Based on the new genetic information that has been analyzed in 2007, ICES presents advice for the following stock units of tusk:
- Subareas I and II;
- Division Va and Subarea XIV;
- the Mid Atlantic Ridge (Division XII excluding XIIb, see Roundnose Grenadier);
- Subarea VIb (Rockall);
- Other areas (IIIa, IV, Vb, VIa, VII, VIII, IX, and XIIb). This latter grouping is a combination of isolated fishing grounds and these areas are grouped due to their mutual lack of data.
The new perception of the stock structure is based on considerations of new genetic information in 2007 (ICES, 2007). Studies using recently developed microsatellite primers detected highly significant genetic differentiation in tusk within its North Atlantic range. In particular, tusk around Rockall, the Mid-Atlantic Ridge, and off Canada, most likely represent different biological populations that clearly warrant separate management considerations. For the remainder of the North Atlantic this study uncovered limited genetic differentiation, and no firm conclusion can be reached at present regarding the number of populations and assessment units. Samples from Storegga and Tromsøflaket (ICES Area II) are similar and may represent a single population in this area, and likewise for Greenland and Iceland. Tusk from around Faroe Islands differ significantly both from those from Iceland and Tromsøflaket, but the differences are small and it is at present unclear how these differences should be interpreted in terms of management implications. The geographical coverage of this study is insufficient to exclude the possibility of additional genetically differentiated populations of tusk. More samples are required from around Iceland, Faroe Islands, Hatton Bank, and Western Scotland to disentangle potential structure within and among these areas.
Previously, ICES has advised for three management units proposed on the basis of apparent isolation of fishing ground: I and II (Arctic), Va (Iceland), and IIIa, IVa, Vb,VI, VII, VIII, IX, XII, and XIV (other areas).
Tusk is primarily fished in the depth range 200–500 m, though it is also found at shallower depths. Tusk is more vulnerable to overexploitation than ling due to a slower growth rate and higher age at first maturity, but it is still less sensitive than other deep-water species (ICES, 2006, Section 8.2.2).