Cusk (Brosme brosme) COSEWIC assessment and status report 2012: chapter 8

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Habitat

Habitat Requirements

Cusk are commonly taken on hard, rough, and rocky substrates and are described by Bigelow and Schroeder (1953) and Collette and Klein-MacPhee (2002) as being "decidedly fastidious in their choice of bottoms, being found chiefly on hard ground, especially where the sea floor is rough with rocks or boulders; on gravely or pebbly grounds, occasionally on mud with hake (Urophycis) but seldom on smooth clean sand". They have been observed hiding in crevices (Hovland and Judd 1988, Freiwald et al. 2002, Jones et al. 2009). Cuskhave been taken with longlines and gillnets off southwestern Norway at depths of 150 to 300 m in coral (Lophelia pertusa) habitats (Husebø et al. 2002). Fish in coral habitats (Svedevidov 1948) tended to be larger in size than those in non-coral habitats. Cuskwere also observed associated with carbonate-cemented slabs, sometimes hiding beneath them in seabed pockmarks and seepages in the central North Sea (Hovland and Judd 1988).

Cusk prefer relatively warm intermediate depths in the western Atlantic. Cuskare found at temperatures from 2-12°C on the Scotian Shelf and ca.1-10°C in the Gulf of Maine (Scott 1982, Scott and Scott 1988). The preferred temperature range is ca. 6-10°C on the Scotian Shelf (Scott 1982, Scott and Scott 1988). The principal fishing areas are reported to coincide with regions having a minimum bottom temperature of about 4°C (Oldham 1972, Scott 1982, Scott and Scott 1988). In the summer bottom trawl survey, 91% of the sets that caught Cusk were at bottom temperatures above 4°C (Table 5). Of the sets made in this temperature range, 13% had Cusk compared to 2% in sets at lower temperatures.

Cusk are rarely if ever found near the shore or at depths less than 20-30 m (Svedevidov 1948). Regarding Cusk’s upper depth limit, Cohen et al.. (1990) and Hareide and Garnes (2001) indicate a species’ depth range of between 20 to 1100 m while Andriyashev (1954) reports that Cusk are rarely found deeper than 400 m. Scott (1982) and Scott and Scott (1988) report that the depth range on the Scotian Shelf is 73-363 m based on DFO trawl survey sampling. Ninety percent of the trawl survey sets that caught Cusk were at depths between 100 – 400 m(Table 5). For the sets conducted in this depth range, 15% produced Cusk in contrast to 2% in the 0 – 100 m range and 3 % at depths above 400 m. Of the 41 sets in waters > 600 m, no Cusk were caught. Harris and Hanke (2010) report that catch rates in the Halibut industry survey peaked between 400 – 600 m, with Cusk caught at depths as great as 1185 m (no sets were conducted deeper than this).

Habitat Trends

While the large-scale characteristics of the bottom habitat on the Scotian Shelf and in the Gulf of Maine area have been described (Kostylev and Hannah 2007), except for a small area of southwest Nova Scotia (Todd and Kostylev 2011), this is not at a scale which would be informative to this assessment.

In comparison, there is significant information available on the temporal trends of the physical oceanographic conditions that are summarized in Worcester and Parker (2010). Year to year water temperatures on the Scotian Shelf and in the Gulf of Maine area are among the most variable in the North Atlantic. The western Scotian Shelf (WSS) is generally warmer than the Eastern Scotian Shelf (ESS), as warmer Gulf Stream water enters onto the WSS over the southwest slope between Browns and Western banks; whereas, the ESS is more influenced by the cool, low salinity water from the north entering over Misaine Bank near Cape Breton.

Water temperature during 1987-1993 and 2003-2004 were predominantly colder than normal, while 1973-1985 and 1999-2000 was warmer than normal. Variability in water temperature has been increasing in the past decade.

Salinity measurements have been taken since 1924 at a fixed station near Saint Andrews, NB, adjacent to the entrance of the Bay of Fundy. There was a decrease in salinity from the 1940s to the early 1970s. This was followed by an increase in the mid-1970s and another long-term decline to a time series low in 1996. Salinity subsequently increased to 2002, followed again by a decline (Figure 12). This pattern is consistent with the pattern of salinities measured by the US Northeast Fisheries Science Centre on the continental shelf (Gulf of Maine) since the 1970s.

Figure 12. Time series of annual surface salinity anomalies (grey dashed line with dots) and 5-year running means (heavy, black line) (from Worcester et al. 2010).

Long description for Figure 12

On the Scotian Shelf, the average 0 – 50 m stratification index increased considerably in the 1990s. From the mid- to late 1990s, the index was at or near its maximum over the 50-year record (Figure 13). Stratification in 2008 was above normal by one standard deviation, the 4th strongest in 49 years. Important changes in stratification have also been noted over time in the eastern Gulf of Maine and Georges Bank with increasing temperature and changes in salinity. Stratification has increased steadily from the mid-1980s on Georges Bank and in the eastern Gulf of Maine in a similar manner as on the Scotian Shelf. Strong stratification sometimes inhibits vertical mixing enough to cause dissolved oxygen levels in deeper layers to become depressed. However, although the waters of this ecozone do stratify, low dissolved oxygen has not been apparent with the exception of a few coastal locations and potentially some of the deepest basins.

Figure 13. Mean annual (dashed line) and 5-year running mean (solid line) of the stratification index over the Scotian Shelf; anomalies based on 1971-2000 observations; standard error estimates for each annual value are shown (from Worcester et al. 2010).

It is difficult to judge from these trends in temperature, salinity, and stratification if Cusk habitat has changed significantly over time. DFO (2008) concluded that habitat does not appear to be, nor is likely to become, a limiting factor to Cusk survival and recovery. It further stated that there are no known threats that have reduced Cusk habitat quantity and quality.