There is a widely-held paradigm that mangroves are critical for sustaining production in coastal fisheries, through their role as important nursery areas for fisheries species or as a source of outwelling of nutrients. There is also an assumption that the area of mangrove habitat in an estuary translates to production m commercial fisheries and this is supported by studies that have found positive correlations between the extent of mangroves and fisheries catch. However, this approach does not take into account the underlying biological mechanisms of potential mangrove-fisheries links and does not identify cause-and-effect relationships. Without a biological basis, the validity of the correlations and their value for predicting fisheries production in relation to changes in mangrove distribution is questionable. Much of the biological information necessary for evaluating the strength of the paradigm is currently lacking. The primary objective of this thesis was to evaluate mangrove-fisheries links by investigating ways that biological information can be incorporated into these evaluations.
An assessment of distribution and change in mangroves showed that spatial and temporal pattern metrics and change detection analyses were useful measures of mangrove characteristics over a range of spatial scales: local scale (10s km - e.g. the Logan, Fitzroy, Mission and Embley rivers in Queensland and the Berkeley River in Western Australia), a small-regional scale (10 to 100 km - e.g. Moreton Bay, southeast Queensland) and broad scale (1000s km - e.g. Queensland east coast). Change detection analyses demonstrated that, over time, mangrove distribution and extent were more dynamic than indicated by the estimates of total area that have generally been used in the past. The ability to quantify spatial and temporal patterns of mangroves is a necessary first step before integration with fisheries data, and spatial metrics such as area, perimeter and mangrove-water interface length were identified as important parameters to be included in subsequent modelling of mangrove-fisheries links.
Links between mangrove extent and commercial fisheries catch of individual species were modelled and a layer of biological information was added to the models by taking into account the species' life-histories. Strong correlations were found between mangrove extent and the catch of three species that are known to have strong links to mangroves at some stage of their life history (banana prawns Penaeus merguiensis, mud crabs Scylla serrata and barramundi Lates calcarifer). The correlations between mangrove extent and the catch of estuarine (but not necessarily mangrove-related) species (tiger prawns Penaeus esculentus and P. semisulcatus, blue swimmer crabs Portunus pelagicus and blue threadfin Eleutheronema tetradactylum) were much weaker. Furthermore, there was no correlation at all between mangrove extent and the catch of an offshore species group (coral front Plectropomus spp.).
The contrast in the findings for the three life-history categories strengthened the case for causal links between mangrove habitat and fishery production for some species but information was still lacking as to what the underlying biological processes were. If the proposed mangrove-fisheries links are mediated by the use of mangroves as nurseries, then the relationship between mangrove characteristics and juvenile populations should provide a link to fisheries production. However, a detailed investigation of the abundance of juvenile banana prawns in five rivers in tropical and subtropical Australia revealed no clear patterns between the extent of mangroves and juvenile prawn production. These analyses identified high levels of variability in prawn catches between sites within a system and emphasised the need for broad-scale surveys of systems before any attempt to identify the causal basis for links between mangroves, juvenile abundance and fisheries catch.
This thesis demonstrated that steps can be taken towards incorporating biological data into the evaluation of mangrove-fisheries links, but that the process is complex and current data collection methods are inadequate. Validating correlation studies using biological data represents a new approach to evaluating mangrove-fisheries links that can be applied to fixture monitoring of other fisheries in different regions. This approach can be used in predicting the potential impacts of change in mangroves on fisheries and provides vital information for addressing some of the critical questions in fisheries and habitat management.