Rose et al Mortality rates for Chionoecetes opilio, C. bairdi, and Paralithodes camtschaticus after trawls on the seafloor 
51 
locomotion, compared with that of commercial fish spe- 
cies, led us to assume that the number of crabs that 
encountered each part of the trawl system is roughly 
proportional to the area swept by each part. 
Red king crab had higher mortalities (6-32%) than 
2 species of Chionoecetes, snow and southern Tanner 
crabs (4-15%) — a result that was expected given the 
generally smaller size and flatter body shape of Chi- 
onoecetes crabs. Overall mortality rates, weighted for 
the approximate relative areas swept by each trawl 
component for modern Bering Sea flatfish trawls (90% 
sweeps, 6% footrope wings, 4% footrope center) were 
6% for snow crab, 5% for southern Tanner crab, and 
11% for red king crab. The raised sweeps reduce mor- 
tality rate for red king crab to 6%. Such sweep modi- 
fications were required by the North Pacific Fishery 
Management Council for Bering Sea flatfish trawlers 
beginning in January 2011. 
The trawl gear and methods selected for our experi- 
ment represented those gear and methods used in the 
Bering Sea flatfish fisheries. The gear is characterized 
by long, combination rope sweeps and footropes built 
with large-diameter, rubber bobbins or disks to keep 
the net mouth more than 20 cm above the seafloor. 
This footrope selection by the fleet has been driven 
partially by pressure to reduce crab bycatch. Decreas- 
ing bycatch through changes to gear means that more 
crabs pass under the trawl net. Although other Alas- 
ka bottom trawl fisheries (e.g., for Pacific Cod [Gadus 
macrocephalus ]) use similar footropes, they use much 
shorter sweeps. Therefore, although cod trawls cover 
less seafloor (and hence contact fewer crabs) per kilo- 
meter towed than flatfish trawls, a higher proportion 
of the crabs might die because more of them would 
encounter the footrope components. The other major 
trawl fishery that can affect Bering Sea crabs is the 
fishery for Walleye Pollock { Theragra chalcogramma ). 
Pollock trawls must meet a number of requirements 
that allow them to be considered “pelagic” trawls, but 
this fishery commonly has been fished with substantial 
seafloor contact. 
Because regulations disallow any protective bobbins, 
none of the crab mortality estimates for gear compo- 
nents examined in our study can be used to estimate 
mortalities used for the pollock fishery, where chain foo- 
tropes are used. The differences we found in mortality 
rates between different gear components indicate that 
changes in the specific gear configurations could im- 
prove or worsen crab mortality rates. The rates found 
here should not be applied to trawls with substantially 
different ground gear (e.g., chain footropes used in the 
Bering Sea pollock fishery). Component-specific mortal- 
ity differences also present an opportunity to reduce 
crab mortality through identification of less damaging 
footrope configurations that sustain effective capture 
of target species. A companion study where an alter- 
native footrope was tested has been completed (Ham- 
mond, 2009). 
Because crabs were held for periods <14 days, our 
results did not include mortalities delayed over longer 
periods. The rapid drop of new mortalities after the 
first few days and the presence of all reflexes at the 
end of the study suggest that little additional mortality 
would be expected unless some other mechanism, such 
as infection or problems with molting, created a pulse 
of mortalities outside of the time period observed (see 
also Stoner et ah, 2008). Likewise, holding crabs in on- 
deck tanks protected them from predation that would 
have increased delayed mortality if vulnerability to 
predation was enhanced by injury or stress after trawl 
exposure. Predators and scavenger species have been 
observed to move into areas recently swept by bottom 
trawls (Prena et ah, 1999). Although this potential for 
additional mortality was not addressed directly in this 
study, the vast majority of the surviving crabs retained 
their full suite of assessed reflexes, including mobility 
of walking legs and defensive reactions. If predators 
initially focused on the more severely impaired and in- 
jured crabs that ended up as mortalities in our study, 
less impaired crabs might have some respite, allowing 
some time for recovery and reducing any difference be- 
tween our results and the actual unobserved mortality 
due to predation. 
All retained red king crab were held until the end of 
the study, 4 days after the control tows were complet- 
ed. Because control crabs were held for only 4-6 days, 
we examined the proportions of delayed mortalities of 
crabs held for longer periods. For crabs held more than 
10 days, 93% of the mortalities occurred in the first 4 
days and 95% in the first 6 days. Because only 9 of the 
881 red king crab caught in the control net died, the 
possibility of missing one additional mortality because 
of a shortened holding time was not considered to in- 
troduce a significant potential bias. Short holding time 
was even less of a concern for southern Tanner and 
snow crab because all of those crabs were held 7 days 
or longer and the low proportion of deaths after the 
first days noted during the pilot project (Stoner et ah, 
2008) continued during our 2008 observations. 
In this study, the RAMP procedure (Stoner et ah, 
2008) was successful in prediction of mortality rates 
for many more crabs than we could have held to ob- 
serve delayed mortality. Of all crabs assessed, 85% had 
either all reflexes present ( Chionoecetes spp.) or were 
uninjured with all reflexes present (red king crab). 
Holding only one-eighth of these crabs provided gener- 
ous samples (>350 crabs per species) for estimation of 
their low mortality probabilities. If we had followed a 
conventional study method and held all crabs regard- 
less of reflex state, more than 4 times as many crabs 
would have been held, with minimal reductions in 
uncertainty. 
Although only representing a small proportion of the 
observed crabs, the RAMP procedure also allowed us 
to efficiently account for crabs with intermediate reflex 
assessments (reflex scores of 1 to 5). Because significant 
mortalities occurred to injured red king crab with all 
