Variation in age compositions and growth rates of Acanthopagrus butcheri (Sparidae) among estuaries: some possible contributing factors

Age compositions and growth rates have been determined for populations of A can thopagrus butcheri in four estuaries and a saline, coastal lake, all of which differ markedly in their morphological, physicochemical, and biotic characteristics. Because the opaque zones in otoliths were shown to Form an nually, the number of these zones could be used to age individual fish. How ever, the otoliths of fish that were more than six years old had to be sect ioned in order to consistently reveal all opaque zones. The number of annul i on scales did not provide reliable estimates of age. Acanthopagrus butche ri, which typically completes its life cycle in estuaries, was represented in each of the five water bodies by fish greater than or equal to 15 years old and lengths and weights >365 mm and >860 g, respectively. The maximum l ength and weight of A. butcheri recorded in any of the five water bodies we re 485 mm and 2196 g, respectively. The values for L-infinity in von Bertal anffy growth equations differed significantly between females and males in three of the four estuarine populations (P<0.001 or <0.01), whereas those f or both It and t(0) differed significantly between the sexes in only one po pulation and then only at P < 0.05. The values for k and L-<infinity> in th e von Bertalanffy growth equations differed significantly among both female s and males in the four estuaries at either P < 0.001 or P < 0.05. These pa rameters also differed significantly between the males in Lake Clifton and the males in each estuary, except the Swan River Estuary. Growth rates in t wo of the more northern water bodies were greater than those in the two sou thern and cooler estuaries. The pattern of growth in the Moore River Estuar y, as reflected by changes in length with time, differed from that in the o ther four water bodies in that it was initially slower and subsequently did not show such a marked tendency to form an asymptote. The slow initial rat e of increase in length in the Moore River Estuary may be related to partic ularly high densities of juvenile A. butcheri in nearshore, shallow water, as well as to a relatively lower abundance of appropriate food or very low salinities, or to both of the latter The percentage contribution made by fi sh greater than or equal to5 years was the lowest by far in the Swan River Estuary, which was subjected to the greatest fishing pressure.