Cusk (Brosme brosme) COSEWIC assessment and status report 2012: chapter 6

Wildlife Species Description and Significance

Name and Classification

The Cusk, Brosme brosme, belongs to the Order Gadiformes, of which species classification has been controversial (Van Guelpen 2011). Cohen (1984) elevated Gadinae, Lotinae and Phycinae to the family level, which was followed by Markle (1989) but not followed by Nelson et al.. (2004) who kept them as subfamilies. The latter placed Cusk in the subfamily Gadinae while Eschmeyer (2011) places the species in the family Lotidae.

Cusk is a monotypic genus. Common names include cusk, tusk, Brosme (Fr), torsk (No) and menek (Ru) (Scott and Scott 1988).

Morphological Description

Cusk are a slow swimming, relatively robust demersal species with a heavy head and elongate body reaching a maximum total length (TL) of greater than 100 cm (Cohen et al. 1990). The longest Cusk reported in the DFO Maritimes Science port sampling database is 115 cm while the longest specimen in the Halibut industry survey is 118 cm. There is one dorsal and one anal fin, both of which are elongate and extend posteriorly to a rounded caudal fin (Scott and Scott 1988, Collette and Klein-MacPhee 2002) (Figure 1).

Figure 1. Cusk (Brosme brosme) (from COSEWIC 2003).

Illustration of the Cusk, lateral view.
Long description for figure 1

Illustration of the Cusk (Brosme brosme), lateral view. There is one dorsal and one anal fin, both of which are elongate and extend posteriorly to a rounded caudal fin. There is a single barbel on the underside of the chin. The small pelvic fins are located below the rounded and brush-like pectoral fins.

Long description for Figure 1

The combination of a single barbel on the underside of the chin in addition to the single dorsal fin is diagnostic and identifies this species as Brosme brosme. The relationship of the anal and dorsal fins with the caudal fin is also distinctive. The dorsal and the anal fins are continuous with the caudal fin at the base but are separated from it by distinct notches (Scott and Scott 1988, Collette and Klein-MacPhee 2002). The small pelvic fins have 4-5 rays and are located below the rounded and brush-like pectoral fins. All fins are thick and fleshy at the base (Collette and Klein-MacPhee 2002). Individual rays are only evident at the margins. The body contains minute, deeply embedded scales (Wheeler 1969). Colour is variously described, being light grey with a brownish tint, paler on the sides, changing to greyish white on the belly in the northeast Atlantic to dark reddish or greenish brown, sometimes lighter brown, shading to cream or white on the belly in the northwest Atlantic (Bigelow and Schroeder 1953, Scott and Scott 1988, Collette and Klein-MacPhee 2002).

Population Spatial Structure and Variability

Cusk are a demersal fish spread widely throughout the North Atlantic. Based upon observations from egg and larval surveys, spawning appears to be widespread and the species does not appear to form spawning aggregations. Given that Cusk from different spawning grounds are reported to have different colouration, growth rate, number of vertebrae and fin rays, length distributions and length/weight relationships (Hareide 1988 in Knutsen et al. 2009), it is likely that spawning, while widespread throughout the distributional range, is also localized. Little is presently known about the migration and dispersal capacity of the species although it appears to be fairly sedentary, suggesting limited seasonal movement (Halliday 2006).

Knutsen et al. (2009) undertook an analysis of the genetic structure of Cusk in the North Atlantic. Using a combination of research and commercial vessels, tissue samples were obtained throughout the species’ distributional range (Figure 2) and a microsatellite DNA analysis of 7 loci was undertaken. The overall magnitude of genetic differentiation was quite low, with a global Fst of 0.0014. Pairwise Fst estimates between the only Canadian location surveyed and locations in the remainder of the trans-Atlantic range averaged only 0.0042. Spatial genetic variability was only weakly related to geographical distance between study sites or the separation of study sites along the path of major ocean currents. Rather, a significant effect of bathymetry was found. Limited adult migration across bathymetric barriers in combination with limited inter-site exchange of pelagic eggs and larvae due to site-specific circulatory retention, or poor survival during drift phases across deep basins, was hypothesized as reducing gene flow. Furthermore, the scarcity of catch records from the extremely cold waters off Labrador suggests a discontinuity in Cusk distribution in the northwest Atlantic which likely severely restricts genetic interchange between Cusk off West Greenland and from the Grand Banks and further south (Halliday 2006).

Figure 2. Sample locations of the Knutsen et al. (2009) study, together with ocean topography and water masses of the North Atlantic Ocean; Rockall (RA) and Mid-Atlantic Ridge (MAR) are located on sea mountains surrounded by deep areas (white areas >1000 m depth and beyond the maximum depth range of Cusk); Greenland (GR), Iceland (IS), Faroe Island (FI), Storegga (SE) and Tromsøflaket (TF) are interconnected with depth <1000 m (grey shaded areas), and are within the recorded depth range of Cusk (from Knutsen et al. 2009).

Map showing sample locations of the Knutsen et al.

Designatable Units

Cuskin the North American waters is treated as a single designatable unit (DU) in this report. This is supported by the restricted spatial distribution, with the bulk of the population being located between 41° to 44°N latitude in the Gulf of Maine and southern Scotian Shelf (Figure 3) as well as the studies of barriers to gene flow in Cusk discussed above.

Figure 3. Distribution of Cuskin the Northwest Atlantic (from Brown et al. 1996).

Map of the distribution of Cusk in the Northwest Atlantic based on Fisheries and Oceans Canada and National Oceanic and Atmospheric Administration catch per tow data.

The species is transboundary with the US in the Gulf of Maine area. However, given the limited apparent mobility of Cusk, focus in the report is placed upon analysis of Canadian data on the species with examination of US data sources to corroborate observations.

Both the potential discontinuity in Cusk distribution off West Greenland and areas further south (Halliday 2006), and the analyses of and barriers to gene flow in Cusk in the North Atlantic (Knutsen et al.. 2009), indicate that Cusk in West Greenland may potentially be part of a different population.

Special Significance

Cusk is a monotypic genus. In Canadian east coast waters, it does not have any particular importance as a targeted species but is bycatch in some fisheries (e.g. Cod (Gadus morhua), Haddock (Melanogrammus aeglefinus), Pollock (Pollachius virens), Atlantic Halibut (Hippoglossus hippoglossus), and Atlantic Lobster (Homarus americanus)). It does not appear to have any socio-economic significance in trade or ceremonial uses.

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