European hake 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 Black sea. ICES assumes since the end of the 1970s two different stocks units (ICES, 2013a):
• The Northern stock is distributed throughout the Kattegat, the Skagerrak, the North Sea, the English Channel, to the west of Scotland and Ireland and into the Bay of Biscay (EC, 2004)
• The Southern stock, in Divisions VIIIc and IXa along the Spanish and Portuguese coasts.
However, there is still no consensus on the stock structure of European hake in the Northeast Atlantic. Several studies have raised the issue that there is no genetic evidence of multiple populations in the Northeast Atlantic (Roldán et al., 1998, Castillo et al., 2005, Pita et al., 2010). On the other hand, analyses by Lundy et al. (1999), suggested a differentiation between Bay of Biscay and Portugese samples, both considered by ICES as part of the Southern stock. In a recent study Pita el 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. However, the Cap Breton canyon (close to the border between the Southern part of Division VIIIb and the eastern part of Division VIIIc, i.e. approximately between the French and Spanish borders) is still considered to be a geographical boundary limiting exchanges between the two stocks (ICES, 2013a).