Size-related movements of Rhabdosargus sarba in three different environments and their influence on estimates of von Bertalanffy growth parameters

Abstract

In temperate, marine waters on the lower west coast of Australia, Rhabdosargus sarba settles in unvegetated nearshore areas and then, as it increases in size, moves progressively to nearby seagrass beds and then to exposed unvegetated nearshore areas and finally to areas around reefs where spawning occurs. The von Bertalanffy growth curves fitted to the lengths at age of R. sarba collected from the different habitats mentioned above were significantly different from each other (P<0.001). These significant differences were at least partly attributable to the fact that the size-related movement of R. sarba between habitats affects the composition of the lengths at age of this species in each habitat. The problem of producing a sound quantitative description of the overall growth of the individuals in populations such, as that of R. sarba in coastal marine waters, was overcome by incorporating logistic functions within the von Bertalanffy growth model that accommodated the probability of a fish of a given length occurring in a particular habitat. R. sarba also underwent size-related changes in habitats in two other environments, namely a temperate estuary and a subtropical embayment. Thus, as this species increases in size, it moves from nearshore, shallow waters to offshore, deeper waters in the former environment and from mangroves to areas around reefs in the latter environment. Although descriptions of the growth of individuals in the populations of R. sarba in these latter two environments were improved statistically by using the adjusted von Bertalanffy growth equation, these improvements were so small that they were considered not to be of biological significance. This was because, in contrast to the situation in temperate coastal marine waters, the samples collected from the different habitats in the estuary and subtropical embayment provided, as a whole, better representations of the compositions of the lengths at age of those two populations. The growth of R. sarba was slightly faster in the lower reaches of the temperate estuary than in either temperate coastal marine waters or the subtropical marine embayment, where food was almost certainly less abundant.