Preliminary spatially disaggregated stock assessment of northern hake, a widely distributed stock of the north-east Atlantic

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European hake (Merluccius merluccius) is widely distributed over the north-east Atlantic shelf, from Norway to Mauritania. Despite its large distribution (covering ICES Division IIIa, Subareas IV, VI and VII and Divisions VIIIa,b,d, from Norway to the Bay of Biscay) and a complex population structure, the so-called northern stock of hake is currently assessed as a single unit, using a spatially aggregated implementation of the Stock Synthesis assessment model. In this study, a preliminary spatially explicit implementation of Stock Synthesis is developed, which accounts for the distribution of the hake population during its life cycle (nurseries and spawning areas) and the distribution of the fleets (trawlers, gillnetters and long-liners) exploiting the stock at various life stages. By explicitly incorporating spatial structure, the model can make consistent use of survey indices that only cover part of the stock distribution area. The model is fitted to commercial fishing data (landings, discards and quarterly size composition) and survey indices corresponding to the geographical areas defined in the model. Parameter estimation is carried out using the maximum likelihood estimation approach. Little information is currently available on exchange rates between main distribution areas and population structuring, so several hypotheses are tested. Implications in terms of stock assessment and management are discussed.