between M1997 and M2018,and there was an increase in low-TL fish,the biomass of the planktivores increased from 11.06%of the total fish biomass in M1997to 34.5%in M2018.The keystone species switched from the planktivores/piscivores dominated by Decapterus maruadsi in M1997 to planktivores in M2018.This variation in keystone species is consistent with findings in the Bohai Sea and the northern South China Sea,where the dominant species were also low-TL organisms such as cephalopods and mollusks.This might be ascribed to the policy on the minimum mesh size.Since 2004,the minimum mesh size of fishing nets in the China Sea has been 5cm,but B.pterotum,the dominant planktivore species,has only a 3.5 cm maximum body length（Fishbase）;thus,this species can benefit from this policy.4)The Ecosim model was used to construct two models of seasonal fishing moratorium（SFM）under actual fishing pressure,Then two Ecosim models that did or did not integrate the SFM under the actual fishing pressures were built,which are expressed by M2018_SFM and M2018_no-SFM.The results illustrated that the SFM has positive effects on fishery resources recovery,especially for commercial fishes（i.e.,large yellow croakers and hairtails）,as reflected by the significantly higher predicted biomass of fish in M2018_SFM compared to M2018_no-SFM and M1997,although the bioaccumulation was consumed by the intense fishing pressure after the SFM.However,the M2018_SFM prediction for nektons was still lower than the actual value,especially for planktivores,which display a sharp increase in biomass.5)Using Ecosim to dynamically simulate the effects of different fishing strategieson the recovery of important commercial species populations in the next 30 years,decreasing the total amount of fishing,increasing the abundance of forage species,and no fishing was all beneficial to the recovery of ecosystem structure and function,with the greatest effect of controlling the total amount of fishing,with the increase ranging from about 10%to 300%.Shrimp had a significant effect on the recovery of small yellow croaker and large yellow croaker,indicating that the available food was a critical factor in the recovery of yellow croakers.6)The climate results showed that SST and NPP had the greatest impact on resources and there were synergistic effects among climate factors,i.e.,the combined climate effects were greater than the cumulative effects of climate single factors.Functional groups responded differently to climate factors,and groups with high fishing pressure were more sensitive to climate change,such as piscivores and large yellow croaker,etc.The interactions between fishing activities and climate change were influenced by trophic relationships and showed different mechanisms of action,and reducing fishing activities could effectively mitigate the effects of climate change on species with high fishing pressure.