Applying biomass dynamic models to the southern horse mackerel stock (Atlantic waters of Iberian Peninsula). A comparison with VPA-based methods

Abstract

The horse mackerel (Trachurus trachurus) fishery of the Northeast Atlantic, is currently subject to assessment and management in the ICES area. Assessment of the Southern horse mackerel (ICES Divisions VIIIc and IXa) is made with Virtual Population Analysis (VPA), using time series of catch-at-age data and catch-per-unit-of-effort (CPUE) from 1985 to present. The application of biomass-dynamic models to the assessment and catch prediction of this stock has never been attempted before. In this paper, a production model is applied to the Southern horse mackerel stock. To quantify uncertainty in parameter estimates, bootstrap confidence intervals were computed, which showed that estimates could be considered as reliable. The bootstrap standard deviations of F_t, r, MSY and F_MSY were relatively small, despite the lack of trends in the available effort series. On the contrary, the catchability, q, showed a coefficient of variation of 74%. The current level of fishing mortality for 1998 was considered as unsustainable for the resource, being well above F_MSY according to the biomass-dynamic models, and above F_pa according to the age-structured model. Both models were in close agreement on the time series of fishing mortality and in the perception of the state of the stock. Differences existed in the changes in stock biomass especially over the last years. Over this time period the age-structured model showed an increasing trend in biomass. The estimates of MSY and F_MSYwere in accordance with the precautionary approach philosophy. The biomass-dynamic model used here was usefully applied to the Southern horse mackerel stock, giving complementary information to the age-structured model, both in the perception of the state of the stock and in the definition of management targets.