Chapter 1
European hake (Merluccius merluccius) in the Northeast Atlantic Ocean

Maria Korta1, Dorleta García2, Marina Santurtún2, Nerea Goikoetxea2, Eider Andonegi2, Hilario Murua1, Paula Álvarez1, Santiago Cerviño3, José Castro3 and Arantza Murillas2

1AZTI, Herrera Kaia Portu-aldea z/g, 20110, Pasaia, Basque Country, Spain

2AZTI, Txatxarramendi Ugartea z7g, 48395, Sukarrieta, Basque Country, Spain

3Instituto Español de Oceanografía, Subida Radio Faro 50, 36390, Vigo, Galicia, Spain

1.1 Distribution

The European hake (Merluccius merluccius) is widely distributed over the Northeast Atlantic shelf. Its range extends from Mauritania to off the western coast of Norway (between 21°N and 62°N) and the waters south of Iceland (Casey and Pereiro, 1995), being more abundant from the British Isles to the south of Spain (ICES, 2008). It is also found in the Mediterranean and Black Sea (Casey and Pereiro, 1995). M. merluccius is a demersal and pelagic species; it lives mostly between the depths of 70 and 370 m, although it is found in both shallower and deeper waters, from coastal waters (30 m) down to the depths of 1000 m (Lloris et al., 2003; Meiners, 2007).

The spatial behaviour of M. merluccius is strongly linked to its biology. Mature fish aggregate during their spawning season (Casey and Pereiro, 1995; Poulard, 2001) over the continental shelf slope from Portugal to Scotland. Besides, there is a northward displacement of mature fish, followed by a coastward displacement of the peak of spawning, as the season progresses along the French waters (Álvarez et al., 2004). This general pattern of distribution is consistent with previous data available on M. merluccius spawning periodicity (Arbault and Lacroix-Boutin, 1969; Solá and Franco, 1985; Valencia et al., 1989; Motos et al., 2000). The area around the shelf edge appears as a preferred region for spawning, the shelf break being a natural barrier for spawning of this species. Individuals of M. Merluccius appear to be dispersed during non-spawning season (Drouineau et al., 2010). In Portuguese continental waters, the abundance of small individuals of M. merluccius is higher in autumn and early spring. In the Southwest area, main concentrations of M. merluccius occur at 200-300 m depths, while in the South, they are mainly distributed at coastal waters. In the north of Portugal, juveniles are more abundant between 100 and 200 m depths. In contrast, northern juveniles concentrate in two main nursery areas: the Grande Vasière in the Bay of Biscay and the continental shelf of the Celtic Sea (Álvarez et al., 2001, 2004; Kacher and Amara, 2005).

Depth preferences also vary throughout the life of the individuals of M. merluccius showing an ontogenetic distribution pattern (Woillez et al., 2007). Vertical distribution of M. merluccius eggs and larvae from the northern Bay of Biscay and Celtic sea shelf-edge region shows an occurrence down to 150 m depth with a sub-surface peak of larvae at about 50-60 m depth (Coombs and Mitchell, 1982). The maximum egg abundance occurs close to the isobaths of 200 m depth (Motos et al., 2000). Larvae of M. merluccius remain resident in plankton about 2 months in depths from 50 to 150 m until they metamorphose to the juvenile stage (Motos et al., 1998). Individuals of age 0 years concentrate almost exclusively between 75 and 125 m depths, whereas individuals of M. merluccius of ages 1 and 2 years are found in shallower waters on the shelf (25-125 m depth), but a fraction of older fish is distributed on the slope at 200-500 m depths (Figure 1.1) (Woillez et al., 2007). Moreover, young individuals of M. merluccius (age 0 and 1 years) prefer muddy beds on the continental shelf, while large adults are found on the shelf slope where the bottom is rough and associated with canyons and cliffs (Murua, 2010).

Figure 1.1 Preferential distribution of M. merluccius individuals of age 0, 4 and 5 years for the period 1987-2004 (adapted from Woillez et al., 2007).

(Source: Woillez et al. 2007. Reproduced with permission of authors and ICES.)

1.1.1 Distribution change

Despite its wide distribution, M. merluccius is not evenly distributed over the Northeast Atlantic continental shelf. Furthermore, the abundance of individuals of age 0 years in the two main nursery areas varies inter-annually (Figure 1.2 for the period 1997-2007; Figure 1.3 for the period 2007-2012). In the Celtic Sea, the inter-annual variation in abundance of individuals of age 0 years of M. merluccius is higher, whereas concentration of M. merluccius over the French continental shelf does not significantly change over time (except for 1998, 1999 and 2003). Sánchez and Gil (2000) reported that in Galician-Cantabrian Sea, nursery areas remained stable and extent independently on the inter-annual density variability. Individuals of age 1 years remain on nursery grounds until spring. Afterwards, some seasonal displacement is detected in individuals of age 2, 3 and 4 years. Younger hakes (age of 2 and 3 years) are preferentially found in shallow waters in spring and scattered over the shelf in autumn.

Figure 1.2 Spatial distribution of individuals of age 0 years in M. merluccius in the two main nursery areas (Bay of Biscay and Celtic Sea) from 1997 to 2007 (ICES, 2008).

(Source: ICES 2008. Reproduced with permission of ICES.)

Figure 1.3 Spatial distribution of individuals of age 0 years in M. merluccius in the two main nursery areas (Bay of Biscay and Celtic Sea) from 2007 to 2012 (ICES, 2013).

(Source: ICES 2013. Reproduced with permission of ICES.)

1.2 Physical environment and hydrography

The hydrography of the Northeast Atlantic Ocean is affected by the North Atlantic current, which is a warm ocean current that carries the Gulf Stream northeast towards the European coast (OSPAR Commission, 2000). The North Atlantic current splits into two branches around west Ireland. While the Canary current takes a southerly direction, the other warm-water branch continues north along the coast of northwestern Europe, heating the cold northern atmosphere.

In the North Atlantic Ocean, two major zonally orientated water masses are found. In general, the water masses in the upper layer (down to 1000 m depth) are characterised by Northeast Atlantic Central Water (ENACW), with temperatures ranging between 10.5 and 12°C and salinity between 35.45 and 35.75 (Koutsikopoulos and Le Cann, 1996; Valencia and Franco, 2004). Two sources of ENACW masses are identified in this area: colder water mass with a sub-polar origin (ENACWp) and warmer water mass with sub-tropical origin (ENACWt) (Ríos et al., 1992). Upper waters are influenced by seasonal variations in the atmosphere, whereas below 1000 m, the oceanic waters are characterised by the Mediterranean Sea overflow water, which spreads into the Northeast Atlantic Ocean from the Gibraltar Strait. It moves northwards from the Portuguese continental slope, entering the Bay of Biscay (Lavin et al., 2006). Most of the water masses of the Bay of Biscay and the Atlantic coast of the Iberian Peninsula are the result of a mixture of North Atlantic water masses and waters of Mediterranean Sea origin (OSPAR Commission, 2000).

The oceanic circulation in the Bay of Biscay is characterised by weak clockwise circulation (Figure 1.4), with a mean geostrophic current of about 1-2 cm/s at a depth of 4000 m (Koutsikopoulos and Le Cann, 1996). However, while the deep regions of the bay might be influenced primarily by general oceanic circulation, the shelf is characterised by the dominance of mesoscale structures, which are strongly influenced by regional or local hydroclimatic conditions (Planque et al., 2003). Slope currents in the Bay of Biscay show a consistent poleward transport (Pingree and Le Cann, 1990). Instead, the currents over the shelf are mainly driven by wind, buoyancy and tides.

Figure 1.4 Main physical features in the Celtic Sea and the Bay of Biscay (after Mason et al., 2006).

(Source: Mason et al. 2006. Reproduced with permission of IPMA.)

In addition to the general clockwise circulation, and depending on the season, the circulation can become cyclonic along the Portuguese, Galician and northern Spanish and French continental slopes due to the slope current. In winter, when the northerly wind component relaxes, a warm and saline polewards surface flow (ENACWt) takes place at 20-30 m/s off the Iberian Peninsula, moving eastwards along the Cantabrian coast to enter into the Bay of Biscay (Figure 1.4). As this warm water usually reaches the Bay of Biscay near Christmas, it has been referred to as the Navidad or Christmas current (Pingree and Le Cann, 1992). Moreover, in winter, clockwise and anticlockwise eddies are formed in the continental slope due to the interaction between the slope current and topography. These eddies are named 'swoddies' (slope water oceanic eddies) (Pingree and Le Cann, 1992).

On the French coast, the general northerly wind circulation over the shelf...