50 
Fishery Bulletin 1 1 1 (1) 
40% 
35% 
30% 
25% 
20% 
15% 
10% 
5% 
0% 
□ Snow crab 
■ Tanner crab 
S Red king crab 
0 Red king crab (raised sweep) 
Footrope wing 
Footrope center 
Sweep 
Figure 4 
Estimates and 95% confidence intervals of rates of mortality for snow crab (Chionoecetes 
opilio), southern Tanner crab (C. bairdi ), and red king crab ( Paralithodes camtschaticus 
that resulted from contact with 1 of 3 different components of a bottom trawl represen- 
tative of the gear used bottom trawl fisheries in the Bering Sea — the footrope wings or 
extensions, the center of the footrope, or the sweep — and, for red king crab only, a sweep 
raised off of the seafloor (Rose et al., 2010). 
1 
I 
snow crab or as any size of southern Tanner crab, and 
this difference persisted across all gear components 
and control catches. 
Large red king crab had higher mortality than 
smaller king crabs (PcO.OOl), although this effect ex- 
plained <1% of the variability in mortality, compared 
with 12% for the difference between gear components. 
The interaction between crab size and gear component 
was not statistically significant; therefore, there was no 
indication that this difference in vulnerability between 
sizes varied between gear components. Mortality of red 
king crab did not vary significantly between sexes or 
between new-hard-shell and old-shell crab. Although 
the percentage of mortalities was high for soft-shell 
crab (4 of 5 died) and crab with very old shells (3 of 5 
died), those shell types were too rare for a statistical 
validation of difference. 
Discussion 
Our study provides the first reliable estimates of mor- 
tality rates following noncapture (not bycatch or dis- 
card) bottom trawl encounters for 3 commercially im- 
portant crab species. Mortality rates varied by species 
but depended mainly on that part of the trawl system 
they encountered. 
Crabs that passed under the trawl footrope, particu- 
larly in the wing sections, died at higher rates than 
those crab struck by the sweeps. Effective herding by 
sweeps greatly expands the area of seafloor from which 
flatfishes are captured. Mortality rates were substan- 
tially lower for crabs that encountered these herding 
devices in that expanded area than for crabs that en- 
countered the trawl net itself, specifically the footrope. 
Therefore, enhancement of fish capture rates through 
effective herding can also reduce overall crab mor- 
talities (i.e., capture of equivalent quantities of fishes 
without herding would expose more crabs to footrope 
components). The effective reduction of crab mortality 
through use of sweeps was further augmented for red 
king crab with modifications to raise sweeps a few cen- 
timeters above the seafloor (Rose et al., 2010). 
The lower rates of unobserved crab mortalities from 
herding devices (i.e., sweeps), compared with mortality 
rates from trawl footropes, only partially indicate the 
potential of herding to reduce crab mortalities. Mortali- 
ties of crabs that encounter the footrope also would in- 
clude those crabs retained in the net (bycatch). Stevens 
(1990) found that mortality rates were much higher for 
both captured red king crab (79%) and captured south- 
ern Tanner crab (78%) than for escaping crabs. Some 
herding of crabs is conceivable, but their much slower 
