Evaluating The Effects Of Summer Fishing Moratorium In The Bohai Sea And Its Influence On The Population Dynamics Of Japanese Anchovy(Engraulis Japonicus)

The Bohai Sea serves as important spawning,nursery and feeding grounds for various economic fishery species in China's offshore waters,and thus has great ecological and economic benefits to humans along the coast.Since the 1980 s,deterioration of marine environment and overfishing have severely depleted fishery resources in this local ecosystem,resulting in a series of declines in trophic levels,individual miniaturization and the low quality of fishery products.In order to conserve fishery resources,China has introduced a series of fisheries management and conservation measures.Among such measures,the summer fishing moratorium is the most extensive,deepest and thorough policy so far.A scientific evaluation of its implementation effect is conducive for further improving this policy.This study is based on sporadic bottom trawl surveys data for fisheries resources in the Bohai Sea(1992-2015),and wild fisheries products data from China Fishery Statistical Yearbooks(1979-2018).We compared and analyzed temporal changes of wild fisheries products,fishery resources,and the biomass of major fishery species in different seasons before and after the summer fishing moratorium in the Bohai Sea.Meanwhile,we took Japanese anchovy as a case study,based on bottom trawl surveys data in the Bohai Sea(2014-2017),and analyzed its stock abundance distribution and growth characteristics.The main results are as follows:The summer fishing moratorium initiated in 1995-19998 in the Bohai Sea,and wild capture fisheries productions increased by 43.76%,reaching a maximum of 1.6225 million tons in 1999;after 1999,summer fishing moratorium was prolonged for several times,and productions from wild capture fisheries displayed a general downward trend,and kept relatively stable at 1 million tons after 2008.Recently,fisheries resources in the Bohai Sea showed a sign of recovery after a sharp decline in 1992-2015,based on scientific survey data.The summer resources in 2010-2014 were generally higher than that in 1998-2004.In summer,fisheries resources in 2010-2014 were generally higher in amount than those in 1998-2004.In autumn,fisheries resources in 2014 were 2.88% higher in abundance than that in 2004,but lower than that in 1998.The abundances of Japanese anchovy,silver pomfret(Pampus argenteus),swimming crab(Portunus trituberculatus)and some other economic species in spring and autumn of 2014-2015 were higher than that in 2004-2010,but lower than that in 1992-1993.After several months of recruitment and growth in the closed fishing season under the circumstance of no fishing,the abundance of fishery resources generally increased.The total abundance of fisheries resources in autumn in 1998 and 2004 increased by 321.69% and 13.79% than those in spring,respectively.The abundances of half-fin anchovy(Setipinna taty),dotted gizzard shad(Konosirus punctatus),silver pomfret increased by more than five times from May to October in 2004.In addition to the increases in abundance and individual size,the implementation of summer fishing moratorium also brought certain economic and social benefits,including lower operating costs,economic compensation from resource recovery,improving the fishing nets structure,and raising the awareness of resource and environmental protection.Unfortunately,the trend of resource decline,individual miniaturization and lower quality has not been substantially halted.It is necessary to further improve the summer fishing moratorium system and its supporting policies,to make rational use and management of fishery resources.Density distribution characteristics of Japanese anchovy in the Bohai Sea: The abundances of anchovy in 2017 increased from 0.07 kg/h in May to 0.85 kg/h in July,and the frequency of occurrence also showed an upward trend.In spring and summer,anchovy enter the Bohai Sea for spawning and feeding,which mainly dispersed in the central Bohai Sea and the mouth of Laizhou Bay,with the water temperature ranges from 14-16?,19-21?,and 24-26?,respectively.The relative density of anchovy decreased in autumn,which mainly distributed in the 18-19 ? isotherm near the Bohai Strait.The center of gravity of the resource is often used to describe the change and migration of the spatial distribution of fishes.The latitude change of the center of gravity of the relative resource density in the fall of 2014-2017 is consistent with the change in the average latitude of the isotherm at 18-19 ° C.Based on historical data,the relative density of anchovy has fluctuated significantly in recent years,but it has generally increased.Japanese anchovy population dynamics in Bohai Sea: The abundance and occurrence frequency of Japanese anchovy increased from May to July 2017,with an overall increasing rate at 8.7%.Spring and summer are the spawning and feeding periods of Japanese anchovy.The spatial distribution of this species gradually expanded from the off-sea of Qinhuangdao and Laizhou Bay in June to the Central Bohai Sea and Liaodong Bay.In October,the abundance of Japanese anchovy dropped sharply,some colonies had swam out of the Bohai Sea,and most of the rest were distributed in the mouth of Laizhou Bay.In 2017,the summer fishing moratorium was prolonged for one more month,while the abundance continued to decline in 2014,2016 and 2017,with a decreasing rate of up to 93.92%.And the latitudinal change of its distribution center is consistent with the average latitudinal change of 18-19 ? isotherm,showing the trend of moving northward first and then southward.From the perspective of growth characteristics,the average fork length and body weight of anchovy showed a continuous downward trend from June to October in 2017,and the frequency density distribution of fork length changed from single peak type to multi peak type.This is in line with the law of population growth and change.The average fork length increased and the average weight decreased from August to November in 2016.In 2016 and 2017,the parameter b range of fork length-weight relationship was 3.07-3.12 and 2.86-3.32 respectively,and both of them have the opposite trend of seasonal change,and their potential influences need further study.Relative weight is an important indicator to reflect the growth and energy budget of fish.In 2016 and 2017,relative weight showed a common decreasing trend in summer and autumn,with a maximum of 117.41% in June.It is related to the growth stage and the feed conditions.