Methods for monitoring the abundance of the Northern Australian mud crab Scylla serrata

The final outcomes of the 1996 Northern Territory mud crab (Scylla serrata) fishery assessment recommended the development of alternate methods for monitoring the abundance and habitat of this species; citing difficulties with analyses of simple CPUE data. In particular the assessment team recommended the testing of a model where: Abundance = area of critical habitat x density of animals per unit of habitat Determination of the suitability of this model required the estimation of two parameters: the area of each critical habitat type and the abundance of crabs per unit of critical habitat type. Consultation with fishery stakeholders, review of historical fishery information and a survey of the literature provided information on the key habitat associations of the mud crab. The two habitats of primary interest, mangroves and mud flats, are generally remote and difficult to access in northern Australia. An existing regional wetland dataset collected using remote-sensing techniques (Landsat ETM+ imagery) was identified and additional analyses were completed providing estimates of area of mangrove, mangrove lined waterways, and mud flat foreshore for each fishing grid. Methods to estimate mud crab abundance within habitat type were reviewed for fitness against species behaviour and environmental conditions. Due to the dynamic nature of this fishery my initial work focused on gaining an understanding of the potential uncertainties in meeting the analysis assumptions. Preliminary studies were undertaken to test various assumptions and uncertainties, such as environmental and biological effects on catchability. An examination of crab movement patterns during short term study periods demonstrated little evidence of large scale movement and a general acceptance of the assumption of a closed population. Mark recapture techniques previously utilised in terrestrial animal abundance estimation were applied for the first time in a marine environment. A new study approach incorporating depletion and mark recapture methods was utilised for mangrove-lined streams and a novel trapping web design for the foreshore mudflat areas. The biomass estimates obtained from these studies were then assessed against four commonly used fishery assessment metrics, commercial catch, commercial effort, commercial CPUE, fishery independent CPUE. Commercial CPUE performed poorly and fishery independent CPUE data partially supported observed trends in the catch. These results provide support that CPUE data performs poorly in assessment of this fishery. Biomass estimates obtained from the mark recapture studies demonstrated very similar temporal trends at both study sites providing good support for the fishery independent methods tested in this project.