Longspine thornyhead (Sebastolobus altivelis) COSEWIC assessment and status report: chapter 6

Biology

Life cycle and reproduction

Populations of longspine thornyhead off Oregon and California spawn primarily in the oxygen minimum zone between 600 and 1,000 m (Jacobson and Vetter 1996). Each female longspine thornyhead releases from 20,000 to 450,000 eggs (Wakefield 1990) in a buoyant gelatinous matrix. This oviparous characteristic contrasts with the ovoviviparous nature of Sebastes species. Pearcy (1962) first described the appearance of these gelatinous egg masses in surface waters off the Oregon coast. He collected specimens, incubated them in vitro, and described the embryonic development. Best (1964) then determined a weight-length relationship from California trawl specimens. Moser (1974) later described the larval and juvenile morphology changes in detail, and determined that spawning generally occurs from February to May with a peak in April.

Recent data from the MOCNESS[1] program in southern California suggest that longspine thornyhead larvae move away from surface waters (12% at 0-100 m, 30% at 100-150 m, 58% at 150-200 m; Bill Watson[2], pers. comm.). Juveniles remain in the water column for approximately 1 year (Moser 1974). Smith and Brown (1983) noted the highest abundance of juvenile longspine thornyheads at 600 m in the vicinity of a deep-scattering layer. They also noted that the primary prey species of juveniles were omnivorous euphausiids. Thus, the chronology of the longspine thornyhead pelagic phase (18-20 months) can be generalized:

  1. eggs float to the surface (Feb-May) where they hatch and develop for 3-4 weeks;
  2. larvae move away from the surface but remain in the upper 200 m for 6-7 months;
  3. juveniles remain in the mesopelagic zone (~600 m) for another year.

After the pelagic phase, longspine thornyhead juveniles (55 mm average total length, Wakefield and Smith 1990) settle directly into the benthic adult habitat between 600 and 1,200 m, where they reportedly remain (Wakefield 1990). In comparison, shortspine thornyheads settle as juveniles at 100 m (Moser 1974) and migrate deeper as they get older and, presumably, bigger (Jacobson and Vetter 1996). Consistent size gradients with depth occur along the WCVI for the two thornyhead species, where shortspine thornyhead length increases with depth while longspine thornyhead length decreases (Haigh and Schnute 2003). The two species co-occur between 600 and 1,100 m, where the median longspine thornyhead volume never exceeds one half the comparable measurement for shortspine thornyhead. The larger shortspine thornyheads commonly consume longspine thornyheads (observations cited in Jacobson and Vetter 1996). The primary food species for longspine thornyhead comprise the omnivorous brittle star[3] Ophiophthalmus normani (Smith and Brown 1983) and other benthic fauna (see below).

Low productivity (Vetter and Lynn 1997) and low diversity (Haigh and Schnute 2003) characterize the depths at which longspine thornyheads live. Vetter and Lynn (1997) used enzymatic analyzes to compare various slope rockfish and estimated that the time between feeding for longspine thornyhead is 130-180 days (80-90 days for shortspine thornyhead). Furthermore, Yang and Somero (1993) found similar metabolic rates of laboratory-fasted shortspine thornyhead to those for freshly collected specimens. Deep-water thornyheads have adapted to this food-limited environment by adopting a sedentary adult phase with planktonic larval and juveniles phases (Smith and Brown 1983).

The literature suggests that longspine thornyheads stop growing at a length of about 300 mm, corresponding to an estimated age of 25 to 45 years (Jacobson and Vetter 1996). Off WCVI, 99% of observed longspine thornyheads measure less than 300 mm (median length = 240 mm). Haigh and Schnute (2003) estimated the recruitment (to the fishery) length at 170 mm. Wakefield and Smith (1990) estimated the size-at-first-maturity at 150 mm. The Pacific Biological Station (PBS) has developed an ageing protocol for this species; however, its practical application remains elusive. An experimental sample thus far yields a maximum age of 71 y (mean = 23.6 y, median = 22.0 y, n = 204). In contrast, radiometric work at Moss Landing Laboratories indicates a provisional maximum age for this species of 45 years (Kline 1996). Given the paucity of the PBS data, length-at-age relationships remain poorly defined (Figure 5) for the BC population. Jacobson (1991), Kline (1996), and Fay (2006) give estimates of von Bertalanffy growth parameters for populations in Oregon/California (Table 2).

Figure 5. Length-at-age relationship for Sebastolobus altivelis fitted using the von Bertalanffy growth equation: Lt = L(1-e-k(t-t0)). Source: Haigh et al. (2005).

Figure 5.  Length-at-age relationship for Sebastolobus altivelis fitted using the von Bertalanffy growth equation: Lt = L∞(1-e-k(t-t0)). Source: Haigh et al. (2005).

 

Table 2.  Reported von Bertalanffy growth parameters for Sebastes altivelis along the west coast of North America.
Population Source n LInfinito(mm) K To
BC Haigh et al. (2005)
198
315
0.0314
-16.0
Oregon Jacobson (1991)
192
338.6
0.0585
-0.38
California Kline (1996)
478
300.6
0.072
-1.9
California Fay (2005)
815
312
0.064
-2.02

The PBS data suggest an age at 50% maturity of 20 years (Haigh et al. 2005).  The generation time using the formula tgen = K+ 1/M), where K=20 (age of 50% maturity) and M= 0.10 (natural mortality rate, Ianelli et al. 1994), is 30 years.

Interspecific interactions

The primary prey species of longspine thornyhead pelagic juveniles is probably omnivorous euphausiids, while settled adults eat the omnivorous brittle star Ophiophthalmus normani (Smith and Brown 1983). The adults also consume a variety of other benthic organisms including grooved tanner crabs (small specimens <30 mm carapace width, moulting adolescents <70 mm CW, and moulting sub-adults <110 mm CW), myctophids (lanternfish), and small thornyheads (Greg Workman[4], pers. comm.).

A known predator is shortspine thornyhead S. alascanus (observation by P. Adams, cited in Jacobsen and Vetter 1996). Other fish species are presumed to prey on S. altivelis, including cannibalism of newly settled juveniles by larger individuals (Love et al. 2002).

Physiology

Longspine thornyheads have adaptations that enable them to live in the oxygen minimum zone (600-1000 m). For instance, Sebastolobus altivelis has twice as many white-muscle metabolic enzymes than the shallower-living S. alascanus, which might reduce oxygen requirements two-fold (Siebenaller and Somero 1982). Also, enzymes in S. altivelis are adapted to functioning under pressure through conformational changes (Somero 1982). Unlike rockfish of the genus Sebastes, thornyheads do not have a swim bladder, and may be able to survive being brought to the surface.

Dispersal/migration

Information on movements once the adults settle into their territories at depth does not exist.

Interspecific interactions

Longspine thornyheads co-occur primarily with shortspine thornyheads Sebastolobus alascanus and sablefish Anoplopoma fimbria (Figure 6). The former frequently consume longspine thornyheads. As depths increase from those where the two co-occur most densely (500-800 m) to those with oxygen minima and high pressures, S. altivelis gains a competitive advantage through physiological adaptation to extreme conditions, and eventually predominates.

Figure 6. Concurrence of species in trawl tows (1996-2004) that captured longspine thornyhead in the preferred depth range (274-1056 m). Abundance expressed as a percent of total catch weight. Source: Haigh et al. (2005).

Figure 6.  Concurrence of species in trawl tows (1996-2004) that captured longspine thornyhead in the preferred depth range (274-1056 m). Abundance expressed as a percent of total catch weight. Source: Haigh et al. (2005).

Adaptability

Longspine thornyhead are highly adapted to deep-sea environments characterized by high pressure, low oxygen and low productivity. Large-scale environmental changes to this environment, either anthropogenic or natural, would likely have detrimental effects on the species. Unlike rockfish of the genus Sebastes, they do not suffer obvious depressurization effects (e.g., organ extrusion) when brought to the surface. We do not know their ability to survive the temporary exposure to low pressure upon capture and release.

[1] The MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) is a computer controlled net system used to collect zooplankton samples from specific depths in the water column. As the net system is towed through the water, individual nets can be opened within target depth zones.
(http://swfsc.nmfs.noaa.gov/frd/CalCOFI/TT/MOCNESS.htm).

[2] Southwest Fisheries Science Center, 8604 La Jolla Shores Drive, La Jolla, California 92037-1508.

[3] Class Ophiuroidea.

[4] Groundfish surveys, Pacific Biological Station, 3190 Hammond Bay Road, Nanaimo, BC, V9T 6N7.

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