Longspine thornyhead (Sebastolobus altivelis) COSEWIC assessment and status report: chapter 3

Species Information

Name and classification

The genus Sebastolobus comprises three species in the north Pacific Ocean, including longspine thornyhead, sébastolobe à longues épines (Sebastolobus altivelis Gilbert 1893).  The taxonomic name stems from the Greek sebastos (magnificent) and lobos (lobe – of pectoral fin), and the Latin alutus (high) and velum (sail – dorsal fin) (Hart 1973). Longspine thornyhead differs from its congener shortspine thornyhead (S. alascanus) by an elongated third dorsal spine, a mostly black gill chamber, and usually 15 dorsal spines (Love et al. 2002).  Other common names for the longspine thornyhead include channel rockfish, hardhead, idiotfish (Love 1996).

Morphological description

Hart (1973) details this species’ morphology. Generally, longspine thornyheads exhibit a reddish colour with some black on the fins (Figure 1).  The elongate body, measuring up to 35 cm, has a terminal mouth with a large upper jaw that overhangs the lower one.  The eyes appear large and oval. The strong, sharp head spines project backwards. The dorsal fin exhibits 15-16 spines, 8-9 of which are deeply and broadly notched with the 3rd being the longest.  The pectoral fins have two distinct lobes.  A strong spiny ridge runs along the suborbital bone.  The length-weight relationship describes a typical cubic exponential curve with no difference between the sexes (Figure 2).

Figure 1. Longspine thornyhead Sebastolobus altivelis – ink drawing (Hart 1973) and photo (http://www.afsc.noaa.gov/groundfish/RockfishGuide/Rockfish_Pages/Longspine_thornyhead.htm).

Figure 1.  Longspine thornyhead Sebastolobus altivelis – ink drawing (Hart 1973).

Figure 2. Longspine thornyhead weight vs. length fitted using a lognormal linear model: log W= log α + β log L. Source: Haigh et al. (2005)

Figure2.  Longspine thornyhead weight vs. length fitted using a lognormal linear model: log W= log α + β log L. Source: Haigh et al. (2005)

Genetic description

Currently, no genetic information on longspine thornyheads in Canada’s Pacific waters exists.  A mitochondrial DNA analysis by Stepien et al. (2000) based on 55 samples from five sites (Seward Alaska to Southern California--no Canadian sites) suggests genetic mixing of longspine thornyheads along the Pacific coast, with some evidence for genetic structuring, possibly due to larval retention caused by currents and gyres around prominent bathymetric features.

Designatable units

The species may be distributed continuously along the continental slope between 500 and 1,600 m.  There are three distinct fishing areas, WCVI (west coast Vancouver Island), Tidemarks, and Rennell Sound (Figure 4).  The population structure of these three areas has not been investigated due to the lack of a feasible aging protocol.  For the purpose of this report, British Columbia (BC) is assumed to have one designatable unit.

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