Emigration of juvenile banana prawns Penaeus merguiensis from a mangrove estuary and recruitment to offshore areas in the wet-dry tropics of the Gulf of Carpentaria, Australia

Abstract

Four yr sampling (1975 to 79) of emigrating juvenile Penaeusmerguiensisat the mouth of the Norman River estuary in the southeastern Gulf of Carpentaria clearly demonstrated the importance of rainfall to the emigration processes of this penaeid prawn. Most emigration occurred during the wet summer months (Dec to Mar) when 80 % of annual precipitation occurred. The majority of prawns were caught within 0.5 m of the water surface across the width of the estuary. Marked tidal and die1 rhythms in emigration were observed, the two interacting to give different patterns of emigration depending on whether the hours of darkness coincided with the ebb tide. Emigration was extremely variable over short time scales (weeks) and was affected by the rainfall of the previous 3 wk, the number of resident juvenile prawns and the phase of the neap/spring cycle. When results were pooled over longer time scales (months and years), the effects of tide phase and luvenile population numbers became less important. Rainfall alone accounted for over 70 % of the observed variation in monthly emigration rate. The size at which prawns emigrated also depended on rainfall and estuarine salinity and was inversely related to the number of emigrants. In wet years, a large number of prawns of all sizes emigrated, whereas in dry years, only a few larger prawns migrated. Recruitment to the offshore fishery occurred approximately 8 wk after the prawns left the estuary. Over the 4 yr of study, the inter-annual differences in rainfall and prawn emigration from the Norman River were reflected in the commercial catch of P. merguiensis in the southeastern Gulf.