Benchmarking the ability of different stock-assessment models to capture the highly-fluctuating dynamics of small pelagics

Small pelagics dynamics are characterised by extreme variability owing to environmental factors, fishing and natural mortality. Because of highly-fluctuating dynamics, it is difficult to evaluate the stock status through models. To assess these evaluation difficulties, a model comparison framework based on the Management Strategy Evaluation (MSE) approach has been developed and tested in the Gulf of Cadiz anchovy stock. We have used a minimum realistic model (MRM) as operating model, including well documented environmental drivers for this stock to simulate abundance indexes and catches, and also a TAC value based on population size that works as a reference. Outputs from simulations were used as inputs for the implementation of a Gadget integrated model and some data limited methods. This simulation approach allows testing how well Gadget and data limited methods capture the highly-fluctuating dynamics of anchovy measured as the distance from the estimated TAC value (by different models) to the known reference. The results indicate that Gadget TAC estimate was closer to the reference compared with the other methods in all the simulations. This high estimation power of Gadget suggests its suitability for the stock assessment of other small pelagics. This work presents a measure of how well this model accounts for external sources of variability coming from the effect of the environment and a methodology that is flexible enough to be used with different models in other fisheries assessments.