Ecology of juvenile and adolescent banana prawns, Penaeus merguiensis, in a mangrove estuary and adjacent off-shore area of the Gulf of Carpentaria. II. Emigration, population Structure and Growth of Juveniles

Abstract

Emigration of juvenile P. merguiensis from the Norman River was largely confined to the wet season; three main waves of emigration were observed, each corresponding to a period of heavy rain in December, January, and February. Following each period of rain, the prawns migrated in the surface waters of the estuary and became spread laterally across the river's width. The size of emigrating prawns decreased over the wet season, reflecting changes in both the size of prawns in the resident population and the strength of the stimulus to emigrate. During periods of low rainfall only a small number of larger prawns tended to migrate, whereas during high rainfall periods a mass movement of prawns of all sizes occurred. Examination of weekly changes in population structure showed that three main cohorts ofjuvenile prawns entered and left the river during the spring-summer period (November-February). The resident time of these prawns ranged from 1 to 9 weeks, depending on the timing of heavy rain during the period. Three smaller cohorts reached the river during March and April and the majority of these prawns did not emigrate but overwintered until the following spring. Growth during the summer period averaged an increase in carapace length of 1.2 mm/week, whereas growth in winter was slower in response to lower temperatures. After leaving the river, adolescent P. rnerguiensis spent a short time in the shallow beach zone near the mouth of the river before migrating off shore. The migration up until the time the prawns recruited into the off-shore fishery was estimated to take approximately 2 months. During this time the maximum growth was estimated as 15-mm increase in carapace length. The significance of these results is discussed in relation to the life history of the prawn in the south-eastern area of the Gulf of Carpentaria.