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Fishery Bulletin 11 5(4) 
A C 
Time (min) 
Figure 4 
Results of the behavioral experiment quantifying responses of pairs of Pacific halibut (Hippoglossus stenol- 
epis) to a visual stimulus (i.e., a white jig that simulated prey) at 4 ambient light levels (photon flux den¬ 
sity): (A) lxl0~ 3 , (B) lxlO -4 , (C) lxl0~ 5 , and (D) lxl0~ 6 pmolrrr 2 -s~h For each experiment, ambient light 
level was set with a rheostat and fish were allowed to acclimate for 2 h before the next trial began. Each 
trial consisted of a 5-min period before and a 5-min period after presentation of the visual stimulus. After 
the initial 5-min period, the jig was rapidly moved up and down within a Plexiglas column for 60 s and 
then allowed to sink back to the level at which it was out of sight of the fish (i.e., to the masked bottom of 
the column). A reaction was considered positive if the fish 1) moved one body length, 2) made oral contact 
with the column as it attempted to bite at the jig, or 3) reoriented itself such that its long axis was directly 
pointing toward the jig. Scores were recorded at 10-s intervals as either 0 (no reaction by either fish in the 
pair), 1 (reaction by one fish), and 2 (reaction by both fish). For each minute, the scores were summed to 
arrive at an activity index. 
Using both ERG and a behavioral assay, we tested 
the hypothesis that Pacific halibut recover from retinal 
damage and visual function resulting from exposure to 
direct sunlight. Our ERG data indicated damage to the 
Pacific halibut visual system and no significant recov¬ 
ery during the 10 weeks after exposure. Even after 10 
weeks, it took approximately 17 times the light inten¬ 
sity to elicit a response 50% of maximum than with 
control fish. This result equates to an approximate 
94% reduction in retinal sensitivity. In contrast, our 
behavior assay (which occurred 2-6 d after exposure 
to simulated sunlight) could not reveal impairment of 
the ability of Pacific halibut to detect visual cues as¬ 
sociated with simulated prey across a broad range of 
ambient light levels. 
Electroretinography is a procedure in which the 
summed electrical responses from the retinal photore¬ 
ceptors are recorded by placing electrodes on the cor¬ 
neal surface and skin adjacent to the eye. In our study, 
we exposed fish to 15 min of simulated sunlight, an 
intensity equivalent to ambient sunlight under clear 
skies at noon (Newport, Oregon, 5 October 2007; the 
same exposure used by Brill et ah, 2008). Light-ex- 
posed fish required approximately 5 times the amount 
of light to generate an ERG response equal to control 
fish. This was manifest as a depression in both volt¬ 
age and p-max voltage plotted against log illumina¬ 
tion. These curves remained depressed over a 10-week 
post exposure period, compared with controls that in¬ 
dicated no recovery of retinal sensitivity. Brill et al. 
(2008) speculated that the mechanism of damage was 
disruption of photoreceptor cells and predicted that 
the process would be progressive and permanent. Our 
ERG data support this contention. The illumination re¬ 
quired to stimulate a 50% maximum response, shows 
that vision deteriorated from 2 weeks to 10 weeks 
