Structure and estimation framework for Atlantic Bluefin tuna operating models.

A preliminary spatial, multi-stock statistical catch-at-length assessment model is developed as a basis for defining operating models for Atlantic bluefin tuna. The modifiable multi-stock model (M3) aims to improve upon previous multi-stock models such as MAST (Taylor et al. 2011) in three core areas. The first iteration of the model: (1) makes use of indices of abundance specific to time-area strata (e.g. for a given ocean area and month of the year), (2) does not use conventional tagging data to inform exploitation rates, (3) is fitted to samples of length composition data and therefore avoids established problems related to ageing individuals based on a growth curve and length data only. In this paper we provide a full account of preliminary M3 model equations and discuss the results of simulation evaluations of model estimation performance. Limitations of the current approach and future areas for model development are also discussed