Growth parameter estimation and the effect of fishing on size composition and growth of snappers and emperors - implications for management

Background

Fisheries targeting snappers and emperors are important in tropical countries as sources of employment and protein. However, the long-lived, slow-growing nature of these species renders them vulnerable to over-fishing. Sustainable exploitation through accurate management is required. However, the length-based methods for growth estimation commonly used in the tropics have been shown to be inappropriate for these types of species. Age-based methods are considered to be more reliable, but are used less frequently in the tropics due to the cost, and perceived difficulty in ageing tropical fish.

There is therefore a need to evaluate whether the use of age-based estimates of growth parameters leads to more appropriate management advice than the use of length-based methods.

Approach

The project developed advice for the assessment and management of fisheries targeting long-lived, slow-growing species. It involved both a practical study, and desk-based assessments and simulations. Firstly, it validated the use of hard parts (otoliths) collected from Indian Ocean study locations for ageing two tropical fish species. Simulations were then performed to examine the effects of fishing on estimates of growth parameters derived using both length and age-based methods, and the effects of the subsequent use of these estimates in further assessments. The effect on management performance of uncertainty in growth parameter inputs was simulated, comparing age-based and length-based assessment methods.

Assessments were performed for the study fisheries, to validate the simulation findings. Finally, a cost-benefit analysis was undertaken to determine whether the additional costs required for age-based assessments were compensated for by increased yields and hence revenue.

Findings

The project developed guidelines for the data collection, assessment and management of fisheries targeting long-lived, slow-growing species (see Final Technical Report).

It successfully validated the use of otoliths for ageing two species common in tropical latitudes, helping to dispel the view that otoliths cannot be used to age tropical fish species. Local staff at collaborating institutions were trained in the preparation and ageing of study species.

The simulations indicated that age-based methods for assessment of growth parameters were both more accurate and precise than length-based methods. Subsequent management simulations indicated that uncertainty and variability in growth parameter estimation led to both age-based and length-based assessment methods performing poorly, although the age-based methods were better. Even where accurate age-based growth parameter estimates were used, the subsequent use of length-based components in the assessment process diluted the benefits of using accurate growth parameter estimates. It was concluded that the management performance of fully age-based assessments should investigated. Despite the uncertainty, the cost-benefit analysis indicated that age-based growth estimation methods are the most accurate and cost-effective for the study species.