309 
Abstract— We estimated size-specific 
depth distributions and commercial 
bottom trawl fishery selectivities for 
Dover sole (Microstomus pacificus), 
shortspine thornyhead (Sebastolobus 
alascanus), longspine thornyhead (S. 
altivelis), and sablefish ( Anoplopoma 
fimbria) along the U.S. west coast. 
Depth distributions are size-specific be- 
cause fish migrate ontogenetically to 
deep water. With ontogenetic migra- 
tion, fishery selectivities of commercial 
bottom trawls depend on depth of fish- 
ing because large fish are most common 
in deep water. Depth distributions were 
similar for northern and southern areas 
and for males and females. Results show 
ontogenetic migration in sablefish, sug- 
gest a possible weak ontogenetic mi- 
gration in longspine thornyhead, and 
confirm ontogenetic migration patterns 
already reported for Dover sole and 
shortspine thornyhead. Fishery selec- 
tivities varied among species, between 
areas, and changed dramatically over 
time for most species as fishing effort 
moved into deep water. Our approach 
used biological data collected during 
research bottom trawl surveys but was 
generally not affected by size selectiv- 
ity of bottom trawl survey gear. Uncer- 
tainty in our commercial bottom trawl 
selectivity estimates was mostly from 
length-specific capture probabilities (or 
vulnerabilities) for fish in the path of 
commercial bottom trawls. Our esti- 
mates complement selectivity estimates 
from stock assessment models. The 
approach may be useful whenever the 
geographic distribution of fish depends 
on size or age, fishing effort is not ran- 
domly distributed geographically, and 
survey estimates of fish density, bathy- 
metric data, and commercial fishing 
effort information are available. 
Manuscript accepted 11 October 2000. 
Fish. Bull. 99:309-327 (2001 ). 
Depth distributions and time-varying bottom trawl 
selectivities for Dover sole {Microstomus pacificus), 
sablefish {Anoplopoma fimbria), and tliornyheads 
{Sebastolobus alascanus and 5. altivelis) 
in a commercial fishery 
Larry D. Jacobson 
Southwest Fisheries Science Center 
National Marine Fisheries Sen/ice, NOAA 
P.O Box 271 
La Jolla, California 92038 
Present address: Northeast Fisheries Science Center 
National Marine Fisheries Sen/ice, NOAA 
166 Water Street 
Woods Hole, Massachusetts 02543-1026 
E-mail address: larry.|acobson@noaa gov 
Jon Brodziak 
Jean Rogers 
Northwest Fisheries Science Center 
National Marine Fisheries Service, NOAA 
2030 SE Marine Science Drive 
Newport, Oregon 97365-5296 
In our study, we estimated depth dis- 
tributions and fishery selectivities for 
four demersal fish species taken in com- 
mercial bottom trawls: Dover sole ( Micros- 
tomus pacificus), shortspine thorny- 
head ( Sebastolobus alascanus), long- 
spine thornyhead (S. altivelis ), and sable- 
fish (Anoplopoma fimbria). The fishes 
in our study were all valuable com- 
ponents of the deep-water commercial 
bottom trawl fishery off Washington, 
Oregon, and California (Pacific Fishery 
Management Council, 1998). Depth dis- 
tributions for many fishes in the deep- 
water fishery depend on length and age 
because of ontogenetic migration (move- 
ment to deep water as fish grow and age, 
Jacobson and Hunter, 1993; Jacobson 
and Vetter, 1995). Depth distributions 
and ontogenetic migration are impor- 
tant because they affect many aspects 
of the deep-water fishery, including se- 
lectivity of commercial bottom trawls, 
which are the primary fishing gear. 
Fishery selectivities measure the rela- 
tive intensity of fishing mortality on fish 
of different size or age (Megrey, 1989). 
Fishery selectivities depend on size for 
fishes in the deep-water bottom trawl 
fishery (Perez-Comas, 1996) because of 
factors that include size and shape of 
mesh, size and shape of fish, orientation 
of netting, twine material (Wileman et 
al., 1996), and (as shown below) depth of 
fishing. In many length-structured stock 
assessment models, for example, the size- 
specific fishing mortality rate (F yL ) in 
year y for fish in length class L is sep- 
arated into the product of year-specific 
fishing mortality (F ) and size-specific 
selectivity parameters (s L ), so that F L = 
F v s L (Megrey, 1989). Selectivities are 
typically scaled so that the selectivity for 
a reference size or age is one (Deriso et 
al., 1985; Methot, 1990). By convention, 
we scaled selectivities so that the length 
group with the highest fishing mortality 
rate had a selectivity of one. 
Selectivities determine how fishing af- 
fects the size and age structure of a fish 
stock. They are used in stock assessment 
models to relate length and age compo- 
sition data from catch samples to length 
and age composition of the stock. They 
are important in predicting effects of 
harvest rates (Legault, 1998) and in cal- 
culating biological reference points (e.g. 
*0.1> F r*p> *35*. F n,a.V See Clark ’ 1991 > 
used to recommend catch levels. At the 
policy and legal levels, they are often 
