Humborstad et al.: Reflex impairment as a measure of survival potential for Gadus morhua 
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Figure 1 
Fish restraining device used to test reflex actions in Atlantic cod ( Gadus morhua). In this 
photograph, the fish is being tested for operculum reflex action. 
response to hand stroking of the tail flanks along the 
long axis while being held under the abdomen. The en- 
tire sequence for reflex testing in a replicate fish was 
conducted within 60 seconds. 
Repeated reflex testing was conducted with a group 
of ten fish to determine if cod were stressed by the 
reflex testing procedure itself. These fish were netted, 
restrained, and their reflexes were tested at 0, 60, and 
180 minutes after the initial netting and restraint. 
Between each sampling time, fish were placed back 
into a tank with sea water and recaptured for the next 
reflex test. 
The effect of recovery time (0-15 min) on reflexes af- 
ter exposure of fish to stressors was tested to determine 
an appropriate time to sample cod reflexes. Replicate 
fish (n = 35) were exposed to standard stressors (see de- 
tailed description of stressor administrations below in 
step 2) of 5 minutes of swimming, 1 minute of net abra- 
sion, and 5 minutes in air (all at 9.0°C). Then fish were 
returned to the stock tank and reflex testing was begun 
at 0, 5, 7, 10, or 15 minutes after exposure to stressors 
and completed within 8 minutes from starting time of 
testing for each exposure group [n-1 per group). 
It was possible that disturbance from repeated net- 
ting of fish from a stock tank before netting replicate 
fish for stressor treatment could have affected the level 
of reflex impairment resulting from exposure to stress- 
ors, because fish could have been repeatedly stressed 
by being chased before capture. To test this possibility, 
fish were exposed to 5 minutes of swimming, 1 minute 
of net abrasion, and 10 minutes of air exposure (all at 
9.0°C) after being either captured from a tank (n = 8) in 
which four fish had been previously captured or from 
a tank (n- 8) in which 16 fish had been previously cap- 
tured. Then fish were returned to the tanks and reflex 
testing began 5 minutes after exposure to stressors 
and was completed within 9 minutes from the start of 
testing for each exposure group. Fish were then held 
to determine mortality. Immediate and delayed mor- 
tality resulting from stressors was observed by holding 
fish in tanks for 60 days and removing and counting 
dead fish daily (mortality observed as common death 
signs, e.g., motionlessness, static flaring of operculum, 
nonresponsiveness to touch stimuli, rigor mortis). The 
rearing regime for the stressed fish was similar to stock 
tank fish. 
Step 2: Stressors, exposure experiments, 
and stress induction 
Stressors were selected to mimic the stressors cod expe- 
rience during capture during Danish seine operations 
and to create a gradient of stress from low to moderate 
to lethal. Exposure to air was chosen for its known 
importance as a stressor for fish sorted on deck. Forced 
swimming is a relevant capture stressor. Net abrasion 
