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Fishery Bulletin 99(2) 
R 
from the length composition of the population (Gunderson, 
1993). Our estimates of depth distributions for the total 
population s p(d\L) were generally unaffected by bottom 
trawl survey gear selectivity because selectivity of the sur- 
vey gear affects both PJL,d) and PJL) equally and “can- 
cels out.” This important point is explained further below, 
after other terms in Equation 1 are defined. 
For each bottom trawl survey, species, and depth stra- 
tum, length composition of the surveyed population pjL ) d) 
was calculated as a weighted average of length composi- 
tion data from each tow in the stratum: 
^p sl (L\d)w sdl 
p (L\d) = — 
where p st (L \ d) = the length distribution from tow t, and 
w sdt = the tow catch rate (fish/m 2 ). 
Tow catch rates were computed as w s d t = n sd t l a sd t , where 
n sdt is the total number of fish caught and a sd t is the area 
swept (width of the net times distance towed). 
