Growth, Mortality And Reasonable Utilization Of Fat Greenling Population In Haizhou Bay And Its Adjacent Waters

Fat greenling, Hexagrammos otakii, is a commercially important species anddistributes widely in coastal China sea area. However, its stock status in Haizhou Bayand its adjacent waters has not been reported. To assess the stock status of fatgreenling in Haizhou Bay and its adjacent waters scientifically and to use itreasonably, bottom trawl surveys has been carried out in Haizhou Bay and its adjacentwaters in Mar., May, Jul., Sept., Dec,2011and additional surveyin the northern areaof Haizhou Bay in Sept.,2012. Based on the biological data collected from thesurveys, the growth of fat greenling has been researched using length frequencyanalysis method and otolith-based age identification, respectively. In addition, theeffects of big individual samples on the growth function were examined. Mortalitycoefficients Z (total mortality coefficient), M (natural mortality coefficient) and F(fishing mortality coefficient) have been calculated by the method of length-convertedcatch curves and the nethod of Pauly empirical function. Together with the historicalcharacteristics of fat greenling and its length-weight relationship, biological referencepoints Fmaxand F0.1and the yield per recruitment (YW/R) isoline have been employedto study the status of fat greenling in Haizhou Bay based on the Beverton-Holt model.The effects of F and tc(age at first capture) on YW/R has been studied and propermanagement measures have been suggested. In addition, based on the method ofMonte Carlo simulation, the effect of uncertainty in fishery on stock assessmentresults has been studied.Fat greenling sample structure: The standard length ranged44~292mm, with theaverage of88mm and dominant range of54~74mm, while the body weight ranged0.35~574.3g, with the average of17.94and the dominant range of0~10g. Thelength-weight relationship was W=4.23*10-6L3.31(R2=0.9786, n=1514). In addition, the structure of length and weight and their relationships show a certain monthlyvariations.Based on the fat greenling samples captured in2011only, the growth functioncalculated by otolith-based age identification method and ELEFAN Ⅰ areL262.0*(1e（0.62(0.14)t t）)and Lt229.4*(1e（0.91(t0.10)）),respectively. While, based on the samples with big individual samples captured in2012added, the growth function computed by ELEFAN Ⅰ isL328.0*(1e（0.36(0.43)t t）). Differences exist between this three growthfunctions when the age is low and high, and the result calculated from ELEFANⅠissmaller than the one from otolith-based age identification method. The growthfunction may change with the inclusion of big individuals or not.Z is estimated as2.06by length-converted catch curves, M is calculated as0.62by Pauly empirical function, and F is1.44. E (exploitation rate) is0.70.Using the model of Beverton-Holt yield per recruitment, we estimate FmaxandF0.1as0.63and0.41, respectively, both of which are much lower than Fcur. Thisshows the current fat greenling is overfished. The YW/R isoline analysis indicates thatthe current fishery point is under the proper yield area, which means a too early andquite strong fishing existing at current. YW/R will increase at first and then decrease atlast with the increase of both F and tc. However, when tcis increased, YW/R willincrease faster. Considering flexion age, critical age, recruitment and economicinterest, this paper finally suggests standard length of191mm as the optimumcatchable size of Hexagrammos otakii in Haizhou Bay and its adjacent sea.Monte Carlo simulation analysis shows that uncertainty in fishery may affect theprobability distributions of Fmaxand F0.1. With the increase of uncertainty, distributesof Fmaxand F0.1may spread around. F0.1is more stable than Fmax. Theprobability(Fcur>FBRP) is nearly100%no matter under which certainty levels,reflecting the high risk of overfishing for fat greenling in Haizhou Bay.