42 
Quantification and reduction of unobserved 
mortality rates for snow, southern Tanner, and 
red king crabs (Chionoecetes opilio, G bairdi, 
and Paralithodes camtschaticus ) after 
encounters with trawls on the seafloor 
Craig S. Rose (contact author ) 1 
Carwyn F. Hammond 1 
Allan W. Stoner 2 
J. Eric Munk 3 
John R. Gauvin 4 
Email address for contact author: craig. rose@noaa gov 
Abstract — Unobserved mortalities 
of nontarget species are among the 
most troubling and difficult issues 
associated with fishing, especially 
when those species are targeted 
by other fisheries. Of such concern 
are mortalities of crab species of 
the Bering Sea, which are exposed 
to bottom trawling from groundfish 
fisheries. Uncertainty in the man- 
agement of these fisheries has been 
exacerbated by unknown mortality 
rates for crabs struck by trawls. In 
this study, the mortality rates for 3 
species of commercially important 
crabs — red king crab, ( Paralithodes 
camtschaticus), snow crab ( Chion- 
oecetes opilio ) and southern Tanner 
crab (C. bairdi ) — that encounter dif- 
ferent components of bottom trawls 
were estimated through capture of 
crabs behind the bottom trawl and 
by evaluation of immediate and de- 
layed mortalities. We used a reflex 
action mortality predictor to predict 
delayed mortalities. Estimated mor- 
tality rates varied by species and by 
the part of the trawl gear encoun- 
tered. Red king crab were more vul- 
nerable than snow or southern Tan- 
ner crabs. Crabs were more likely 
to die after encountering the foot- 
rope than the sweeps of the trawl, 
and higher death rates were noted 
for the side sections of the footrope 
than for the center footrope section. 
Mortality rates were <16%, except 
for red king crab that passed under 
the trawl wings (32%). Herding de- 
vices (sweeps) can expand greatly 
the area of seafloor from which flat- 
fishes are captured, and they subject 
crabs in that additional area to low- 
er (4-9%) mortality rates. Raising 
sweep cables off of the seafloor re- 
duced red king crab mortality rates 
from 10% to 4%. 
Manuscript submitted 7 March 2012. 
Manuscript accepted 9 November 2012. 
Fish. Bull. 111:42-53 (2013). 
doi:10.7755/FB. 11 1.1.4 
The views and opinions expressed 
or implied in this article are those of the 
author (or authors) and do not necessar- 
ily reflect the position of the National 
Marine Fisheries Service, NOAA. 
’ Conservation Engineering Program 
Alaska Fisheries Science Center 
National Marine Fisheries Service. NOAA 
7600 Sand Point Way NE 
Seattle, Washington 98115 
2 Fisheries Behavioral Ecology Program 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
2030 Marine Science Drive 
Newport, Oregon 97365 
The potential for unobserved mor- 
tality of crabs that encounter bot- 
tom trawls but are not captured has 
long been a concern for the manage- 
ment of groundfish fisheries in the 
Bering Sea (Witherell and Pautzke, 
1997; Witherell and Woodby, 2005). 
Fisheries on the crab and groundfish 
stocks of the wide continental shelf 
of the eastern Bering Sea have made 
Dutch Harbor, the principal port for 
that area, the highest port by ton- 
nage in the United States and 1 of 
the 2 highest ports by dollar value 
for more than 20 years. 1 Three ma- 
jor crab species — red king crab ( Para- 
lithodes camtschaticus), snow crab 
( Chionoecetes opilio), and southern 
Tanner crab (C. bairdi ) — are targets 
of large commercial fisheries (Otto, 
1990). The 2 Chionoecetes species 
1 U.S. Department of Commerce. 1995- 
2011. Fisheries of the United States 
1995 (1996, ...,2011). Current Fishery 
Statistics 1995 ( 1996, ...,2011). U.S. 
Dep. Commer., NOAA, Natl. Mar. Fish. 
Serv., Fisheries Statistics Division, Silver 
Spring, MD. [Available from http:// 
w w w. st.nmfs.noaa.gov/commercial- 
fisheries/fus/index. ] 
3 Shellfish Assessment Program 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
301 Research Court 
Kodiak, Alaska 99615 
have similar low, flat body shapes 
and inhabit deeper water with mud- 
dier substrates than that of the red 
king crab, which has a thicker body 
and inhabits shallower, sandier areas 
(Jadamec et al., 1999; Donaldson and 
Byersdorfer, 2005). Groundfish spe- 
cies, particularly gadids and flatfishes 
are targeted with trawls. Overlaps be- 
tween crab habitat and areas trawled 
by groundfish fisheries can result in 
some mortality for crabs that encoun- 
ter groundfish trawls, either through 
capture and discard (bycatch) or as 
unobserved mortality of crabs that 
remain on the seafloor (Witherell and 
Pautzke, 1997). 
The current management mea- 
sures to control and reduce bycatch 
of the major Bering Sea crab spe- 
cies in Alaska groundfish fisheries 
include extensive year-round trawl 
closure areas (Fig. 1) and bycatch 
limits outside these areas. The year- 
round closure areas were established 
to protect areas of known concentra- 
tions of female and juvenile crabs. 
Armstrong et al. (1993) and Witherell 
and Pautzke (1997) cited unobserved 
trawl-induced mortality, along with 
4 Alaska Seafood Cooperative 
4241 21 s1 Avenue W, Suite 302 
Seattle, Washington 98199 
