FIELD ESTIMATES OF GROWTH AND MORTALITY OF JUVENILE BANANA PRAWNS (PENAEUS-MERGUIENSIS)

Postlarval and juvenile Penaeus merguiensis de Man from the Embley Riv er estuary on the north-eastern Gulf of Carpentaria were sampled every 2 wk from September 1986 until August 1989, using a small beam trawl. Settlement of planktonic postlarvae peaked during the pre-wet season (October to December), and declined through the wet season (January to March). Using length-frequency analysis between 12 and 14 cohorts of juvenile prawns were identified each year. Length-frequency analysis a nd modal progression were used to derive growth rates during the estua rine phase of the life cycle. Growth rates, which could be described b y a linear model, ranged from 0.63 to 1.65 mm CL (carapace length) wk - 1. Growth rates were positively influenced by water temperature and negatively influenced by prawn density. Salinity had no effect on grow th rates. Prawns spent between 6 and 20 wk in the Embley River before emigrating offshore from the estuary. Weekly instantaneous rates of na tural mortality (M) ranged from 0.23 to 0.94, and in general were lowe st during the dry season (July to September) and highest during the pr e-wet and wet seasons. Only temperature significantly influenced morta lity rates, with mortality rates increasing with temperature. By proje cting juvenile growth rates forward through time, we established which cohorts contributed to the offshore fishery each year. In 1987 and 19 88 the April fishery consisted of prawns which had settled in the rive r before the end of January each year. Slow growth rates during the pr e-wet season of 1988 meant that only cohorts that were settled before early December 1988 contributed to the fishery in April 1989. Whether a cohort contributes to the fishery depends on the settlement date, wa ter temperature and prawn density.