The potential for improved management performance with increased age-based stock assessment components: extension of the management strategy simulations to incorporate age-based assessments

Background

Fisheries targeting long-lived, slow-growing species such as snappers and emperors are an important source of employment and protein in many tropical countries. However, these species are vulnerable to overexploitation, and appropriate management is required. Historically, management has been based on the use of length-based growth parameter estimates, which are highly uncertain for these species.

Previous FMSP project R6465 showed that management performance improved using age-based methods of growth parameter estimation. However, the improvements resulting from the use of more accurate age-based growth parameter estimates were diluted by the subsequent use of length-based methods later in the stock assessment process. It was suggested that a greater use of age-based approaches in the stock assessment process could improve management performance further.

Approach

The project derived guidelines for the management of demersal tropical fisheries, through examining whether management performance of long-lived, slow-growing fish species improved using age-based methods of total mortality estimation. This was achieved by using management strategy simulations to compare the performance of management based on age-based total mortality estimates with that based on length-based methods of mortality estimation (using length-converted catch curves). The Yield software developed under FMSP project R7041 was used to conduct the simulations.

The comparison of management performance was assessed against conservation measures and fleet performance. Guidelines for management were developed based on the outputs of the simulation studies.

Findings

The outputs from the management strategy simulations were used to develop stock assessment and management guidelines for fisheries in developing countries targeting long-lived, slow-growing species. In addition to the use of age-based growth parameter estimation methods for these types of species, as recommended by FMSP Project R6465, the current project recommended the use of age-based methods of total mortality estimation. This resulted in improved management performance compared to the use of length-based total mortality estimates.

Despite the improvements in management performance gained by using age-based growth and total mortality estimates, considerable uncertainty in management remained. It was suggested that the use of empirical estimates of natural mortality (using Pauly's or Ralston's formulae) was a possible cause. Thus a further recommendation of the project was to derive an independent estimate of natural mortality where possible.