Mant et al Biological characteristics and mortality of Pentapodus vitic 



517 



Females 



03 



I I ^^m^m^ 



S O N D J F M 



Month 



t=d 



A M 



Figure 4 



Mean monthly gonadosomatic indices (GSIl (±1 standard 

 error) for females and males of western butterfish (Pen- 

 tapodus vitta) that were >1 year old. Monthly sample 

 sizes are given above each mean. 



that had developed to at least stage III by September 

 would have progressed through to maturity during the 

 ensuing months of the spawning period if it had not 

 been removed from the population. 



In each month, the distribution of oocyte diameters 

 in the mature ovaries of P. vitta produced a well-de- 

 fined mode between -20 and 80 am, which represented 

 chromatin nucleolar- and perinucleolar-stage oocytes. 

 Cortical alveolar oocytes were present in some ovaries 

 in July and September, when oocyte diameters had 

 reached a maximum of 200 and 150 jam, respectively. 

 These larger oocytes and also yolk granule oocytes were 

 abundant between October and December and reached 

 a maximum diameter of 380 \xva.. The contribution of 

 cortical alveolar and yolk granule oocytes declined in 

 January. Hydrated oocytes were found in several ova- 

 ries in October, December, and January and postovu- 

 latory follicles were present in some ovaries during 

 November. 



Mortality 



The age-frequency distributions for both female and 

 male western butterfish caught during extensive trawl- 

 ing in November and December of 1997 had a modal 

 peak at 2 years of age (Fig. 3B). Because the age at full 

 recruitment was thus considered to be 3 years, catch 



curve analysis was undertaken with data for the 3-i- 

 and older age classes. The resulting point estimate for 

 the instantaneous rate of total mortality, Z, was signifi- 

 cantly greater (P<0.05) for males (1.15/year) than for 

 females (0.91/year) (Table 2). Likewise, the simulation 

 method of Hall et al. (2004), based on the number of 

 individuals above 5 years of age (2 males and 13 females 

 for sample sizes of 130 and 253, respectively), yielded a 

 greater point estimate of Z for males (1.34/year) than 

 females (0.89/year). The point estimate for Z, derived 

 by inserting the maximum ages for female and male 

 butterfish (8 years for both males and females), into the 

 refitted Hoenig (1983) equation for fish, was 0.55/year 

 for the two sexes (Table 2). The point estimates for the 



