Jacobson et al : Depth distributions and time-varying selectivities for various bottom fishes 
325 
(K[ ) were reduced by escapement of small fish through 
3-inch meshes so that vulnerabilities estimates for small 
fish were biased high as well. The extent of the potential 
bias is unknown but bias was probably low for large fish. 
It might be possible to refine estimates of vulnerability 
parameters by carrying out field studies similar to those 
analyzed by Perez-Comas, but by using small mesh liners 
in the codends of commercial bottom trawls as the ref- 
erence standard. Small mesh can cause problems under 
commercial fishing conditions (Erickson et al., 1996) and 
it might therefore be preferable to use codend covers to 
capture small fish as they escape through codends with 
commercial-size mesh instead. 
As described above, fish may move away from or into 
the path of a commercial bottom trawl (herding) to an ex- 
tent that depends on size (Gunderson, 1993). Herding is 
complex and depends on net design, size, towing speed, 
and other factors (Ramm and Xiao, 1995). These variables 
would affect our calculations of size-specific selectivities in 
commercial bottom trawls to the extent that the vulner- 
ability estimates used in our study would fail to measure 
the relatively probability of capture for fish of different 
size in front of commercial bottom trawls. We have little 
information on this topic for west coast groundfish. 
Loss of small fish through 3-inch codends during the 
paired bottom trawl experiments used to estimate vulner- 
abilities likely biased our estimates of bottom trawl selec- 
tivities in commercial fisheries. Length-composition data 
for sablefish, Dover sole, and thornyheads from commer- 
cial gear with 3-inch codends include fewer small fish than 
length-composition data from survey bottom trawls with 
small mesh (3.2 cm) liners (Lauth 3 ). We have no other infor- 
mation about size-specific probabilities of escape of small 
fish, but studies with survey bottom trawls and a variety 
of species (e.g. Engas and Godp, 1989a; 1989b; Walsh, 1992) 
show that small fish do escape. We recommend field studies 
with commercial bottom trawls and video equipment. 
Acknowledgments 
J. A. Perez-Comas (School of Fisheries, Univerisity of 
Washington) provided advice, information, and unpub- 
lished estimates of vulnerability parameters for sablefish. 
R. Lauth and M. Wilkins (Alaska Fisheries Science Center, 
National Marine Fisheries Service) provided data and 
advice. D. Sampson (Oregon State University) stimulated 
our sensitivity analyses. R. Methot (Northwest Fisheries 
Table 8 
Average depth distributions, i.e. probabilities of depth given length or p(cl | L), and CVs for longspine thornyhead (sexes combined) 
between 36°00'N and 48°30'N lat and between 200 and 699 fm (longspine thornyhead were seldom taken at 100-199 fm) during 
October-December based on eight National Marine Fisheries Service bottom trawl surveys. Length groups and depth intervals 
defined as in Table 5. All eight surveys took 12-30 cm longspine thornyhead 
Depth intervals (fm) 
Fork length ( cm ) 
200-299 
300-399 
400-499 
500-599 
600-699 
Depth distributions 
12 
0.0684 
0.2512 
0.3288 
0.1845 
0.1671 
14 
0.0701 
0.2082 
0.3252 
0.2028 
0.1938 
16 
0.0817 
0.2024 
0.3155 
0.2078 
0.1925 
18 
0.0652 
0.1831 
0.2875 
0.2289 
0.2353 
20 
0.0401 
0.1701 
0.2801 
0.2590 
0.2509 
22 
0.0334 
0.1545 
0.2970 
0.2957 
0.2195 
24 
0.0250 
0.1344 
0.2894 
0.3488 
0.2024 
26 
0.0112 
0.1081 
0.2784 
0.4152 
0.1871 
28 
0.0080 
0.1225 
0.3151 
0.3610 
0.1934 
30 
0.0034 
0.2403 
0.2711 
0.2777 
0.2076 
cv 
12 
0.34 
0.09 
0.09 
0.07 
0.09 
14 
0.38 
0.11 
0.13 
0.09 
0.13 
16 
0.37 
0.14 
0.10 
0.12 
0.13 
18 
0.33 
0.14 
0.09 
0.10 
Oil 
20 
0.25 
0.12 
0.09 
0.11 
0.11 
22 
0.27 
0.13 
0.07 
0.07 
0.13 
24 
0.34 
0.09 
0.06 
0.04 
0.14 
26 
0.26 
0.13 
0.08 
0.03 
0.17 
28 
0.69 
0.15 
0.15 
0.07 
0.23 
30 
0.46 
0.19 
0.23 
0.16 
0.25 
