Magel et al.: Recovery of visual function in Hippoglossus stenolepis after exposure to bright light 
567 
2001). Trawl fisheries are, however, required to discard 
all Pacific halibut, thus subjecting a significant portion 
of the Bering Sea and Gulf of Alaska population to 
capture stress (Williams and Wilderbuer 1 ). Methods to 
determine health and condition of Pacific halibut des¬ 
tined for discard are based on the physical condition of 
the fish and variables related to the actual fishing pro¬ 
cess (Kaimmer and Trumble, 1998). Information on fish 
condition, stress, and variables related to the fishing 
process are collected by fisheries observers, but these 
data can vary greatly owing to subjective differences 
in assessment of fish condition and trawl tow charac¬ 
teristics (e.g., catch weight, depth of tow, tow speed) 
(Pikitch et al. 2 ). Therefore, the amount of time on deck 
may be a better indicator of condition at release than 
the means of capture (i.e., trawl or longline) (Davis and 
Schreck, 2005). 
Recent studies indicate that Pacific halibut biomass 
remains relatively stable, although recruitment re¬ 
mains weak (Stewart and Hicks 3 ), and bycatch mor¬ 
tality is approximately 20% within directed groundfish 
fisheries (Benaka et al., 2014). Also, bycatch has been 
slowly decreasing, although rates fluctuate depending 
on the location of the fishery itself (Dykstra 4 ). Contin¬ 
ued reductions in bycatch mortality could be facilitated 
by a better understanding of both the physiological and 
behavioral mechanisms that are compromised at the 
time of release of bycatch and affect survival. 
Pacific halibut are visual predators (Hurst et al., 
2007) and frequently live in turbid coastal waters at 
depths ranging from 90 to 900 m (i.e., on the conti¬ 
nental shelf) (IPHC 5 ) and therefore under low ambient 
light levels. After capture in trawl fisheries, individual 
fish are often left on deck for tens of minutes before 
they are discarded (Trumble et al., 1995; Davis and 
Olla, 2001). During this time, they can be exposed to 
direct sunlight (i.e., at light levels orders of magnitude 
above ambient levels on the seafloor) that potentially 
1 Williams, G. H., and T. Wilderbuer. 1992. Revised esti¬ 
mates of Pacific halibut discard mortality rates in the 1990 
groundfish fisheries off Alaska. In Int. Pac. Halibut Comm, 
report of assessment and research activities 1991, p. 191-209. 
Int. Pac. Halibut Comm., Seattle, WA. [Available from 
website.] 
2 Pikitch, E. K., D. L. Erickson, C. K. Mitchell, and J. R. Wal¬ 
lace. 1997. Practical applications of fishing and handling 
techniques in estimating the mortality of discarded trawl- 
caught Pacific halibut ( Hippoglossus stenolepis). ICES C.M. 
1997/FF:05, 18 p. [Available from website.] 
3 Stewart, I. J., and A. C. Hicks. 2017. Assessment of the 
Pacific halibut stock at the end of 2016. In Int. Pac. Halibut 
Comm, report of assessment and research activities 2006. 
IPHC-2016-RARA-26-R, p. 365-394. Int. Pac. Halibut 
Comm., Seattle, WA. [Available from website.] 
4 Dykstra, C. L. 2017. Incidental catch and mortality of Pa¬ 
cific halibut, 1990-2016. In Int. Pac. Halibut Comm, report 
of assessment and research activities 2006. IPHC-2016- 
RARA-26-R, p. 71-89. Int. Pac. Halibut Comm., Seattle, 
WA. [Available from website.] 
5 IPHC (International Pacific Halibut Commission). 
1998. The Pacific halibut: biology, fishery, and manage¬ 
ment. Tech. Rep. 40, 64 p. Inti. Pac. Halibut Comm., Se¬ 
attle, WA. [Available from website.] 
causes impaired visual function (Loew, 1976; Meyer- 
Rochow, 1994; Wu et al., 2006). Previous research has 
documented a reduction in retinal sensitivity to light 
in Pacific halibut after 15 min of exposure to simulat¬ 
ed sunlight (Brill et al, 2008). This reduction in sight 
could have consequences for foraging success after re¬ 
lease by diminishing the ability of a fish to perceive 
and capture prey. It is unknown, however, whether this 
deficit is permanent or whether it reduces the ability 
of Pacific halibut to detect and capture prey. Our objec¬ 
tive was to extend previous research (Brill et al., 2008) 
and to assess specifically whether retinal sensitivity 
and overall visual function can recover after exposure 
to simulated sunlight. 
We addressed these objectives by using both elec- 
troretinography (ERG) and behavioral methods. ERG 
measures the summed potential of electrical signals 
within the retina, providing a technique for rapidly 
and quantitatively assessing retinal function (Brown, 
1968). An evaluation of the behavior of Pacific halibut 
subjected to bright light, namely an evaluation of their 
ability to accomplish essential tasks, such as perceiv¬ 
ing and capturing prey, will help determine the effects 
of bycatch on somatic growth, fecundity, and survival. 
Materials and methods 
All fish capture, maintenance, handling, and experi¬ 
mental procedures followed accepted protocols and 
were in compliance with all relevant laws and regula¬ 
tion. Age-0 Pacific halibut (40-70 mm in total length 
[TL]) were acquired by trawl net in Chiniak Bay, Ko¬ 
diak Island, Alaska (57°40'N, 152°30'W) and delivered 
to the Hatfield Marine Science Center, Newport, Or¬ 
egon. Pacific halibut were kept in 3.1-m diameter fi¬ 
berglass tanks (at a 1-m depth) with flowing seawater 
at 8-10°C degrees for 2 or 3 years before use in the 
experiments. The tanks were maintained under low- 
illumination fluorescent lighting (photon flux density of 
0.01 pmol-m _2 -s _1 ) and day time and night time were 
set on a 12-h photoperiod. Fish were fed 3 times per 
week during the first year and twice per week during 
the second year with a gel food consisting of gelatin, 
vitamins, amino acid supplements, krill (Euphausia su¬ 
perha), pelleted food, Pacific herring (Clupea pallasii), 
and squid. 
Exposure to bright light 
Individual 2-year-old Pacific halibut (13-17 cm TL) fish 
were lifted by dip net from their holding tank, lightly 
anesthetized with a tricaine methanosufonate (Tricaine- 
S 6 [MS-222], Western Chemical, Inc., Ferndale, WA) 
solution of ~5 mg/L to reduce movement and stress, 
and held in a shallow seawater bath (12°C). They were 
6 Mention of trade names or commercial companies is for iden¬ 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
