Connectivity between juvenile and adult fish populations: do adults remain near their recruitment estuaries?

Abstract

Juvenile snapper Pagrus auratus (Sparidae) were collected from 15 estuaries during the recruitment season of 1998–99 and their otoliths were analysed to determine concentrations of chemical elements. Differences in elemental composition of juveniles were found among estuaries or groups of estuaries suggesting that the nursery or recruitment estuary of adult fish could be determined by analysing the juvenile region of adult otoliths. Adult fish from the commercial fishery in the vicinity of Sydney, New South Wales, Australia were then collected, their ages estimated and fish with birth years during 1998–99 selected for microchemical analysis to determine their recruitment estuary. Maximum likelihood analyses were used to determine the proportion of juvenile fish and adult fish from different regions (Wallis Lake, Sydney estuaries, Eden, and other estuaries). For juvenile fish, the actual composition ranged from 7 to 53% depending on the estuary or group of estuaries and the estimate of proportion of juveniles from the different estuaries ranged from 7.24 to 48.21% suggesting an error rate of <1 to 4.79%. Most (89%) adult fish caught as part of the snapper fishery in the Sydney region originated from local estuaries, although about 9% of fish had Eden as their recruitment estuary and 2% had come from the remaining estuaries excluding Wallis Lake, which contributed no fish. These results suggest that adults on reefs outside estuaries in the Sydney region have come from the estuaries closest to them with little transfer from other estuaries. For other regions, 2 scenarios are possible, namely, adults originated from estuaries close by with little transfer from other estuaries, or alternatively estuaries in the vicinity of Sydney supply the majority of adult fish along the coast. The latter situation is considered unlikely. Results show a link between juvenile and adult populations and suggest that populations of snapper may be self-sustaining or relatively closed.