234 
Fishery Bulletin 115(2) 
variability within regions and by depth (Echave et ah, 
2012; Head et ah, 2014). Size and age at maturity have 
also been found to differ with latitude and depth; for 
females in the southern stock, Head et al. (2014) esti¬ 
mated 50% maturity at ages of 4.9-11 years and lengths 
of 48.5-58.5 cm FL. 
The separation between sablefish stocks along the 
coast of Vancouver Island occurs at the boundary be¬ 
tween the California Current upwelling ecosystem to 
the south and the Gulf of Alaska downwelling ecosys¬ 
tem to the north. The associated differences in a broad 
suite of oceanographic factors are likely to affect sa¬ 
blefish biology. Genotypic differentiation, however, is 
weak (Tripp-Valdez et ah, 2012), indicating significant 
exchange between the 2 stocks of fish. Prior tagging 
studies have reported small but persistent migrations 
from Alaska to west coast waters and vice versa (Fu- 
jioka et ah, 1988; Kimura et ah, 1998; Echave et ah, 
2013). 
Patterns in depth distribution 
The broad bathymetric distribution of sablefish is 
thought to arise from an ontogenetic migration to pro¬ 
gressively deeper depths with age, on the basis of in¬ 
creasing proportions of mature fish at greater depths 
(Fujiwara and Hankin, 1988), increasing mean length 
with depth (Hunter et ah, 1989), and increasing abun¬ 
dances of older or larger fish with depth (Saunders et 
ah, 1997; Sigler et ah, 1997; Jacobson et ah, 2001; Head 
et ah, 2014; Johnson et ah^). Maloney and Sigler (2008) 
examined depth changes of tagged juveniles and found 
a clear pattern of movement to deeper waters by age 3 
and a decline in occurrence at depths <500 m as fish 
aged. However, at ages from 3 to 20 years the largest 
concentration of each age class was at depths of 500- 
700 m. Kimura et ah (1998) compared depth at initial 
capture with depth at recapture and found that north¬ 
ern fish congregated at depths of 400-800 m, whereas 
southern fish were more likely to be recaptured in the 
same depth zone as that of tagging. 
An alternative to the hypothesis of ontogenetic 
migration to deeper habitats was proposed by Norris 
(1997), who suggested that different depth distribu¬ 
tions are a consequence of adaptive radiation of en¬ 
zyme systems and differing physiological efficiency at 
different depths depending on genotype. Under this 
scenario, upon reaching sexual maturity, adults of dif¬ 
ferent ecotypes migrate to depth ranges appropriate to 
their physiology and then remain at those depths. Fu¬ 
jiwara and Hankin (1988) likewise suggested the pos¬ 
sibility of depth-related population structure for sable¬ 
fish in California on the basis of contrasting maturity 
schedules. 
The depth range of adult sablefish includes the per¬ 
sistent oxygen minimum zone (OMZ) present along 
the eastern Pacific slope. Gilly et al. (2013) defined the 
OMZ as waters with dissolved oxygen levels <20 pmol/ 
kg, or approximately 10% of the saturation of oxygen 
in surface waters. The position of the OMZ varies lati- 
tudinally, but along the Oregon coast it spans depths of 
about 500-1500 m and is enveloped by an oxygen lim¬ 
ited zone of slightly higher dissolved oxygen concentra¬ 
tions (Pierce et al., 2012; Gilly et al., 2013). Sablefish 
residing within the OMZ appear to be well adapted to 
the harsh physical conditions and potentially limited 
food availability of deep slope habitats (Sullivan and 
Smith, 1982; Brazen, 2007). 
Spatia! movement patterns 
Spatial movements of sablefish have been extensive¬ 
ly documented in tag-recapture studies. Within the 
northern stock, multiple studies spanning several de¬ 
cades have found widespread movement away from the 
location of tagging and that the likelihood of movement 
increases from southeast Alaska waters through the 
central Gulf of Alaska and into the Aleutian Islands 
and Bering Sea (reviews in Echave et al., 2013; Han- 
selman et al., 2015). In contrast, southern sablefish ex¬ 
hibit more limited movement away from the general 
tagging location (Fujioka et al., 1988; Kimura et al., 
1998). Movement patterns of fish tagged in British Co¬ 
lumbia waters suggest a more limited dispersal from 
southern tagging locations than from northern tagging 
locations (Beamish and McFarlane, 1988; McFarlane 
and Saunders, 1997). 
Growth 
Age-0 sablefish exhibit extremely rapid growth rates 
(Boehlert and Yoklavich, 1985; Sigler et ah, 2001; So- 
gard and 011a, 2001; Sogard, 2011), but growth slows 
markedly in mature adults, and fish reach asymp¬ 
totic size within their first decade (Johnson et al.^). 
Males grow more slowly than females (McFarlane 
and Beamish, 1983; Sasaki, 1985; Kimura et ah, 1993; 
Saunders et al., 1997; Echave et ah, 2012; Morita et al., 
2012), and northern fish appear to have faster growth 
rates and attain larger sizes than southern fish (Kimu¬ 
ra et ah, 1993; Kimura, 2008). Depth-related differ¬ 
ences in growth rates are suggested by the pattern of 
smaller size-at-age with depth (Saunders et al., 1997; 
Head et al., 2014). 
Management impacts 
The southern stock has been heavily exploited since 
the 1970s, although regulations have reduced annual 
landings to <10,000 metric tons (t) in recent years 
(Johnson et al.^). In conjunction with a size-related 
price structure, discarding of smaller fish was com¬ 
mon practice. Although sablefish lack swimbladders 
and do not suffer barotrauma, they are susceptible to 
the rapid temperature increases associated with cap¬ 
ture in cold, deep water and retrieval to warm surface 
waters. Mortality rates in experiments simulating cap¬ 
ture at 4-6°C and discarding at surface temperatures 
