Report of the second horse mackerel otolith reading workshop
Age estimation of commercially exploited fish species in the Benguella region. Stocks of the two species of horse mackerel that occur in the Benguela region, namely Cape (Trachurus trachurus capensis) and Cunene horse mackerel (Trachurus trecae) are being exposed to increasing levels of commercial exploitation, highlighting the requirement for effective management measures to ensure their sustainable utilization. Such measures can only be implemented with confidence if they are based on reliable stock assessment procedures. Sizestructured stock assessment models have been found to be unsuitable for Southern African horse mackerel stocks, and the decision has recently been made to move to an age-structured VPA approach. A fundamental requirement for this approach to stock assessment is the availability of robust information concerning the age structure of the population(s). This requirement has been highlighted in the BENEFIT Science Plan, and more recently during the International Workshop on the Research and Management of Horse Mackerel in Namibian Waters convened in Swakopmund, Namibia (26 – 30 March, 2001). The lack of representative age length keys for horse mackerel in the Benguela has been identified as a major shortcoming of current stock assessment models employed in the region (the age length key currently used for Namibian horse mackerel dates back to 1987). To overcome this obstacle, effort has been directed towards developing an age estimation program for horse mackerel using periodic features in sagittal otoliths. The first step in implementing such a program is to develop the best method of preparing, examining and analyzing otoliths. Once the method been developed, it has to be validated. The validation process is comprised of two phases, the first testing the accuracy of the age estimates obtained using the method, and the second testing the precision and consistency with which researchers obtain the age estimates. The first phase of validating age estimation from horse mackerel otoliths is the subject of an otolith-marking project currently being conducted at the Marine and Coastal Management Research Aquarium in Cape Town, South Africa, under the auspices of BENEFIT.