A stock assessment model for Northeast Atlantic spurdog, incorporating fecundity data to estimate the extent of density dependence in pup production

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An exploratory assessment model for Northeast Atlantic spurdog (Squalus acanthias) is presented. The model is based on an approach developed for school shark (Galeorhinus galeus) off southern Australia. It is essentially age- and sex-structured, but is based on processes that are length-based, such as maturity, pup production, growth (in terms of weight) and gear selectivity, with a length–age relationship to define the conversion from length to age. Pup production (recruitment) is closely linked to the numbers of mature females, but the model allows deviations from this relationship to be estimated (subject to a constraint on the amount of deviation), and parameter estimates to be based on fitting fecundity data. The model fits to a combined Scottish groundfish survey index of abundance, and to proportion-by-category data from both the survey and commercial catches (aggregated across gears). The only estimable parameters considered are total virgin biomass (B0), Scottish survey selectivity-by-category (three parameters), commercial selectivity-by-category for two fleets (four parameters, two reflecting Scottish selectivity, and two from English and Welsh fisheries), pup-production parameters (up to three) and constrained recruitment deviations (1905–2005). The model assumes two commercial catch exploitation patterns that have remained constant since 1905, which is an oversimplification given the number of gears that take spurdog, and the change in the relative contribution of these gears in directed and mixed fisheries over time. This simplifying assumption allows stock dynamics to be taken back to near-virgin levels. The model estimates current depletion levels of around 5% relative to 1905, and 7% relative to 1955.