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Fishery Bulletin 11 5(4) 
A 
~ B 
© 
C/3 
log (illumination) 
Figure 1 
Comparison of responses to increasing illu¬ 
mination or light intensities (I, measured in 
log candela/m 2 by using electroretinography) 
between right and left eyes of previously un¬ 
exposed Pacific halibut (Hippoglossus stenol- 
epis) (n= 4). To construct voltage in relation to 
log light intensity (V-log I) response curves, 
light intensities were increased in 0.2 log 
units from levels that produced no measur¬ 
able responses to those that produced maxi¬ 
mal responses. The data are reported either 
as voltage or as log-normalized by expressing 
the average response to an intensity step as 
a percentage of the maximum observed aver¬ 
age response (p-max). V-log I response curves 
were created with both (A) voltage data and 
(B) log-normalized data expressed by the av¬ 
erage response to an intensity step as a per¬ 
centage of the maximum observed average 
response. Data points represent mean values, 
and error bars indicate standard errors of the 
means. 
max, a significant difference was no longer present 
between left and right eyes (F'n, 2 p0.00, P=0.963), nor 
was there a significant interaction between eye and 
light intensity (F (16 32 p0.90, P=0.575). P-max contin¬ 
ued to increase with increasing test light level (F 16 32] 
=17.68, PcO.OOl). 
Exposure to simulated sunlight for 15 min resulted 
in a visual deficit that did not improve during the 10 
weeks of recovery. Voltages measured from the right 
eyes of control fish (i.e., no exposure to simulated sun¬ 
light) were generally greater than those of the right 
eyes of fish that were exposed to simulated sunlight 
and allowed to recover for 2-10 weeks. This was par¬ 
ticularly evident at lower test light levels, as evidenced 
by a significant interaction between treatment and 
light intensity (Fig. 2A; F[ 64 j 2 72]=1-55, P=0.009). Con¬ 
version of voltages to p-max did not appreciably change 
this relationship (Fig. 2B). Again, there was a signifi¬ 
cant interaction between treatment and light intensity 
(^64,2721=2.04, PcO.OOl). 
There were significant differences in light intensi¬ 
ties required to produce a response 50% of maximum 
(F [4 ,i 7]=11.4, PcO.OOl) between treatments (control, and 
2, 4, 6 and 10-weeks recovery) (Fig. 3). The light inten¬ 
sity required to produce a response 50% of maximum 
was significantly lower for control fish, than for fish in 
any of the recovery treatments (Tukey’s HSB: P<0.05). 
Among the recovery treatments, the light intensity 
required to produce a response 50% of maximum in¬ 
creased over the 10-week recovery period and was low¬ 
er at week 2 than at week 10 (Tukey’s HSD: P<0.05). 
The light intensity required to produce a response 
50% of maximum at week 2 did not differ from those 
at either weeks 4 or 6, and similarly, the response at 
week 10 did not differ from responses at weeks 4 or 
6 (Tukey’s HSD: P<0.05). In context, it took approxi¬ 
mately 17 times the photons to produce a response of 
50% of maximum in fish exposed to simulated sunlight 
after 10 weeks than it did for control fish. 
Behavioral experiment 
There was no effect of exposure to simulated sunlight 
on the behavioral response of Pacific halibut to the 
visual cues associated with a simulated prey (F[ 2 7 , 324 ] 
=0.40, P=0.539). This lack of difference between control 
and treated fish was consistent throughout the trials, 
as well as across ambient light levels, because ANOVA 
showed no significant interactions between treatment 
(control vs fish exposed to simulated sunlight) and any 
of the other factors (e.g., time, ambient light level). 
Pacific halibut were generally active and responded 
strongly to the appearance of prey (presented at the 
beginning of minute 6) at the highest ambient illumi¬ 
nation • 3x1(5“ 3 pmol-m _2 -s _1 ), but responsiveness pro¬ 
gressively declined at lower ambient light levels (Fig. 
4). This finding is supported by a significant interac¬ 
tion between time and ambient light level in our ANO¬ 
VA for Pacific halibut activity (F| 27 - ;2 4;-4.'i.G, PcO.OOl). 
At the 2 highest ambient light levels, fish would ori¬ 
ent themselves toward the simulated prey when it ap¬ 
peared, swim toward it, and repeatedly strike at the 
sides of the Plexiglas tube containing the simulated 
prey. This behavior was characterized by a sharp in- 
