Study On Fishery Resources And Their Succession In China's Coastal Region

Fishery resources are constantly updating and changing biological resources,and the rational development and utilization of them will achieve economic,social and ecological benefits,and fulfill the purpose of sustainable use.However,due to the excessive exploitation and utilization by mankind,coupled with the deterioration of the ecological environment,many fishery resources have been damaged and in a serious recession.In recent decades,given the intensive fishing activities,China's coastal fishery resources have been damaged to varying degrees,which is similar to the world's fishery development trend.Elucidating the impact of fishery activities on coastal biological communities will provide the basis for the management and protection of inshore fishery resources.Based on the data collected from three large marine ecosystems(LME)in China's coastal waters,i.e.,Yellow Sea,East China Sea and South China Sea,this thesis explored the overall characteristics of the catch,the growth parameters and mortality and the yield-per-recruit for certain fish populations,the marine trophic index(MTI)and the mean temperature of the catch(MTC)for each regional marine ecosystem.The specific research contents and results are as follows:1)With the increasing fishing effort in China's inshore waters,the overall status of fishery resources in three regional marine ecosystems have been in recession and continuing to deteriorate,despite that China's inshore catch was still at a high level.The trend of the dominant species in China's coastal waters was that the traditional bottom layer species were gradually replaced by the middle and upper ones,and the number of jellyfish in the catch have increased in all three regional marine ecosystems.At a long term,the proportion of developing resources in total fishery resources of each regional marine ecosystem was gradually reducing,and the proportion of exploited,over-exploited and collapsed resourcesrose.Rebuilding resources began to occur in recent years,while its proportion was inconsequential.Prior to the 1980s,the three regional marine ecosystems were mainly composed of developing resources,afterwards,the exploited resources became a major component of the catch,with the proportion of developing resources significantly reduced.2)The stock assessment of fish populations can provide a strong basis for the management of fishery resources,however,the data requirements for stock assessment are critical.Given the limited application of stock assessment,ELEFAN software can utilize the length frequency data of a single population,to evaluate the growth parameters and total mortality of fish and evaluate the status of fishery.Based on length frequency data and ELEFAN method and software,the growth parameters and mortality of ten species of fish in China were estimated.Combined with the data on the FishBase website,the growth parameters estimated by ELEFAN were used for yield-per-recruit analysis.The results of the analysis showed that,in view of the small mesh size of the Chinese trawler,all of these ten species had high fishing mortality rates,and were experiencing growth overfishing.The situation was particularly pronounced in large fish stocks.If the fishing intensity was kept constant,but increased the average size of the fish at the first capture,i.e.,increasing the size of the mesh net,then the yield-per-recruit for all ten species would increase,with the highest increase,i.e.2.5 times of the existing level,occurring in largehead hairtail.3)Fishery activities impact not only target,but also whole marine ecosystems.MTI is an indicator of the biodiversity of large fishes,and the decline of MTI means that the intensive fishing pressure leads to changes in catch compositions,which shifts from a dominance of large,high-trophic level species to relatively small,low-trophic level species.Based on the catch of the three regional marine ecosystems in China and the corresponding trophic levels,the MTIs in China's inshore waters were analyzed,and showed a decreasing trend,that is,a phenomenon called 'fishing down marine food webs'(FDMW)occurred.FDMW was underestimated due to data aggregation,neglecting of fish length changes caused by fishery activities,and geographical expansion of fishery activities.When aggregated the original data to order,the absolute value of the FDMW slope in the Bohai and Yellow Sea and the South China Sea would be reduced from 0.006 and 0.007 to 0.003,respectively,and the FDMW phenomenon in East China Sea disappeared completely.After removing the masking effect of geographical expansion to MTI in three LMEs,FDMW intensified.4)Global warming caused by human activities has exerted an impact on the oceans from the beginning of the 20th century and has affected the composition of the catch species,and the average temperature of the catch has provided an indicator that can measure this effect.Since 1950,the MTC in the Bohai and Yellow Sea and the East China Sea regional marine ecosystems have shown upward trends,with starting years occurred in 1968 and 1962,respectively.Meanwhile,there was a good correlation between MTC and SST in these two areas,especially when taking a 3-year time-lag into account.In the Bohai and Yellow Sea and the East China Sea regional marine ecosystems,compared with the 1950s,the proportion of fish with higher temperature preference in the 2000s increased,while the proportion of species with lower temperature preference decreased.The MTC in South China Sea LME had no significant change since 1950,which might be attributed to that,when the temperature of the tropical ecosystem(e.g.,the South China Sea)rises,the tropical fish whose temperature preferences are no longer suitable will leave here to find areas suitable for their growth,while,at the same time,there is no fish with higher temperature preference can replace them and survive in this area.