Considering the prominence of the NSW prawn resources, it is important that managers be equipped with the information necessary to manage the school prawn resource in a sustainable way that provides equity for all stakeholders. Despite its prominence as one of the two most important contributors to the prawn production in NSW, little is known about the population dynamics of this species.
There is evidence to suggest that the school prawn populations in NSW may be overfished. Available information shows that the size of spawning populations in ocean waters may be declining and that prawns are being caught at sizes far shorter than those that would optimise biological “yield per recruit” under an F0.1 fishing policy.
The data which was used in this population modeling of the species had relatively low levels of precision. Estimates of growth and mortality with acceptable levels of precision are needed so that population models can be used with confidence to predict the outcomes of scenarios aimed at optimising the use of the school prawn resource. Out of a list of ten topics needing research, attendees at the Juvenile Prawn Summit assigned the highest priority for research to the study of growth and mortality of school prawns.
Final report
Information about growth and mortality are important in the management of resources because these provide us with an understanding about the productivity of the target population. Estimates are used to assess the impact of fishing upon the target population and the effectiveness of various scenarios in achieving the management objective which is usually the sustainable harvest of resources. The school prawn, Metapenaeus macleayi, is one of three target penaeid species of commercial and recreational importance in estuaries of NSW. It contributes around 64% by weight and 46% by value to prawn production in NSW and is harvested by three commercial fisheries; namely the ocean trawl (8% by weight of commercial landings), estuary prawn trawl (64 %) and the estuary general fishery (28%). Because all stocks could not be studied, we adopted the approach of choosing those that were expected to include the greatest variability about growth and mortality parameter estimates. Growth was investigated by doing monthly fishery independent surveys on the Clarence and Hunter Rivers. Monthly length frequencies were separated into groups of prawns of similar age and these data were then fitted to the Schnute growth models. Female prawn growth was best fitted by a special case of the Schnute model which is equivalent to the von Bertalanffy growth function (VBGF; L∞ = 36.6 and 40.2 CL mm and κ = 0.005 and 0.005 day -1, for Clarence and Hunter, respectively), whilst male growth was best fitted by a four parameter Schnute curve (L∞ = 21.3 and 33.5 CL mm and κ = 0.025 and 0.009 day-1, for Clarence and Hunter, respectively). Male school prawns grew to smaller maximum lengths and had faster rates of growth than females and lived for less than two years. While female growth data fitted the VBGF, much of the observed growth was linear and female prawns never reached the maximum lengths predicted by the growth model, probably because of high rates of mortality. Male growth differed between stocks but female growth did not.
