Effects of misreporting landings, discards, and Catch Per Unit of Effort index in state-space production models: the case of black hake in northwest Africa

Recently, various state-space implementations of surplus production models (SPMs) have been developed for data-limited stocks. Often, catches and fishing effort are underestimated and discards are ignored. This results in biased estimates of stock status and reference points. Therefore, we conduct a sensitivity analysis for different under-reporting scenarios (due to non-declared landings, by-catch, and discards) on model estimates and thus advice for the black hake species in northwest Africa. Two modelling frameworks were used, namely a stochastic SPM in continuous time (SPiCT) and Just Another Bayesian Biomass Assessment (JABBA). A common set of diagnostics was developed to allow comparison across modelling frameworks. Scenarios correspond to hypotheses about misreporting and assumptions and priors that were kept consistent. The ratio of current fishing mortality over the fishing pressure that gives the maximum sustainable yield, F/F_MSY, is most affected by under-reporting. Results are sensitive to the prior assumed for the initial depletion level, B₀/K, and research is needed. If the misreporting is changing over time, relative quantities (e.g. F/F_MSY) and trends are biased, while if misreporting (or at least a part of misreporting) is constant, relative quantities are unbiased. Therefore, the nature of any trend in misreporting should be investigated.