Penaeid prawn fisheries are a highly valuable resource in Australia and many other regions, including the Gulf of Mexico, South America, Africa and the Indian Ocean. In the past it was generally considered that recruitment in penaeid fisheries was largely a function of environmental influences. For many years there was no evidence of a relationship between spawning stock size and recruitment. When this was considered with the fact that penaeid prawns are highly fecund, it was widely considered the group was not susceptible to recruitment overfishing. In the mid 1980s, Western Australian researchers published what is considered to be die first statistically robust evidence of a relationship between spawning stock size and recruitment in a penaeid prawn fishery. Since then additional evidence has been demonstrated in the southwestern Gulf of Mexico and precautionary measures, designed to prevent recruitment overfishing, have been introduced in other major penaeid prawn fisheries.
These recent developments clearly indicate that penaeid prawn spawning stocks can be overfished. It is therefore necessary to identify critical components of penaeid spawning stocks, and to determine the relationships between spawning stock size and recruitment, for effective management. This thesis addressed these needs for die commercial penaeid prawn stocks in southeast Queensland coastal waters.
Spawning stock dynamics and recruitment processes for the three main commercial penaeid prawns differed markedly. Penaeus esculentus was concluded to be the most prone to recruitment overfishing because it displayed a short annual spawning period that was dependent upon few, adult females. Size at maturity, and selectivity of the mesh used by fishers in Moreton Bay, resulted in a high retention of P. esculentus spawners in commercial catches, which was also likely to attribute towards overfishing. Geographic variation in P. esculentus spawning stocks indicated the likelihood of recruitment overfishing increased with latitude. Egg production by Metapenaeus bennettae was 20-30 times higher than that of P. esculentus and extended over several months each year. Metapenaeus bennettae was capable of reproducing at sizes smaller than those retained by the commercial fishing gear. These characteristics of the spawning stock indicated the likelihood of recruitment overfishing M bennettae was low.
Penaeus plebejus produced eggs all year-round in comparatively deep, oceanic waters. Females ranging in size from 35-48 mm CL were responsible for most the population's egg production, but no obvious temporal or spatial spawning patterns were detected. This was despite the fact that timing of recruitment by P plebejus to the fishing grounds was succinct and limited to only 2-3 months each year. Effective spawning periods were identified, and some generalisations about the spatial distribution of egg production were made, for each species. The effective spawning period for P. plebejus was largely deduced from a knowledge of the annual timing of recruitment, and juvenile growth rates. Temporal patterns in recruitment were consistent with, and corroborated the effective spawning periods for M bennettae and P. esculentus.
Results were used to develop spawning stock and recruitment indices for each species, based on Iogbook catch rate data. An experiment to test hypotheses pertaining to short-term cyclic variation in abundance and reproductive condition (critical components of the spawning stock index) of aduh P. plebejus revealed significant lunar and diel effects. Catch rates of adults varied within lunar cycles by up to 2-3 fold. Ovary condition also varied significantly with lunar phase. These results were used to refine the spawning stock index for P. plebejus and should be considered in order to minimise spawning stock measuremoit error.
Characteristics of P. esculentus' spawning stock, and preliminary examination of the stock-recruitment relationship for this species in southeast Queensland indicated it was recruitment overfished. To remedy this situation, a model of the Moreton Bay fishery was deployed to simulate the prawn populations under exploitation and to identify the optimum time of year (January- December) for a one-month fishery closure designed to maximise egg production. The model considered many of the population parameters determined in previous chapters, as well as current size class market values, so that the effects of the various closure scenarios on the fishery's value could also be determined.
Results of the simulations indicated that maximum increase in egg production would be obtained by closing the fishery during the peak of P. esculentus' spawning period (October). However, the loss in revenue incurred closing the fishery at this time is likely to be unacceptably high to fishers because it would prevent them from capturing large numbers of M bennettae and P. plebejus that recruit in October. A second, slightly less effective closure could be introduced at the end of P. esculentus' annual recruitment period (May) at a fraction of the cost. This exercise demonstrated how simulations can be used to further study penaeid spawning stock dynamics and to determine optimal closure timing. The general approach could be applied in other similar fisheries to mitigate the effects of overfishing.
The thesis has provided insight into the spawning stock and recruitment dynamics of the commercial penaeid prawns in southeast Queensland. Recruitment overfishing, and its associated detrimental effects now have a much greater chance of being detected, avoided and/or reduced. Indices of the fishery's spawning stock and recruitment can be measured and monitored with greater accuracy. Long term trends in these indices will provide information on the status of the fishery and eventually a clearer understanding of the relationship between spawning stock size and recruitment.