Dynamic changes of population parameters of Nemipterus bathybiusin the Beibu Gulf

Abstract

Nemipterus bathybius is a small-size demersal species,which is mainly distributed in tropical and subtropical oceanic waters.The species is mostly in habitat offshore areas(60 m) and caught commercially by hook-and-line and bottom trawl.The output of N.bathybius ranked first in the catch compositions of demersal fishes during bottom trawl survey conducted in 1964—1965.Recently,the stock density and yield of N.bathybius have changed dramatically,and the catches mainly consisted of young fish and the population structure was low-aged and miniaturization.Therefore,the aims of this research are to elu-cidate the effects of fishing activities and environment variations on its biological parameters,and provide theoretic basis for reasonable utilization and exploration.Based on the biological data obtained from bottom trawl conducted in the Beibu Gulf during 1992—1993(1992),1997—1999(1997) and 2009—2010(2009),population structure,growth and mortality parameters,as well as its annual variations of N.bathy-bius were analyzed.The results presented miniaturization and maturation at younger ages(and/or smaller sizes) of N.bathybius,as well as the growth rate increased and sexual maturity becameearlier.The average body length of N.bathybius lessened from 138.25 mm in 1992 to 112.70 mm in 1997,and 118.28 mm in 2009,respectively,the corresponding average body weight were 60.06 g,39.03 g and 48.85 g.The parameters,asymptotic length(L∞),growth coefficient(k) and zero-length age(t0) in the von Bertanaffy growth equation in 1992,1997 and 2009,were 242.2 mm,0.39 and -0.39 a;220.4 mm,0.45 and -0.35 a;235.5 mm,0.48 and -0.32 a,respectively.The inflexion age reduced from 2.19a in 1992 to 1.97a in 1997 and 1.86 a in 2009.Body length at 50% sexual maturity(L50) diminished from 128.6 mm in 1992 to 116.5 mm in 1997,and 110.5 mm in 2009.The exploitation rates(E) in 1992,1997 and 2009 were 0.57,0.62 and 0.58,respectively.It suggested that the stock has been over-exploited since the 1990s.A series of changes of population parameters of N.bathybius in the Beibu Gulf indicated that in order to sustain their populations,N.bathybius has undergone adaptive changes under high fishing pressure.