How far do prawns and fish move into mangroves? Distribution of juvenile banana prawns Penaeus merguiensis and fish in a tropical mangrove forest in northern Australia

It is accepted that mangroves are important nursery areas for prawns and fish, including some of major commercial importance, but Little is known about how these mobile animals use the mangrove forests. We recorded the distribution of juvenile banana prawns Penaeus merguiensis and of fish in an intertidal mangrove forest adjacent to a small creek in northern Australia in November 1992 and in March 1993. Four discrete areas of the forest were enclosed with a 100 m long, 2 mm mesh stake net: 2 at the creek mangrove fringe and 2 at further distances into the mangroves. The mean distance of each sampling site inland from the creek mangrove fringe ranged from 13 to 59 m and the area of the sites ranged from 480 to 640 m(2). Two mangrove communities, one dominated by the structurally complex Rhizophora stylosa, the other by the more open Ceriops tagal were sampled. A large size range of juvenile prawns and small fish moved at least 43 m into the mangroves at high tide, and the density of prawns near the creek mangrove fringe was inversely related to the maximum tide height. The highest density of juvenile P. merguiensis recorded in the mangroves in November was 18.3 prawns 100 m(-2) and in March was 334.5 prawns 100 m(-2). Mean fish density over all samples was 83.0 fish 100 m(-2) and mean fish biomass was 3.9 g m(-2); 55 species of fish were caught during the sampling. P. merguiensis showed no apparent preference for either of the 2 mangrove communities sampled; however, more fish (101 fish 100 m(-2)) and more fish species (26) were caught at the creek mangrove fringe site than at the other more inland sites; the lowest numbers of fish (27 fish m(-2)) and species (13) were caught at the inland site (Ceriops). On average, fish caught at the fringe site were also longer and heavier than fish caught at the other sites. By moving well into the mangrove forest, prawns and small fish are probably less vulnerable to predation by larger fish.