ManI et al Biological characteristics and mortality of Pentapodus vitta 



519 



October and January implies that P. vitta spawns during 

 these latter four months. This conclusion is consistent 

 with hydrated oocytes or postovulatory follicles or a com- 

 bination of both being found only in fish caught between 

 October and January. Furthermore, the precipitous 

 decline in the mean monthly GSIs between December 

 and February and the presence of only stage-II ova- 

 ries in February strongly indicates that spawning does 

 not extend into this month. Because spawning activity 

 peaked between October and January, the western but- 

 terfish was given a birth date of 1 December when an 

 age was assigned to each fish. Because mature ovaries 

 of P. vitta often contained a relatively wide size range 

 of vitellogenic oocytes and, in many cases, also either 

 hydrated oocytes or postovulatory follicles, this species is 

 considered a multiple spawner serisu deVlaming (1983), 

 namely individual females release eggs on more than 

 one occasion in a spawning season. 



Is there evidence that Pentapodus vitta 

 is hermaphroditic? 



Our data demonstrate that, in the population of P. vitta 

 in Shark Bay, the females are relatively more abundant 

 than males in length classes below 160 mm, whereas the 

 reverse is true of length classes above that length (Fig. 

 3A). In contrast, the prevalence of females was always 

 greater than that of males in each age class. The trend 

 towards an increasing prevalence of males vs. females 

 with increasing body length can thus be attributed to 

 the males growing faster. The fact that there was not 

 a progressive shift towards an increase in one sex with 

 increasing age and none of the gonads of the 178 fish 

 examined contained both ovarian and testicular tissue 

 provides overwhelming circumstantial evidence that P. 

 vitta is gonochoristic. Like P. vitta, other nemipterid 

 species also exhibit a size-related skew in their sex 

 ratios that can be explained by differences in growth 

 rates between the two sexes (e.g.. Young and Martin, 

 1985; Lau and Sadovy, 2001; Raje, 2002; Granada et 

 al., 2004). The trend among gonochoristic nemipterids, 

 for their males to grow larger than their females, pre- 

 sumably reflects a selective advantage of being large 

 during courtship or mating (Granada et al., 2004). In 

 contrast to these gonochoristic species, some nemipterids 

 are protogynous hermaphrodites, e.g., Scolopsis mono- 

 gramma, S. taeniopterus and S. bilineatus (Young and 

 Martin, 1985). 



Mortality 



The point estimates for natural mortality, M, derived for 

 males and females of P. vitta with Pauly's (1980) equa- 

 tion, were inconsistent with the point estimates derived 

 for total mortality, Z. This conclusion is drawn from the 

 fact that the point estimates for M were greater than 

 those for Z from both Hoenig's (1983) equation for fish 

 and catch curve analysis, thereby paralleling the situ- 

 ation with several other species (see Hall et al., 2004), 

 and also those derived from the simulation and Bayesian 



methods of Hall et al. (2004). The use of the method of 

 Hall et al. (2004) reconciled the inconsistencies between 

 M and Z for P. vitta and provided more precise estimates, 

 particularly for M. Because the point estimates for Z 

 were only slightly greater than those for M, the popula- 

 tion of P. vitta in Shark Bay does not appear to have 

 been subjected to heavy fishing pressure. In this context, 

 it is relevant that, because the vast majority of P. vitta 

 attain maturity by the end of their first year of life, most 

 individuals will have had the opportunity to spawn at 

 least twice before they become fully recruited into the 

 trawl fishery. Furthermore, recreational anglers return 

 about 73% about of the western butterfish they catch in 

 Shark Bay live to the water (Sumner et al.^). 



Pentapodus vitta lives for up to eight years, but grows 

 very rapidly early in life and attains maturity by the 

 end of the first year of life. The males of P. vitta grow 

 more rapidly and reach a larger size than their fe- 

 males. The western butterfish is a gonochoristic species 

 that, in Shark Bay, spawns between mid-spring and 

 mid-summer. Although commercial and recreational 

 fishermen obtain large numbers of P. vitta as bycatch 

 in Shark Bay, this fishing does not appear to have a 

 marked impact on the numbers of this very abundant 

 species in this large subtropical embayment. 



Acknowledgments 



Gratitude is expressed to many colleagues in the Depart- 

 ment of Fisheries, Western Australia, and the Centre for 

 Fish and Fisheries Research, Murdoch University for 

 help with sampling. Comments and constructive criti- 

 cisms were kindly provided by Michael Mackie, Peter 

 Stephenson, and Gary Jackson. Financial and technical 

 assistance were provided by the Department of Fisher- 

 ies, Western Australia, and Murdoch University. 



Literature cited 



Beamish, R. J., and D. A. Fournier. 



1981. A method for comparing the precision of a set 

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 38:982-983. 

 Cerrato, R. M. 



1990. Interpretable statistical tests for growth com- 

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 equation. Can. J. Fish. Aquat. Sci. 47:1416-1426. 

 deVlaming, V. L. 



1983. Oocyte development patterns and hormonal 

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 England. 



2 Sumner, N. R., P. C. Williamson, and B. E. Malseed. 2002. A 

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