Inter-annual changes on body weight-length relationship and relative fatness of small yellow croaker (Larimichthys polyactis)

Abstract

Based on data from 14 bottom trawl surveys in the Bohai Sea and the southern Yellow Sea during spring(from April to May) of 1960-2004 and spring of 1960-2010,inter-annual changes on body weight-length rela-tionship(WLR) and relative fatness of two geographical stocks of small yellow croaker Larimichthys polyactis [the northern Yellow Sea-Bohai Sea stock(NYBS) and the southern Yellow Sea stock(SYS)] and their impact factors were analyzed.WLR was linearly regressed by logarithm-transformed body length and its corresponding mean body weight;relative fatness was computed by relative weight by years,sex and stocks.The results showed that the allometric factor(b) was not significantly different between sexes in most years,and significantly negative correlations(P0.01) were found between the condition factor(a) and b in two stocks.The(lga)/b ratio may be connected with fish body density,and its value was little influenced by environment.The b values almost ap-proached 3,which indicated isometric growth in the NYBS stock in most surveys from 1960 to 2004,and they increased during the regression stage and the early recovery stage of the stock(1982-1993).However,most of b values were significantly smaller than 3(P0.05),indicating strongly negative-allometric growth in the SYS stock from 1960 to 2010,and they decreased during the regression stage and the early recovery stage of the stock(1986-1994),then increased since the late 1990s.The highest relative fatness of small yellow croaker was found in 1960 in the two stocks.The fatness clearly decreased in total from 1960 to 2004 in both male and female fish in the NYBS stock,and it significantly declined from 1960 to 1986 in the SYS stock(P0.05),then slightly and in-significantly increased since the mid-1990s in both male and female fish in the SYS stock(P0.05).According to analysis,based on better food conditions,the relative low fishing pressure was the main reason for the isometric growth of the NYBS during the 1980s and the early 1990s;and the continually heavy fishery increased the pro-portion of low age fish and the growth speed in the SYS,which may be the main reasons for the nega-tive-allometric growth of the stock.The decrease in age at maturity may be the reason of the decline in the relative fatness in both stocks,but the closed seasons and the relative higher water temperature in the southern Yellow Sea made the relative fatness increase since the mid-1990s in the SYS.