Spatial variation in demography of an estuarine teleost: implications for population and fishery assessments

Abstract

Length- and age-based information is used to assess the exploitation status of harvested populations of fish worldwide. This study examined variability in length-at-age, length and age compositions and derived demographic parameters of Acanthopagrus australis (Sparidae) captured in beach seines in 3 estuaries across 5 yr. The method of ageing fish by counting opaque growth zones on sagittal otoliths was validated by field and aquaria studies. Length-at-age relationships varied according to gender, age class and estuary. Females older than 3 yr had a significantly greater mean length-at-age than males, and this was consistent across all estuaries. For both genders, mean length-at-age of fish older than 4 yr was greatest in the southernmost estuary, and least in the northernmost estuary, suggesting differential growth dynamics among estuaries. Observed longevity differed among estuaries, ranging from 14 to 22 yr. Variations in length composition of retained catches were subtler than those for age composition. The average age of fish in catches varied among estuaries from 4.2-5.1 yr to 6.8-8.2 yr even though fish within 5 cm of the minimum legal length dominated catches in all estuaries. Estimates of total and fishing mortality varied 3-fold among estuaries, with fishing mortality exceeding natural mortality. Variability in demographic characteristics was generally greater among estuaries than among years within each estuary. These results show that the population demographics and ensuing assessments should not be extrapolated across estuaries or from one estuary to the entire A. australis population. Length is a poor predictor of age and future sampling of populations must be age-based and stratified to account for estuary-specific variation in demography. Failure to account for such variability could confound assessments and management deliberations necessary for determining the most appropriate harvest and conservation strategies for such species.