Comment on “A new approach for estimating stock status from length frequency data” by Froese et al. (2018)

Abstract

Abstract Potential users of the model proposed by Froese et al. (2018) should be aware of several issues. First, the method to calculate equilibrium numbers-at-length is incomplete and leads to negatively biased estimates of fishing mortality. Second, inadequate simulation testing fails to reveal that the method is highly sensitive to assumptions of equilibrium conditions and that the population average asymptotic length (L∞) can be approximated by the largest observed size (Lmax). Finally, the Froese et al. (2018) model relies on the assumption that the ratio of natural mortality (M) to the von Bertalanffy growth parameter (K; M/K) is typically around 1.5, which they argue is supported by the literature for most fish stocks. We believe that this conclusion is based on an insufficient reading of the literature and, in fact, there is strong evidence to support the claim that M/K is outside the narrow bounds of 1.2–1.8 for many exploited species. Potential users of the method are alerted to these issues and alternative approaches are recommended to avoid these biases.