52 
Fishery Bulletin 1 1 1 (1) 
reflexes present, we held all those crabs, as well as all 
crabs of any of the 3 species with any missing reflexes. 
This procedure maintained the primary advantage of 
our RAMP assessments, accounting for a large group 
with high survival, and avoided the need to rely on 
injury assessments to estimate mortality. Both Stevens 
(1990) and Stoner et al. (2008) applied scoring systems 
for injuries, but the variety of injury types makes in- 
jury assessment more subjective and less likely to be 
repeatable than the reflex assessments. 
We provided specific estimates of the unobserved 
mortality rates of crabs swept over by trawl gear com- 
mon to bottom trawl fisheries in the Bering Sea. How- 
ever, assessment of the effects of such mortalities on 
the populations of those crabs will require estimation 
of the portion of those populations exposed to trawling 
each year. Although the distribution of trawling effort 
is well documented by automated position monitoring 
of vessels and onboard observers, the spatial distribu- 
tion of crabs throughout the year is not well known. A 
reliable estimate of the distribution of crabs, including 
seasonal variability, would be needed to estimate their 
exposure to trawling and allow for use of our mortality 
rate estimates in order to estimate resulting mortali- 
ties to the population. This approach would be subject 
to error from interannual and seasonal variations in 
crab distribution — variations that are not well under- 
stood and would be difficult to monitor. 
The number of crabs captured in bottom trawls 
is monitored through catch sampling by onboard ob- 
servers. Another way to estimate the number of crabs 
encountering trawls would be to learn the proportion 
of crabs that are caught in the path of a trawl. Crab 
bycatch data could then be expanded to estimate the 
number encountered, a value to which our mortality 
rates could be applied to estimate overall, unobserved 
mortality. One significant source of error for this ap- 
proach is variability or changes in the specific foot- 
ropes used across the fishery — differences that could 
substantially alter the proportion of crabs retained by 
the trawl. Also, should the trawl fishery approach its 
goal of eliminating crab bycatch, the base bycatch data 
could become sparse and even more variable. 
Conclusions 
Unobserved mortality is an important component of 
bycatch that is both easily overlooked and difficult 
to assess. Mortality rates for commercial crab species 
overrun by bottom trawls used in the Bering Sea var- 
ied substantially between the different components of 
trawls, with lower mortality for crabs that encountered 
sweeps than for crabs that encountered footropes. Re- 
duction of mortality rates of red king crab from 10% to 
4% by raising the sweeps off the seafloor showed that 
gear modifications can mitigate unobserved mortality. 
Acknowledgments 
This study was primarily funded under a grant from 
the North Pacific Research Board (project 711), with 
additional support from the National Cooperative Re- 
search and National Bycatch Reduction Engineering 
Programs of the National Marine Fisheries Service, 
NOAA. We gratefully acknowledge the substantial con- 
tributions of Captain L. Perry and his crew on the FV 
Pacific Explorer and the invaluable sampling efforts of 
P. Iseri, S. Walters, D. Evans, and K. Lee, and particu- 
larly D. Benjamin, who participated during all 3 sum- 
mers of this study. 
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