Size-dependent natural mortality of juvenile banana prawns Penaeus merguiensis in the Gulf of Carpentaria, Australia

Natural mortality of marine invertebrates is often very high in the early l ife history stages and decreases in later stages. The possible size-depende nt mortality of juvenile banana prawns, P. merguiensis (2-15 mm carapace le ngth) in the Gulf of Carpentaria was investigated. The analysis was based o n the data collected at 2-weekly intervals by beam trawls at four sites ove r a period of six years (between September 1986 and March 1992). It was ass umed that mortality was a parametric function of size, rather than a consta nt. Another complication in estimating mortality for juvenile banana prawns is that a significant proportion of the population emigrates from the stud y area each year. This effect was accounted for by incorporating the size-f requency pattern of the emigrants in the analysis. Both the extra parameter in the model required to describe the size dependence of mortality, and th at used to account for emigration were found to be significantly different from zero, and the instantaneous mortality rate declined from 0.89 week(-1) for 2 mm prawns to 0.02 week(-1) for 15 mm prawns.