Sarre and Potter: Variation in age compositions and growth rates of Acanthopagrus bulcheri 



795 



mate weights offish of a particular length can be estimated. 

 Becasue analysis of covariance (ANCOVA) showed that nei- 

 ther the slopes nor the v-intercepts in the equations for 



female and male fish in each estuary differed significantly 

 (P>0.05), the equations relating total length and weight for 

 both sexes combined in each estuary are also presented. 



(r2=o.99, /i=865). 

 (r2=0.99, «=925). 

 (r2=0.99, n=1790). 



(r2=0.99, n=250). 

 (r2=0.99, ?i=287). 

 (7--^=0.99, ;(=537). 



(/■2=0.99, n=302). 

 (r2=0.99, n=234). 

 (/■2=0.99, n=536). 



(r2=0.99, n=274). 

 (r2=0.99, «=278). 

 (7-2=0.99, ?!=552). 



(/•2=0.98,n=85). 

 (/■2=0.99, n = 100). 



Discussion 



Validation of the method for aging 

 Acanthopagrus butcheri 



Our study shows that a growth zone is not laid down in the 

 otoliths of the 0+ age class of Acanthopagrus butcheri until 

 winter and that this growth zone does not become clearly 

 delineated until late spring. Because spawning peaks in 

 early November in the Swan Estuary (Sarre and Potter, 

 1999), the first growth zone becomes delineated as the 

 individuals of this species become one year old. Further- 

 more, the trends exhibited by the marginal increments on 

 the sectioned otoliths of A. butcheri demonstrate that an 

 opaque zone is laid down annually in this hard structure. 

 Our results also demonstrate that the otoliths of A. butch- 

 eri do not need to be sectioned in order to consistently 

 reveal all of the opaque zones until they had reached a size 

 at which they possessed seven or more such zones. Vali- 

 dation that the opaque zones, which are revealed on the 

 otoliths of A. butcheri by sectioning, are formed annually, 

 implies that the estimates of the ages of individual black 

 bream recorded by Morison et al. (1998) using sectioned 

 otoliths are valid for fish caught in the Gippsland Lakes in 

 eastern Australia. However, because the number of annuli 

 on scales frequently differed from the number of opaque 

 zones on sectioned otoliths from the same fish, the number 

 of annuli on scales do not provide a reliable method for 

 aging A. butcheri. Thus, those estimates of the age of indi- 

 vidual A. butcheri, that have been based on the number 



Table 3 



Significance levels for comparisons between the von Ber- 

 talanfTy growth parameters for female and male Acan- 

 thopagrus butcheri caught in each of the Swan River, 

 Moore River, Nornalup Walpole, and Wellstead estuaries 

 by using the likelihood ratio test.  **=P<0.001, **=P<0.01, 

 ' =P<0.05, NS = Not significant. ^|, is the hypothetical age 

 at which fish would have zero length, k is the growth coef- 

 ficient, and L. is the asymptotic length. 



von Bertalanffy growth parameter 



Estuary 



NS 



NS 

 * 



NS 



NS 



of annuli on scales (Butcher, 1945; Thomson, 1957; Weng, 

 1971; Hobday and Moran'M are, in many cases, probably 

 invalid. 



Differences in age structures amongst populations 



Because the majority of A. butcheri obtained from the 

 Swan River, Moore River, Nornalup Walpole, and Well- 

 stead estuaries were collected by using the same seine and 



