Stoner et al.: An assessment of discard mortality for two Alaskan crab species, based on reflex impairment 
343 
Figure 3 
Time to mortality (days) observed for Chionoecetes 
bairdi (Tanner crab) and C. opilio (snow crab) shown 
as a function of reflex impairment (index A). Values 
are mean, ±standard error (in parentheses). Index A 
was calculated as the sum of completely lost reflexes 
(score = 0) and index B was calculated as the sum of 
reflexes either lost or weak (scores = 0 or 1). 
with increasing impairment, reaching an asymptote 
at an index equal to 3. Mortality also increased with 
injury score, but the effect was smaller and more 
gradual. Probability of mortality in C. opilio was de- 
termined primarily by reflex impairment (with no ob- 
vious or significant effect of injury score). Reflex im- 
pairment and injury scores were correlated in both C. 
bairdi (P-0.514, F l 248 =89.024, P<0.001) and C. opilio 
(P-0.725, F l 3 9 7=44i5.504, PcO.OOl). 
The number of crabs representing each reflex impair- 
ment index (0 to 6) was not uniform, and we wanted to 
be certain that these unequal numbers did not bias the 
resulting relationship between mortality and reflex im- 
pairment. For example, among the 399 C. opilio tested, 
reflex impairments equal to 0, 1, and 6 were over-repre- 
sented. Therefore, we randomly drew up to 20 individu- 
als for each impairment level, reducing the number of 
crabs entering the model to 115. The reduced database 
resulted in a relationship between mortality and reflex 
impairment that was identical to that incorporating the 
larger data set. A similar analysis for C. bair'di produced 
a similar result, showing that the uneven distribution of 
data did not bias the experimental outcome and that the 
logistic models shown in Figure 4 were robust. 
Discussion 
Behavior of animals reflects a host of internal and exter- 
nal conditions, and, in the context of fishing-related 
C. bairdi 
C. opilio 
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Figure 4 
Surface plots, resulting from logistic regression, show- 
ing the probabilities of mortality for Chionoecetes bairdi 
(Tanner crab) and C. opilio (snow crab) with varied 
levels of reflex impairment (index A) and injury. Index 
A was calculated as the sum of completely lost reflexes 
(score = 0) and index B was calculated as the sum of 
reflexes either lost or weak (scores = 0 or 1). 
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