222 



Fishery Bulletin 100(2) 



Length and age at maturity 



The sex oiGlaucosoina hebraicurn was not able to be deter- 

 mined by macroscopic examination of the gonads until it 

 had reached ca. 150 mm in length. During the main part 

 of the spawning period, i.e. December to March, the gonads 

 of all female and male G. hebraiciun <250 mm were at the 

 earliest stages of development, i.e. I-II (Fig. 7). Gonads at 

 stages III-VIII were first found in the 250-299 mm 

 length class of females and in the 300-349 mm length class 

 of males. The presence of such gonads demonstrated that 

 the fish were maturing or that spawning was occurring or 

 had been completed (see earlier). The prevalence of ova- 

 ries at stages III-VIII increased to ca. SO'/r in the 300-349 

 mm length class and to 1009i^ in all females >450 mm. 

 The gonads of all males >450 mm were at stages III-VIII 

 (Fig. 7). The L^^'s for the lengths of female and male G. 

 hebraicum at first maturity, derived from the logistic cui-ve 



4 r Females 



o 



^ 03 



C5 



02 



0.1 



Males 



fitted to the percentage contributions of fish with gonads 

 at stages III-VIII in sequential 50-mm length classes, were 

 301 and 320 mm, respectively (Fig. 7). 



Individual G. hebraicum could first be sexed macroscopi- 

 cally during their second year of life. Although relatively 

 few two-, three- and four-year-old fish were caught, the 

 trends exhibited by the proportion of gonads at stages III- 

 VIII in both sexes during the spawning period were con- 

 sistent. One female and no males at two years of age pos- 

 sessed gonads at stage III or gi-eater (Fig. 8). However, 50'X 

 of three-year-old female and male fish, and all five-year-old 

 females and all six-year-old males possessed such gonads 

 and were thus regarded as mature. The Ar^^s for the age at 

 first maturity of females and males were 3.4 and 3.3 years, 

 respectively. 



Mortality 



Using the regi'ession equation developed by 

 Ralston (1987), in combination with the esti- 

 mated value for the von Bertalanffy growth 

 coefficient, k = 0. Ill/year, we estimated the 

 instantaneous coefficient of natural mortality, 

 M, to be 0.25/year The catch curve analysis 

 of the combined age composition data, for the 

 620 dhufish older than 9 years and longer than 

 the MLL (Fig. 9), produced an estimate of the 

 instantaneous coefficient of total mortality, Z, 

 of 0.21/year (95'^f confidence intei-val: 0.19 to 

 0.23/year). The estimate of Z remained at about 

 this level as the initial age was increased to 15 

 years and then declined to ca. 0.15/year at 24 to 

 27 years (Fig. 9). It subsequently became less 

 precise as the initial age increased. An esti- 

 mate of Z of 0. 10/year was obtained when the 

 obsei-ved maximum age of 41 years was substi- 

 tuted into Hoenig's ( 1983) regression equation. 

 However, when the sample size of 620 fish was 

 taken into account, with the expression for the 

 expected maximum age (Hoenig 1983, Appen- 

 dix A), Z was estimated to be 0.22/year. 



M J J A S O N D J 

 I I 1996 I 



FMAMJJASONDJFMA 

 1997 I 1998 ' 



Month 



Figure 5 



Mean monthly gonadosomatic indices ±1SE for females and males of 

 Glaucosoina hebraicum caught in offshore waters between May 1996 

 and April 1998. Data in this Figure and Figure 6 are restricted to 

 females and males 2Lg„ at first maturity. Numbers offish used to calcu- 

 late each mean GSI arc shown. 



Discussion 



Ontogenetic changes in habitat of 

 Glaucosoma hebraicum 



Extensive sampling for G. hebraicum during 

 the present study, allied with data obtained 

 with an echo sounder and video footage, dem- 

 onstrate that this species changes habitat as 

 it increases in size. Thus, G. hebraicum <150 

 mm was found to live in areas near reefs where 

 the substrate is firm and sponges often occur 

 (Bergquist and Skinner, 1982). The reduction 

 in the numbers of l-t- dhufish caught by trawl- 

 ing in this type of habitat in late autumn, when 

 their lengths were about 130 mm, probably 

 reflects a movement by the members of this 



