Rose et a! : Mortality rates for Chionoecetes opilio, C. bairdi, and Parahthodes camtschaticus after trawls on the seafloor 
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Figure 1 
Sampling locations for snow ( Chionoecetes opilio) (S) and southern Tanner (C. bairdi) (T) crabs in 2008 and 
red king crab (Paralithodes camtschaticus) in 2009 (RK), during our study of unobserved mortality rates from 
bottom trawling. Bottom trawl area closures (shaded) and depth contours are included for reference. 
possible habitat degradation, as principal reasons for 
the establishment of these closures. Crab bycatch lim- 
its (on the basis of numbers caught) have also trig- 
gered additional closures if seasonal, species-specific 
(and sometimes area-specific) limits are reached. These 
bycatch numbers are obtained from onboard fishery ob- 
servers on an in-season basis (Withered et al., 2000). 
The species-specific crab bycatch limits (in estimated 
numbers of crabs brought aboard) are thought to have 
a biologically insignificant effect on the different crab 
populations because these limits have represented as 
little as 0.113% of the abundance index for snow crab 
and 0. 5-1.0% of abundance for southern Tanner crab 
and red king crab (Withered and Pautzke, 1997). 
Critics of the existing framework of measures for 
crab bycatch management have from time to time as- 
serted that, although bycatch limits appear to be suffi- 
ciently conservative, bycatch represents only a fraction 
of the actual mortality of different crab species caused 
by groundfish fisheries. Citing an unpublished tech- 
nical paper, Thompson (1990) estimated actual trawl 
gear mortality for king crabs to be “10 to 15 times the 
number of crabs that are caught in the net (and esti- 
mated by [National Marine Fisheries Service] observ- 
ers).” These concerns cannot be adequately evaluated 
without addition of valid estimates of the unobserved 
mortality rates for these crab species to the assess- 
ments of bycatch and discard. Some crab researchers in 
Alaska (Murphy et al., 1994) also have underscored the 
need for additional research on injury rates and unob- 
served or unaccounted for mortality from both direct- 
ed crab fisheries and groundfish trawl fisheries. Dew 
and McConnaughey (2005) concluded that excessively 
high mortality rates on male Bristol Bay red king crab 
from the directed fishery and unaccounted for mortal- 
ity of females from the groundfish fisheries explain the 
downward population trajectory of this crab species 
through the late 1970s and early 1980s better than 
does the more accepted scientific hypothesis that the 
low population levels of red king crab were explained 
by unfavorable climate conditions. 
Worldwide, the recognition of unobserved mortali- 
ties as a potentially significant element by the fishing 
industry and by fishery managers has increased the 
number of studies that have addressed such mortali- 
ties and the range of methods used in their estimation. 
