518 



Fishery Bulletin 104(4) 



instantaneous rate of natural mortality, M, derived by 

 inserting the parameter values for western butterfish 

 into the refitted Pauly (1980) equation, were higher for 

 females (1.78/year) than males (1.28/year). 



The combined point estimate for Z derived using the 

 method of Hall et al. (2004) was greater for males (1.14/ 

 year) than females (0.89/year; Fig. 6). This method 

 yielded a slightly higher point estimate of M for males 

 (0.80/year) than females (0.71/year) (Fig. 6). 



Discussion 



Validation of aging method, age composition, 

 and growth 



The validation that the opaque zones of P. vitta are 

 formed annually, together with the very high repeatabil- 

 ity of their counts in sectioned otoliths, demonstrate that 

 the number of these opaque zones (annuli) in sectioned 

 otoliths can be reliably used for aging this 

 nemipterid. Stephenson and Hall (2003) 

 demonstrated that the annuli in sectioned 

 otoliths of another nemipterid, Nemipterus 

 furcosus, in Western Australia are also 

 formed annually. 



The vast majority of the P. vitta caught 

 in Shark Bay were less than seven years 

 old and the oldest individual was just over 

 eight years old. The pattern of growth 

 for females and males differed. Thus, the 

 males of each age were, on average, larger 

 than females after the first year of life 

 and had a greater L,. However, most of 

 the growth of both sexes occurs during 

 the first three years of life. 



Spawning period 



The highly elevated mean monthly GSIs 

 of females and males between October and 

 December and the restriction of females 

 with mature ovaries to the period between 



