In many tropical demersal fisheries, such as that within the British Indian Ocean Territory (BIOT/Chagos Archipelago), length-based methods of stock assessment are used to inform management. However, previous project R6465 showed that these methods are not appropriate for long-lived, slow-growing species. That project developed guidelines for the assessment of demersal tropical fisheries, which included promoting the use of age-based methods for growth parameter estimation for long-lived, slow-growing species.
The management authority for BIOT, the UK Foreign and Commonwealth Office, was keen to implement these guidelines for management of the BIOT inshore fishery. Otoliths (fish hard parts) had already been collected during the annual observer programme, which allowed fish ageing to be undertaken.
This short follow-on project implemented the guidelines developed under project R6465 in the inshore fishery of the British Indian Ocean Territory (BIOT).
The guidelines promote the use of age-based growth parameter estimates, which was achieved by examining the otoliths collected to determine fish ages. Using these improved parameter estimates, the project derived biological reference points to be used for management. Total mortality was estimated using catch curves, and natural mortality derived by relating the estimates of total mortality to the level of fishing effort (rather than empirically, for example using Pauly's formula). Stock assessments were then performed by comparing the current level of fishing mortality (ie, total mortality minus natural mortality) to the chosen management reference effort level.
The project assessed whether the guidelines developed in project R6465 were appropriate for use in BIOT, and determined the utility and cost of moving towards an age-based methodology. Overall, the project indicated that it is feasible to employ age-based methods to assess the impacts of fishing on exploited populations of demersal reef fish within BIOT. As such, the outputs from this project have relevance for other locations in the tropics, and will assist institutions in assessing the potential for applying these methods locally.
Further guidelines were developed through the implementation of this project. These included the use of age-based methods to estimate total mortality. The project successfully derived an age-otolith weight relationship for one of the two study species. The cost-benefit analyses performed indicated that where an age-otolith weight relationship is available, otolith weight could be used as a proxy for age, thus reducing the costs of the age-based assessments. Where this age-otolith weight relationship is not available, the most cost-effect approach may be to estimate age-based growth parameters, and then use length-based methods of total mortality estimation.