Frequently, in developing countries, length-based methods of stock assessments are used. However, these have been shown to be inappropriate for long-lived, slow-growing 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.
A previous FMSP project (R6465) found that length-based methods of estimating growth and mortality were inappropriate for long-lived, slow-growing species, and that the use of these estimates in stock assessments led to very poor management performance. Age-based growth estimates performed better, but their use in subsequent length-based assessments diluted the benefits. Follow-on project R7522 showed that using age-based estimates of total mortality improved management performance further.
As has been requested by two partner institutions for R6465, there is a need for advice on whether full age-based approaches (using virtual population analyses) are more appropriate, not only for slow-growing species, but those with different life history strategies.
This desk-based project used management strategy simulations to determine whether age-based or length-based methods of stock assessment are more appropriate for contrasting species types. A moderately fast-growing rabbitfish Siganus sutor, and a slow-growing emperor Lethrinus mahsena were modelled, both species of importance to artisanal fishermen. Simulations were run over a 20-year period, from a range of starting fishing mortalities, simulating lightly to heavily exploited fisheries. Management, simulated to aim for a target fishing mortality of F0.1, was evaluated at the end of the period using three performance indicators, relating to stock conservation and fleet performance.
In addition to the simulation studies, a review was conducted of the use of length-based and age-based methods of assessment in developing countries, the characteristics of the fisheries in which they were used, and the biological characteristics of the fish species targeted. The review helped gauge the relative importance of each stock assessment method currently being used.
The results showed that using fully age-based methods of assessment led to improved management performance compared to fully length-based assessment methods. This was true for both moderately short-lived, fast-growing and long-lived, slow-growing species. For long-lived, slow-growing species, however, fully age-based methods became less effective at very high levels of fishing mortality, and showed little or no improvement over length-based methods. Under these circumstances, it is important to obtain good length-at-age samples to estimate growth parameters and natural mortality.
The review of analytical models found that the most important factors in determining which type of method was used were GNP and latitude (which were positively correlated). The use of age-based parameters appears currently limited to richer nations and important industrial fisheries, both generally found in higher latitudes.
An increasing number of tropical fish species are being validated for ageing studies. The results of this study show that fully age-based assessment methods are more appropriate. Further advances in ageing techniques and validation, and the development of age-otolith weight relationships, which will allow cheaper and faster ageing of large numbers of fish, should encourage more developing countries to use these age-based methods in their fisheries.