Jacobson et al.: Depth distributions and time-varying selectivities for various bottom fishes 
317 
100 300 500 
1.0 
0.8 
0.6 
0.4 
0.2 
0.0 
1.0 
0.8 
0.6 
0.4 
0.2 
0.0 
1.0 
0.8 
0.6 
0.4 
0.2 
0.0 
Depth (fm) 
Figure 2 
Average depth distributions for sablefish (Table 5) from 
October to December by length group. 
1 00 300 500 
100 300 500 
Depth (fm) 
Figure 3 
Average depth distributions for male and female Dover 
sole (Table 6) from October to December by length group. 
this and the expression from Equation 8 for F d into Equa- 
tion 7 gives 
C 
TV 
p(d\L). 
( 11 ) 
The commercial fishery selectivity for fish length L in year 
y is proportional to C vL / N yL in Equation 11 because C vL - 
N l F s l . Thus, commercial fishery selectivities s L were 
proportional to 
s ,x 
oc 
p(d | L). 
( 12 ) 
Following our convention, we rescaled commercial bottom 
trawl fishery selectivity estimates from Equation 12 so 
that the largest was one. 
We computed preliminary sex and subarea-specific depth 
distributions for each species by averaging depth distribu- 
tions for each sex from bottom trawl surveys in each sub- 
area. Depth distributions for male and female sablefish 
and thornyheads were similar, but male Dover sole appear 
to move into deep water at smaller sizes than female Dover 
sole. There were no clear differences between subareas for 
any species or sex. We therefore calculated “best estimates” 
(Tables 5-8; Figs. 2-5) of depth distributions for sablefish 
and thornyheads by combining subareas and sexes (i.e. 
by computing survey-specific depth distributions for males 
and females combined, and then averaging over all sur- 
veys). For Dover sole, we calculated sex-specific depth dis- 
tributions based on combined subareas (i.e. by computing 
survey-specific depth distributions for males and females 
separately and then averaging over all surveys). Depth dis- 
tributions for male and female Dover sole combined (not 
shown) can be approximated by averaging sex-specific val- 
ues (e.g. averaging values and rescaling the averages to a 
maximum of one). 
Our best estimates of commercial bottom trawl fishery 
selectivities for each subarea, year, and species were based 
on subarea-specific fishing effort data (Tables 1-2) and 
our best estimates of depth distributions for the whole 
coast and sexes combined. Preliminary calculations sug- 
gested that commercial bottom trawl selectivities for male 
and female Dover sole were similar (even though depth 
distributions were different); therefore we combined sex- 
es for final selectivity calculations. Apparently, selectivity 
calculations for Dover sole were relatively insensitive to 
differences in depth distributions for males and females 
because of the overwhelming effects of changes over time 
in the distribution of commercial fishing effort. Addition- 
al data may be required to identify clearly differences in 
selectivity patterns for male and female Dover sole in the 
commercial bottom trawl fishery. 
