European hake (Merluccius merluccius) is widely distributed over the Northeast Atlantic shelf, from Norway to Mauritania, with a larger density from the British Islands to the south of Spain (Casey and Pereiro 1995) and in the Mediterranean and the Black Sea. Although, as demonstrated by several studies (Castillo et al. 2005)(Pita et al. 2011)(Pita et al. 2013)(Korta et al. 2015), there is no evidence of multiple populations in the Northeast Atlantic, ICES assumes since the end of the 1970s two different stock units: the so-called the Northern stock in Division 3.a, Subareas 4, 6 and 7 and Divisions 8.a, b, d, and the Southern stock in Divisions 8.c and 9.a, along the Spanish and Portuguese coasts. The main argument for this choice was that the Cap Breton canyon (close to the border between the Southern part of Division 8.b and the more Eastern part of Division 8.c, i.e. approximately between the French and Spanish borders) could be considered as a geographical boundary limiting exchanges between the two populations (ICES 2013). 

On the other hand, in recent studies (Westgaard et al. 2017) small-scale structure was detected among northern samples, all of which were also distinct from Bay of Biscay hake, with the exception of a few individuals from the North Sea and the coast of Norway who clustered genetically together with Bay of Biscay. Besides, hake analyses suggested a differentiation between Bay of Biscay and Portuguese samples (Lundy et al. 1999), both considered by ICES as part of the Southern stock. (Pita et al. 2013) found evidence that a large genetic connectivity exists among Atlantic grounds and is mediated by significant migration rates from the Celtic Sea towards its adjacent Atlantic grounds. Therefore, the spawning biomass of the northern hake population could play a crucial role in ensuring the sustainability of southern hake fish grounds.