Jacobson et al : Depth distributions and time-varying selectivities for various bottom fishes 
319 
al., 1997; Methot et al., 1998). For sablefish, Dover sole 
and shortspine thornyhead, the smallest individuals were 
found almost entirely in shallow water (100-199 fm for 
sablefish and 100-299 fm for Dover sole and shortspine 
thornyhead). The largest individuals occupied the entire 
range of depths but were relatively rare in the shallowest 
water and relatively abundant in the deepest water. As 
indicated by Jacobson and Vetter (1995), depth distribu- 
tions showed little evidence of ontogenetic migration in 
longspine thornyhead (but see below). 
Our depth distribution results for Dover sole, like re- 
sults in Jacobson and Hunter ( 1993, their Figs. 2-3), show 
that males 22-32 cm TL were most common at depths 
of 200-299 fm. Female Dover sole of the same size were 
more often found in shallower water ( 100-199 fm). Larger 
Dover sole (38-44 cm FL) of both sexes were more com- 
mon in deeper water, where males were most common at 
300-399 fm. These patterns seem clear, although, as point- 
ed out by a reviewer, there was substantial overlap in 95% 
confidence intervals for the depth distributions of male 
and female Dover sole. 
Selectivity patterns for groundfish in the deep-water 
commercial bottom trawl fishery were complex and dy- 
namic (Figs. 6-9). As expected, there were differences 
in fishery selectivity estimates for commercial bottom 
trawls among species, between subareas, and over time 
(Figs. 6—9). Selectivities for species with strong ontogenet- 
ic migration (sablefish, shortspine thornyheads and Dover 
sole) changed most over time. For these fishes, commercial 
bottom trawl selectivity curves were dome-shaped with 
peaks that shifted towards larger sizes in later years as 
the relative amount of fishing effort increased on deep-wa- 
ter fishing grounds where large fish were most common. 
Shifts in commercial bottom trawl selectivity patterns 
towards larger sizes (Figs. 6-9) were most pronounced in 
the north because the proportion of total commercial bot- 
tom trawl fishing effort in deep water over the continental 
slope was higher in the southern subarea during the early 
years of our study than in the northern subarea (Tables 
5-6). Higher proportions of total commercial bottom trawl 
effort in deep water (where large fish are most common) 
during the early years caused relatively high selectivities 
for large fish in the southern subarea. 
Changes in commercial bottom trawl selectivity pat- 
terns over time in sablefish, shortspine thornyhead, and 
Dover sole (Figs. 6-8) were large enough to be important 
in stock assessment work (Rogers et al., 1997), particular- 
ly if assessments are carried out for northern fishing ar- 
eas. Most of the changes in selectivity patterns over time 
were for relatively large fish on the right-hand side of 
the selectivity curves. Commercial bottom trawl selectivi- 
ties for smaller fish on the left-hand side of the selectivity 
100 400 100 400 
Figure 4 
Average depth distributions for shortspine thornyhead 
(Table 7) from October to December by length group. 
200 300 400 500 600 
0.4 
0.3 
0.2 
0.1 
0.0 
0.4 
1 0.3 
I 0.2 
o 
£ 0.1 
0.0 
0.4 
0.3 
0.2 
0.1 
0.0 
200 300 400 500 600 
Depth (fm) 
Figure 5 
Average depth distributions for longspine thornyhead 
(Table 8) from October to December by length group. 
12 cm FL 
14 cm FL 
16 cm FL 
18 cm FL 
20 cm FL 
/ 
22 cm FL 
/ - 
24 cm FL 
26 cm FL 
/ X 
/ 
28 cm FL 
30 cm FL 
