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 mobilc animals use the mangrove forests. \Me recorded the dlstnbution of juvenile banana prd\-\,ns Penaeus nierguiensis and of f~s h in an intertidal mangrove forest adjacent to a small creek In northern Australia in November 1992 and In h4arch 1993. Four discrete areas of the forest were enclosed w~t h a 100 m long, 2 mm mesh stake net 2 at the creek mangrove 11-inge and 2 at further distances into the mangroves. The mean distance of each sanlpling s~t e inland from the creek mangrove frlnge ranged from 13 to 59 m and the area of the sites ranged from 480 to 640 m< Two mangrove communities, one dominated by the structurally complex Rhlzophora stylosd, the other b? the more open Ceriops tagal were sampled. A large size range of juvenile prawns and small flsh 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-' and in hlarch was 334.5 prawns 100 m-'. Mean fish density over all samples was 83.0 fish 100 ni--' and mean fish biomass was 3.9 g m 2 ; 55 species of f~s h 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 a t the creek mangrove fringe site than a t 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 a t 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 predat~on by larger fish.

We explore the potential of using data from Australia's northern prawn fishery (NPF) vessel monitoring system(s) (VMS) to examine trawl track, trawling intensity, and stock depletion due to trawling. We simulate VMS data by subsampling global positioning system (GPS) fixes from the NPF fishing vessels at different polling intervals to examine their accuracy in describing trawl tracks. The results of the simulations suggest that VMS data with polling intervals longer than 30 min cannot accurately estimate trawl tracks. The analysis of high-polling-frequency VMS data collected in four (later reduced to three) 6 nautical mile × 6 nautical mile grids that historically received high levels of fishing effort showed that trawling was not random and some areas were trawled up to 28 times in the tiger prawn fishing season and the impact varied among years. The results of a catch-depletion analysis suggest that fishery catch-per-unit-effort and cumulative catch may not be proportional to overall target-species biomass in areas with highly aggregated trawl effort. The VMS data also showed a large number of trawls can occur in productive areas and that trawling impacts on benthos may be quite marked.

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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.