Estimating biological parameters for penaeid restocking in an Australian temperate estuary

Abstract

Robust estimates of growth, mortality and reproduction provide fundamental information for evaluating release programs. Length frequency data and mixture analysis were used to estimate a suite of biological parameters for the Western School Prawn (Metapenaeus dalli). This was an iconic recreational species, which is being evaluated for restocking in the Swan-Canning Estuary in temperate, south-western Australia. Monthly length frequency data, collected from hand and otter trawls over 26 consecutive lunar cycles showed that M. dalli exhibits highly seasonal patterns of growth and reproduction. Growth occurred predominantly during the warmer months (October-March), with little to no growth in cooler months (May-August). A von Bertalanffy growth model, incorporating seasonal growth, estimated that female prawns grew significantly larger (L∞=33.6mm CL) than males (L∞=22.8mm CL), but that the rate of reaching the asymptotic size was the same for both sexes (K=0.98). Gravid females were found only from October to March and spawning activity was greatest from November to February, when surface and bottom water temperatures ranged from 20 to 28°C. The instantaneous rate of total mortality (Z) was greater for females (0.069 week−1≅3.57year−1) than males (0.043 week−1≅2.28year−1). Since fishing mortality is now very low, these estimates provide a close approximation to natural mortality (M). A similar approach was applied to estimate the growth parameters from the length distributions of M. dalli reported in this system 30 years earlier, when the population biomass was likely to be much higher than the current biomass and M. dalli was heavily exploited by recreational fishers. The maximum size and L∞ of M. dalli are now between 10 and 20% larger than 30 years previously, which may reflect the current lower fishing pressure and lower population biomass. From this study, the optimal release times for M. dalli are from December to March, when prawns grow rapidly and can be cultured successfully under current production systems.