Longspine thornyhead (Sebastolobus altivelis) COSEWIC assessment and status report: chapter 4

Distribution

Global range

Longspine thornyheads range from Cabo San Lucas, Baja California, to the Aleutian Islands (Figure 3) at depths recorded from 201 to 1,756 m but typically from 500 to 1,300 m (Love et al. 2002). They prefer soft sand or mud bottoms.

Figure3.  North American distribution of longspine thornyhead. Distribution adjacent to Asian countries is not well documented (Froese and Pauly 2005). Figure from Love et al. (2002). Figure reprinted with permission.

Figure3.  North American distribution of longspine thornyhead. Distribution adjacent to Asian countries is not well documented (Froese and Pauly 2005). Figure from Love et al. (2002). Figure reprinted with permission.

Figure 4. Mean CPUE (kg/h) of longspine thornyheads in 25 km² grid cells along the BC coast.  The shaded cells give an approximation of the area of occupancy (11,700 km²) as seen by groundfish trawl tows between 500 and 1400 m from 1996 to 2004.  Isobaths displayed are 500 m and 1,600 m; the area between these isobaths approximates the extent of occurrence (17,775 km²).  The five DFO management regions for longspine thornyhead are delimited by horizontal red lines.  Source: Haigh et al. (2005).

Figure 4.  Mean CPUE (kg/h) of longspine thornyheads in 25 km2 grid cells along the BC coast.  The shaded cells give an approximation of the area of occupancy (11,700 km2) as seen by groundfish trawl tows between 500 and 1400 m from 1996 to 2004.  Isobaths displayed are 500 m and 1,600 m; the area between these isobaths approximates the extent of occurrence (17,775 km2).  The five DFO management regions for longspine thornyhead are delimited by horizontal red lines.  Source: Haigh et al. (2005).

Canadian range

Longspine thornyheads occur along the continental slope of British Columbia. The most likely “extent of occurrence” lies between the 500 and 1,600 m isobaths with a flat-surface area of 17,775 km². Figure 4 shows the areal extent of longspine thornyheads as described by groundfish trawl fishery tows between 500 and 1,400 m from 1996 to 2004. Assuming an average trawl speed of 4.48 km h-1 (Schnute et al. 2004), tows that catch longspine thornyheads travel 31 km on average (SD=11 km, n=14,837). Often the tow path follows a horseshoe shape. Given these specifics, grid cells with dimension 5 km x 5 km reasonably summarize longspine thornyhead tow information. Using such grids, the known habitat or “area of occupancy” covers 11,700 km², a conservative estimate given the gear limitations of the trawl fleet. Deepwater biomass surveys (2001-2003) off the WCVI found longspine thornyheads living deeper than 1,200 m (Starr et al. 2002, Starr et al. 2004, Krishka et al. 2005). Commercial trawlers do not usually fish greater than 1,200 m. Table 1 offers an alternative summary of available and occupied areas using both commercial and survey tow information; however, these values are considered less accurate and are not used in the Technical Summary.

Table 1.  Bathymetric determination of total available and observed occupied areas by 100-m depth interval for longspine thornyhead. Based on events from commercial fishing and surveys located in 25 km² grid cells. Source: Haigh et al. (2005).
Depth Interval (m) Total Area (km²) Occupied Area (km²) % Occupied
501-600
1,782
1,080
60.6
601-700
1,561
1,187
76.0
701-800
1,413
1,125
79.6
801-900
1,247
955
76.6
901-1000
1,470
1,084
73.7
1001-1100
1,623
1,024
63.1
1101-1200
1,804
948
52.5
1201-1300
1,731
817
47.2
1301-1400
1,692
838
49.5
1401-1500
1,630
552
33.9
1501-1600
1,478
304
20.6
Total:
17,431
9,914
56.9

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