Atlantic wolffish (Anarhichas lupus) COSEWIC assessment and status report 2012: chapter 8

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Habitat

Habitat Requirements

Habitat associations of Atlantic Wolffish in terms of temperature, depth and bottom type were described by Kulka et al. (2004) and Simpson et al. (2011). This species occupies different habitats depending on its life stage: the eggs are deposited on the bottom, the larvae are pelagic, and the juveniles and adults inhabit near-bottom waters. In near-shore waters, the eggs are deposited in crevices on rocky bottoms (Keats et al. 1985). The larvae are pelagic until they reach between 30 and 35 mm (Wiseman 1997 in Kulka et al. 2007). Juvenile Spotted Wolffish have been observed to use shelters (Lachance et al. 2010), and this may be the case with Atlantic Wolffish too.

Juveniles and adults occur on the continental shelf on rocky or sandy bottoms. From diver observations, Atlantic Wolffish has not been observed on soft substrate such as clay or mud in inshore water and they appear to depend on boulders or caves for spawning (Kulka et al. 2004).

The fish are found from near-shore to depths of 918 m (Kulka et al. 2004,2007). In Newfoundland waters, abundance peaks at 250 m year-round (Kulka et al. 2004). Of the three wolffish species, only the Atlantic Wolffish occurs frequently in depths less than 100 m. On the Scotian Shelf and Grand Banks off Newfoundland and in the Gulf of St. Lawrence, they typically occupy depths of less than 150 m. On the Scotian Shelf, they are commonly observed at depths between 100 and 350 m (McRuer et al. 2000).

Temperature is believed to be the primary factor determining habitat selection in this species (Kulka et al. 2004). They are most abundant in shelters located below the thermocline and the influence of tidal and coastal currents (Dutil et al. 2010). Other types of structured bottoms could also provide refuge, such as bottoms covered with dense vegetation or soft bottoms with considerable relief.

From Newfoundland research surveys, the highest densities of Atlantic Wolffish are at temperatures of 1.5°C to 4°C (Kulka et al. 2004). On the Scotian Shelf, the temperature range where the species occurs is 1-9 °C (Simon et al. 2011), although the preferred range is similar to that observed further north. Divers do not observe Atlantic Wolffish at temperatures above 10°C and the fish can tolerate temperatures below 0°C (Kulka et al. 2004). On the Scotian Shelf and likely across its range, they occur in waters with salinity between 32 and 34 ‰ (Albikovskaya 1982). Kulka et al. (2004) noted absence of wolffish in waters with low salinity based on diving observations.

Habitat Trends

An episode of particularly cold water temperatures was recorded in the late 1980s and early 1990s (Colbourne et al. 1997). Figure 10 shows Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) from 1950 to 2010 (Yashayaev and Greenan 2011). A positive index is associated with colder than usual conditions in the northern Labrador Sea. The index for the end of 1980 to mid-1990 showed higher values since 1950. Although the latter part of this cooling period coincided with the rapid contraction in range of wolffish species (Kulka et al. 2004), contractions in area of occupancy began well before then (Fig. 7). Shallower waters may have been abandoned in favour of deeper, warmer waters, though this is conjecture.

Figure 10. Winter Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) indices with 1950-2000 as base period. Data are for the Jan-Mar winter period. Source: Yashayaev and Greenan (2011).

Long-term future trends in water temperature are uncertain. Global climate change will likely affect the habitat and distribution of Atlantic Wolffish. The increase in temperature in the northwest Atlantic may result in a northward shift of several marine fish species, including Atlantic Wolffish (Gucinski et al. 1990), as has been documented in many species in the northeastern Atlantic (Perry et al. 2005).