Incorporating specific change points in catchability in fisheries stock assessment models: An alternative approach applied to the blue shark (Prionace glauca) stock in the south Atlantic Ocean

Abstract

Fishermen frequently switch their target fish species without documenting changes in which species they are targeting and the fishing practices used, generating misleading catchability information about the fish caught. To date, changes in target species have been incorporated in stock assessments at two different levels in analyses. First, these changes are taken into account during the parameterization of generalized linear models used to compute the CPUE index standardization. Second, changes in target species are directly incorporated as a time-varying catchability parameter during the fitting of the dynamic model used for the assessment. Here, we present an alternative method for this incorporation by specifying a single change point in the stationary distribution of the catchability coefficient in a Bayesian state-space production model. Two models were fitted to the time series of the south Atlantic blue shark (Prionace glauca) stock. In one of the models, only one catchability coefficient was estimated. In the other model, a change point was included, and two catchability coefficients were estimated, one before the change point, and the other after. Despite the latter model introducing an extra parameter, it produced a significantly better fit than the modeling approach without the change point. Although including a single change point in the catchability coefficient had no significant impact on the status of south Atlantic blue shark (which is still above BMSY), it provided a robust way of accounting for changes in catchability as a result of fishermen changing target species.