Stoner: Discard mortality for Alaskan crabs after exposure to freezing temperatures 
461 
but considered together, they reflect overall condition. 
For example, chela closure was the most sensitive in- 
dicator of freezing stress observed in this study, but 
chela function was affected in only 14% of crabs (C. 
bairdi and C. opilio ) injured in trawl capture (Stoner 
et al., 2008). Impairment of reflex action in eye stalks 
was also elevated in crabs exposed to freezing tempera- 
tures but was rare in trawl-captured crabs. Conversely, 
leg retraction and leg flaring actions were impaired 
infrequently under freezing temperatures but were com- 
monly impaired in trawl-captured crabs. Despite these 
differences, mortality was closely associated with reflex 
impairment in both the freezing-stress and trawl-stress 
experiments, and the logistic regressions for C. bairdi 
and C. opilio in this study were nearly identical. This 
result indicates that the reflex impairment score is a ro- 
bust tool for predicting mortality in Chionoecetes species. 
Limitations of reflex action mortality predictors 
The RAMP approach does have certain limitations 
because, as with other measures of animal condition, 
some of the indirect and delayed effects of the discard- 
ing process and the behavioral impairments were not 
considered. 1) Predation on discarded catch can occur 
in the water column. Marine mammals, large fish, and 
birds often follow fishing vessels, scavenging injured and 
uninjured discards. 2) Impairments in defensive behav- 
iors can result in predation once a crab has reached its 
benthic habitat. Some of these discards would other- 
wise survive their injuries. 3) Impairments in sensory 
apparatus, feeding appendages, or locomotory functions 
(e.g., because of limb damage or autotomy), can result 
in starvation. Carls and O’Clair (1995) showed that 
sublethal effects of freezing on C. bairdi can include 
reduced feeding rate and growth. In addition, mortality 
observed in laboratory experiments may not reflect the 
lost function of sensory appendages that aid in locat- 
ing food or avoiding predators. Eye stalks are particu- 
larly susceptible to freezing (van Tamelen, 2005), but 
it is unknown how vision may be impaired by exposure 
of crabs to freezing temperatures. The eyes of deep- 
dwelling decapods can also be damaged by exposure to 
sunlight (Gaten 1988). Future experiments should be 
designed to test for losses in vision, chemoreception, 
and other sensory functions after exposures of crabs to 
air and freezing temperature. 4) Freezing temperatures 
can also cause long-term injury resulting in unsuccessful 
molting (O’Brien et al., 1986). Carls and O’Clair (1995) 
reported increased limb loss at molting after exposure 
to freezing, and deaths during molting are a common 
occurrence in Alaskan crabs (Stoner, pers. observ.). 5) 
Freezing and other forms of injury may also raise the 
animal’s susceptibility to disease. 
RAMP curves as currently developed do not account 
for these forms of discard mortality, and do not provide 
an absolute value for discard mortality. However, it is 
likely that the last four sources of mortality discussed 
above will be directly proportional to the reflex impair- 
ments observed. Longer term holding or tagging experi- 
ments could provide greater insight into the relation- 
ships between reflex impairment and long-term survival 
of discarded animals. 
Conclusions 
Fisheries for Chionoecetes spp. in Alaska are centered on 
the winter season, and the threat of cold-related mortal- 
ity in the face of wind chill conditions is real. One could 
employ van Tamelen’s (2005) thermal model to obtain 
crab mortality estimates in the field; however, this would 
require continuous monitoring of a wide array of envi- 
ronmental conditions as well as crab measurements. In 
addition, mortality of crabs in the field results from vari- 
ous combinations of physiological stressors and physical 
injuries (not only thermal stress) for which appropriate 
mortality rates are not known. A simpler and more direct 
measure of crab condition and viability is provided by the 
RAMP approach. Impaired reflexes reveal stress and the 
composite reflex impairment index allows a calculated 
probability of mortality that is independent of crab size, 
physical injuries, and exposure conditions (Stoner et al., 
2008; this study). Representative crabs are observed in 
hand, the reflex actions are summed, and probability of 
mortality for an individual or a treatment population 
can be extracted quickly from the RAMP curve with 
reasonable precision. Once a robust RAMP curve is 
established, presence or absence of reflex actions are 
the only observations required to calculate probability of 
mortality. The approach should be equally applicable to 
the wide variety of crabs, lobsters, and shrimps that are 
routinely discarded as unwanted or illegal bycatch, or 
in instances where an immediate assessment of animal 
condition or discard mortality is required. Greatest use 
for the RAMP approach will occur in experiments with 
fishing gear or handling methods aimed at reducing 
bycatch or discard mortality. 
Acknowledgments 
This research was supported by a grant from the North 
Pacific Research Board (project no. 711). C. Rose, J. E. 
Munk, P. Iseri, D. Benjamin, and C. Hammond assisted 
with the field operations, and the captain and crew of FV 
Pacific Explorer helped to set up experimental systems. 
M. W. Davis provided guidance in experimental proto- 
cols and data interpretation. Helpful suggestions for the 
manuscript were made by M. Carls, M. W. Davis, J. E. 
Munk, and B. G. Stevens. 
Literature cited 
Alverson, D. L., M. H. Freeberg, S. A. Murawski, and J. G. 
Pope. 
1994. A global assessment of fisheries bycatch and 
discards. FAO (United Nations Food and Agriculture 
Organization) Fish. Tech. Paper no. 339, 235 p. 
