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Assessing the potential for restocking the western school prawn Metapenaeus dalli in a temperate Australian estuary

Broadley, A. (2014) Assessing the potential for restocking the western school prawn Metapenaeus dalli in a temperate Australian estuary. Honours thesis, Murdoch University.

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The population dynamics of restocking involves trade-offs between different release scenarios and the conservation of wild stocks and evaluating these trade-offs is an important component of restocking programs. This study used length frequency data and mixture analysis to estimate growth, mortality and reproductive parameters for the western school prawn Metapenaeus dalli in the Swan-Canning Estuary. These biological parameters provide the first quantitative estimates of growth and mortality for M. dalli and were used as inputs into a bioeconomic model developed with the EnhanceFish software used to evaluate the effectiveness of different stocking levels and sizes-at-release. This research had two primary objectives:

1) To use the data from a spatially and temporally comprehensive sampling program for M. dalli in the Swan-Canning Estuary to determine the biological parameters for growth, mortality and size at maturity; and
2) Develop a bioeconomic model using the EnhanceFish software to evaluate the potential increase in population biomass from restocking with different numbers of released prawns and different sizes-at-release.

Monthly length frequency data demonstrated that M. dalli exhibits strong seasonal growth and reproductive cycles. Growth was estimated using Somers (1988) seasonal adaptation of the von Bertalanffy growth curve. This analysis demonstrated that females grew significantly larger (L∞ = 33.72 mm CL, k = 1.06) than males (L∞ = 24.20 mm CL, k = 0.97). Gravid females, or females carrying a spermatophore, were only found between November and April, when surface and bottom water temperatures were between 20 and 27 ºC. Pauly’s (1983a, b, 1984) seasonal catch-curve revealed little difference in total instantaneous mortality between female and males (Z = 0.048 and 0.047 week-1, respectively). Whereas Pauly’s (1990) non- seasonal catch-curve showed a larger range in mortality between females (0.067 week-1) and males and (0.052 week-1). Since recreational fishing effort and thus fishing mortality is very low, these estimates provide a close approximation for natural mortality (M) in the population. The patterns of growth and reproduction were compared with those recorded in this system ≈ 30 years ago by Potter et al. (1986), who found a similar seasonal pattern of growth. However, the maximum size of prawns recorded in that study were about 10% smaller and the estimates of L∞ about 20 % smaller than those from the current sampling. These differences indicate that the far greater fishing pressure in this earlier period may have selectively removed larger prawns from the population.

The release scenarios evaluated by the bioeconomic model varied from 650,000 to 5 million prawns (i.e. 650,000, 1 million, 2 million and 5 million) and the size-at-release from 1 mm carapace length (CL) to 10 mm CL. The greatest potential returns were obtained when 5 million prawns were released at a size of 10 mm CL, however, such an aquaculture effort would require substantial capital expenditure in order to produce the required number of juveniles. The model results also highlighted that density-dependent processes are likely to be important and decrease the relative effectiveness of releases. At the current low population level and without any restocking, the population biomass was projected to remain virtually unchanged over a five-year period. The model results also highlight the need for empirical information on natural morality, the influence of density on growth and survival and how time-at-release might influence survival. These results improve our biological understanding of M. dalli and will be used to help plan any future restocking of this species in the Swan-Canning Estuary.

Publication Type: Thesis (Honours)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Loneragan, Neil and Tweedley, James
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