Magei et al.: Recovery of visual function in Hippoglossus stenolepis after exposure to bright light 
573 
after exposure, indicating a progressive worsening of 
Pacific halibut retinal sensitivity over time. In an en¬ 
vironmental context, a sunlight exposed Pacific halibut 
would have to move to water that is 18 m shallower to 
have the same visual acuity as that of an unexposed 
fish, assuming a light extinction coefficient of 0.15 
(e.g., simulating typical conditions in the Gulf of Alas¬ 
ka). This level of illumination would potentially result 
in a shoaling effect among fish discarded as bycatch. 
Our data further indicate that the visual deficit as¬ 
sociated with sunlight exposure was most pronounced 
at the low end of the Pacific halibut visual range. As 
a consequence, fish captured in and subsequently re¬ 
turned to relatively shallow well-lit waters may be less 
affected than fish captured from and then returned to 
deeper water, where impaired fish may be at the lim¬ 
it of their range of visual sensitivity. Whether or not 
discarded Pacific halibut move to shallower water to 
mitigate visual impairments could be tested in future 
research with mark-recapture techniques. It should be 
noted that Pacific halibut size generally increases with 
depth. The fish used in our ERG were 2 year olds and 
therefore were smaller than most fish encountered in 
commercial fisheries. Although we have no reason to 
believe that the visual systems of our fish differed from 
those of larger Pacific halibut, future work in this area 
would benefit from an examination of a wider range of 
fish sizes. 
The impairment of retinal sensitivity revealed by 
ERG contrasts with the results from our behavioral as¬ 
say that produced no statistical evidence of significant 
visual impairment associated with exposure to simulat¬ 
ed sunlight. The simulated prey bobbed up and down 
within a clear Plexiglass tube that minimized cues as¬ 
sociated with water movements and the possibility that 
Pacific halibut would respond to nonvisual cues. The 
fact that the responsiveness of fish, as measured by 
activity, decreased with decreasing ambient light lev¬ 
els clearly indicates that Pacific halibut use vision to 
detect prey. Yet, across the range of ambient light lev¬ 
els there were no consistent statistical differences be¬ 
tween control fish and those exposed to simulated sun¬ 
light, with the possible exception of a slight reduction 
of behavioral activity (i.e. movement, bait strike, etc) 
among the latter at an ambient light level of lxlO -4 
pmolm -2 -s -1 (Fig. 4B). Pacific halibut are visual preda¬ 
tors and at light levels of lxlO -4 pmol-m -2 -s -1 primar¬ 
ily use visual cues to locate and attack prey, shifting 
to tactile and olfactory cues as light levels fall below 
lxlO -5 pmol-m -2 -s -1 (Hurst et al., 2007). For immobile 
baits, Pacific halibut feeding performance is likewise 
facilitated by vision (Stoner, 2003). We initially rea¬ 
soned that the threshold ambient light level for visual 
foraging would be that at which a deficit would be most 
pronounced. It is possible that we performed tests over 
too wide a range of ambient light levels. For example, 
we might have seen a difference between sunlight- 
exposed and control fish by testing over finer grada¬ 
tions of ambient light levels between lxlO -5 to lxlO -4 
pmol-m -2 -s -1 ). Additionally, conditions in this behavior¬ 
al assay were designed to maximize the probability of 
prey detection. The Pacific halibut were in close prox¬ 
imity to the simulated prey in clear water. Had the dis¬ 
tance between Pacific halibut and simulated prey been 
greater, or the water more turbid, the demands upon 
the visual system may have been magnified in such a 
way that more clearly showed impairment. 
An ancillary discovery from our work was the dif¬ 
ference between left and right eye function in Pacific 
halibut. Left eyes had consistently depressed V-log I 
curves than right eyes (i.e., the former are less light 
sensitive than the latter). Pacific halibut are right¬ 
eyed flounders; the left eye migrates to the right side 
of the head during larval development and metamor¬ 
phosis. This “tortured ontogeny” in flatfish may add 
constraints to optic nerve function. To our knowledge, 
however, little research exists on retinal anatomy or 
physiology in larval flatfish, beyond documentation of 
eye development of Atlantic halibut (Hippoglossus hip¬ 
poglossus) and other flatfish at settlement (Kvenseth 
et al., 1996; Friedman, 2008). Although V-log I curves 
differed between left and right eyes, there were no ap¬ 
parent differences when responses were transformed to 
p-max response curves. Therefore, although voltage re¬ 
sponses to brief light flashes from the left eyes are low¬ 
er, both left and right eyes appear to otherwise func¬ 
tion comparably. In brief, both eyes show comparable 
light sensitivities, although the smaller ERG response 
from the left eye (compared with that of the right eye) 
at the same light intensities implies anatomical and 
perhaps functional differences at the central nervous 
system level. Additionally, because of their unique dex- 
tral morphological features as adults, Pacific halibut 
may be more susceptible to injuries to their right eyes 
owing to hooking injuries in long-line commercial fish¬ 
eries because the right eye is closer than the left eye to 
the jaw. This conclusion would warrant future research 
in hook-induced eye damage and handling practices 
specific to hook-and-line fisheries. 
Hook-and-line fisheries, whether recreational or 
commercial, generally result in the rapid return of dis¬ 
carded fish to the water so that there is a concomitant 
minimal exposure to direct sunlight. In contrast, in 
trawl fisheries Pacific halibut may remain on deck for 
up 30 min and experience significant mortality (Trum- 
ble et al., 1995), although new deck sorting methods 
have decreased that time. For those fish that survive 
aerial exposure, it was postulated that sublethal ef¬ 
fects on visual sensitivity arising from sunlight expo¬ 
sure could further reduce growth and survival (Brill 
et al., 2008). Because flicker fusion frequency (i.e., the 
speed of vision or the ability to detect moving objects) 
and light sensitivity of the Pacific halibut visual sys¬ 
tem are adapted to low-light environments (Warrant, 
1999), Pacific halibut, in particular, are susceptible to 
retinal damage from exposure to direct sunlight than 
are shallow-water fish species. Our ERG data support 
these conclusions and are consistent with the data from 
Brill et al. (2008) in that we found that exposure to 
simulated sunlight exposure reduces retinal light sen- 
