| The Beibu gulf is a semi-closed bay in the northwestern South China Sea (SCS)surrounded by Southern China,Hainan Island,Leizhou Peninsula and Vietnam.The Beibu gulf has high productivity and abundant marine resources.However,with the lack of effective supervisions and the increasing fishing pressure in the past four decades,the 1structure and function of Beibu gulf ecosystem shifted remarkably.The catch rates have been displayed a declining curve,and the demersal catch compositions have been found to transfer from the high-value fish species,to the low-value species.Decapterus maruadsi is one of the most common and commercially species for the bottom trawl and gillnet fisheries in the Beibu Gulf,South China Sea.Until now,reports on fishery biology and population structure of D.maruadsi are limited.So,the population biological parameters of D.maruadsi in Beibu Gulf,including size composition,growth,mortality and maturity,were analyzed based on bottom trawls survey data during 1992~1993(1992),1998~1999(1998),2006,2007,2008~2009(2009),2010 and 2011~2013(2012).In total,635 fish individuals were measured in 1992,1714 in 1998,1049 in 2006,1028 in 2007,1226 in 2009,465 in2010 and 1318 in 2012.Results showed that the size composition of D.maruadsi in the Beibu Gulf has deeply changed after the long-term fishing pressure;the dominant body length groups decreased from 130~170mm in 1992 to 110~140mm in 2007,then showed an increased trend,to 120~130mm and 140~170mm in 2012;The phenotypic traits,such as body length and age at 50%sexual maturity,were also produced adaptive responses due to fishing.The average body length decreased from 160.23mm in 1992to 139mm in 2007,then increased to 149mm in 2012;As two important reference points in fishery management,body length and age at 50%sexual maturity presented a similar tendency with the average body length,which decreased from 180.16mm and 1.860 in1998 to 174.91mm and 0.888 in 2007,respectively;then increased to 180.87 and 2.538in 2012,respectively.However,the growth coefficient(K)decreased gradually after increasing from 0.39 in 1992 to 0.72 in 2007,the overall tendency was upward.The total mortality coefficient(Z)was increasing since the 1990s,which up to the highest of 2.77 in 2009,then it downwards to 1.68 in 2012.The exploitation rate(E)variation trends parallel with the total mortality coefficient,which increased from 0.7 in 1992 to0.78 in 2009,and then reduced to 0.64 in 2012.All of the diversifications exposed miniaturization and maturation at younger ages(and/or smaller sizes)of D.maruadsi,as well as the growth rate increased.A series of changes of biological characteristics in D.maruadsi has declared over-fishing since 1990s until now,but this situation has refined gradually.The over-fishing state of N.virgatus in northern South China Sea(include Beibu gulf)has been revealed,but the variations of population dynamics over time,and the potential factors influenced biological traits hasn't known yet.In this study,we analyzed the time-series of some biological traits,such as population structures,growth and mortality parameters,exploitation ratio,body length and age at median sexual maturity of N.virgatus in Beibu Gulf based on the bottom trawl survey data during1959-1960(1960),1961-1962(1962),1992-1993(1992),1998-1999(1998),2006,2007,2008-2009(2009)and 2011-2013(2012).Results showed that:1)the average body length of in the eight periods fluctuated,but the overall trend was reduced;compared with 155.29 mm in 1960,the average body length in 2012 was only 129.87mm;2)The body length and age at median sexual maturity of N.virgatus in Beibu gulf significantly decreased from 1960 to 2012,but there was an increased trend from 2006to 2012,which decreased from 144.62mm and 2.59 in 1960 to 134.26mm and 2.12 in2006 respectively,and then increased to 2012.138.46mm and 2.95,the overall trend is down,the inflection point is 2006;3)the inter-annual variation of the growth coefficient K is similar to the changes in the average body length,showing fluctuations,but the overall trend is increasing,the minimum value of K is 0.27;4)L_?initially increased from 326mm in 1962 to 341.5mm in 1992,and then decreased gradually until it was reduced to 260mm in 2012,the overall trend is decreasing;5)Exploitation rate E increased from 0.34 in 1960 to 0.61 in 1992,the highest value in 2009 was 0.74,and then dropped significantly to 0.53 in 2012.The overall trend was increased,and the E value in 2010 has been less than the results of the early 1990s.The above results indicated that the phenotypic characteristics of the N.virgatus in the Beibu Gulf have undergone inter-annual changes,the individuals tend to be miniaturized,and the body length and age at median sexual maturity are reduced.It has been in an over-developed state since 1992,but in recent years this state has refined.It was found that both of the D.maruadsi and N.virgatus fish stocks have been over-fished since 1990s.Which results miniaturization and maturation at younger ages(and/or smaller sizes)of the stocks.Although the over-fished state still existed until now,this situation has refined recently.The modification of biological parameters in these two stocks indicates it belongs to phenotypic adaptation.Therefore,the continuously changes need to be considered when the fishery policy posed. |
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