Spawning Stock-recruitment relationships and management of the penaeid prawn fishery in Shark Bay, Western Australia

James W. Penn

Previous management plans for the important Western Australian penaeid prawn fisheries (Bowen and Hancock, 1984) have been based on the assumption that the economic restraints on trawl fishing would make it unlikely that overfishing of penaeid stocks could occur (Neal, 1975). However, a more recent analysis of data for the Shark Bay and Exmouth Gulf fisheries during the early 1980s reported in this thesis indicate that this widely accepted assumption is no longer valid for all penaeid stocks. The present study addresses the fundamental fisheries biology and management issue of the relationships between spawning stock and recruitment for the western king (Penaeus latisulcatus) and tiger prawn (Penaeus esculentus), the two major penaeid species in the Shark Bay fishery. A detailed investigation of the reproductive biology involving the y development of population fecundity measures for western king and tiger prawn stocks is reported, which enable the historical fishery data base to be used to provide measures of past spawning biomass levels. Data on the location, timing and abundance of recruitment of each species to the commercial trawl grounds during recent years has been used in conjunction with historical data to produce indices of annual recruitment for each stock in past years. Assessment of the relationships between spawning stock and recruitment indices has demonstrated that within Shark Bay there is a clear spawning stock recruitment relationship (SRR) for the tiger prawns, but not for the more abundant western king prawns. Using multiple regression techniques, the effective effort (or fishing pressure) has been shown however, to have had a significant impact on the recruitment-spawning stock relationship (RSR) for both species in the fishery. A recruitment model incorporating both SRR and RSR relationships (after Garcia 1983) has been used to demonstrate that prespawning fishing pressure has been the most likely cause of the severely reduced recruitment and catch of tiger prawns since 1979. Fishing effort could not be shown to have had any long terra effect on recruitment to the western king prawn stock. The results from a concurrent study of the adjacent Exmouth Gulf fishery strongly supports this assessment. A comparative study of the burrowing behaviour of the two species in aquaria and an analysis of fishery data support the hypothesis that differences in catchability have been a major contributing factor in the different responses of the species to fishing pressure. The longer period of fishing between the time of recruitment and the onset of spawning, has also been identified as a factor increasing susceptibility of the tiger prawn stock to the impact of fishing. A further relationship between recruitment index, recruitment catch and fishing effort for tiger prawns has been provided and integrated with data on equilibrium recruitment/effort levels from the SRR/RSR model. The resulting equilibrium recruitment-effort curve for the tiger prawn stock and the surplus yield curves (GENPROD) for both species have been combined to provide a basis for future management of the fishery. A number of management strategies to facilitate the recovery of the Shark Bay tiger prawn stock have been proposed and the wider implications of this second documented case of recruitment overfishing of a penaeid stock are discussed.

Spawning Stock-recruitment relationships and management of the penaeid prawn fishery in Shark Bay, Western Australia

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