148 
Fishery Bulletin 108(2) 
in treatment order precluded any bias attributable to 
flatfish habituation or learning. 
Positioned behind and above the footrope were three 
(50W) infrared LED (light emitting diode) lamps, aimed 
forward and down, so that they illuminated the footrope 
and tank bottom immediately in front of the footrope. 
The wavelength of light emitted by these lamps peaked 
at 880 nm, and emissions dropped to 0 below 760 nm. 
Most fish are insensitive to light at those wavelengths 
(Douglas and Hawryshyn, 1990) and results from light- 
threshold feeding studies for all three flatfish species 
used in this study are consistent with this generaliza- 
tion (Hurst et al., 2007). Two underwater video cameras 
(Aqua-Vu, model ZT-120, Crosslake, MN ) were mounted 
alongside the lamps, also directed at the area in front 
of the footrope. This arrangement allowed for visual 
monitoring out to 1.1 m in advance of the footrope. The 
video footage was captured from a remote location by 
digital mini-DV recorders. 
Trials were conducted with three age classes of Pa- 
cific halibut: age-1, age-2, and age-3, as well as age- 
2 northern rock sole. For age-3 Pacific halibut, three 
groups of five fish each were examined. Trials took 
place over two consecutive days. On the first day sweep 
height was randomly set to either the “in contact” or 
“elevated” position. On the second day the alternative 
position was used. During each day, fish were exposed 
to the simulated sweep approach twice; once in the light 
and once in the dark. The order of application of light 
vs. dark trials was also randomly determined. After the 
second day fish were then removed from the tank, their 
total length was measured, and they were replaced by 
a new group. Age-3 Pacific halibut ranged from 37-52 
cm in total length. 
For age-2 Pacific halibut, age-1 Pacific halibut, and 
age-2 rock sole, groups consisting of 10 fish each were 
trialed differently. Each group was trialed for only a 
single day, at one sweep height. For age-2 Pacific hali- 
but, six groups were trialed at each sweep height. For 
age-1 Pacific halibut and age-2 northern rock sole, five 
groups were trialed at each sweep height. As before, 
the order of light and dark trials was randomized. Age- 
2 Pacific halibut ranged from 19-31 cm, age-1 hali- 
but from 8-14 cm, and age-2 northern rock sole from 
9-17 cm. 
Fish behavior was quantified by using the slow-mo- 
tion playback of digital video. First, the number of 
fish encountered, i.e., observed, as the sweep made 
its transit from one end of the tank to the other, was 
recorded from each trial. Then the initial behavioral 
response of each observed fish was assigned to one 
of four categories: 1) pass under, 2) hop, 3) rise, and 
4) herd. Fish characterized by “under” either did not 
react at all to the approaching sweep, or reacted when 
contacted by the sweep, but passed under the sweep 
as it progressed down the tank. “Hop” characterized 
fish that reacted to the sweep with one or two sinu- 
soidal body undulations, typically after being struck 
by the sweep, which resulted in the fish “hopping” off 
the substrate. However, this initial startle reaction 
< 
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Q. 
10 ’ 
• 
Night 
10 2 
10 3 
o 
O O 
■ Qr-O- 
o 
oo 
o 
Day 
10 4 
o 
o c 
o 
1 
Q 
o 
10 s 
• 
10 6 
• • X 
• 
• • 
• % 
• 
• 
• — 
78 
82 
86 
Depth 
90 
94 
Figure 1 
In situ natural log-transformed light data for trawl tows 
conducted during day and night, plotted by mean depth 
over the course of each tow. Regression analysis indi- 
cated no effect of depth upon ambient light over this 
relatively narrow range of depths and hence, regressions 
are plotted as zero-slope lines. 
was not followed by any further swimming, such that 
the fish tended to hang stationary in the water, and 
passed over the sweep as it progressed down the tank. 
“Rise” characterized the motion of fish that departed 
the bottom with sustained swimming in an upward 
direction, such that the distance between fish and bot- 
tom continuously increased as the fish swam. This was 
in contrast to fish characterized by “herd” where fish 
maintained a distance of less than one body length be- 
tween themselves and the bottom as they swam along 
in front of the sweep, i.e. herding behavior. Ryer and 
Barnett (2006) investigated whether initial orienta- 
tion, i.e., the direction fish were facing, influenced 
behavioral response. No relationship was observed, and 
consequently, no data on fish orientation were recorded 
in this study. Categorical data on behavioral response 
were pooled across replicate groups and analyzed by 
contingency table analysis by using log-linear models 
(Fienberg, 1980). 
Results 
At-sea experiment 
Mean ambient light on the seafloor (Fig. 1) was greater 
during daytime tows (2.0xl0~ 3 pmol photons/m 2 /s) 
than during nighttime tows (8.4xl0~ 7 pmol photons/ 
m 2 /s, F (1 33] =352.76, P<0.001). However, over the rela- 
tively narrow range of tow depths used in this analy- 
sis, depth had no influence upon bottom ambient light 
level (F u 33] =0.27, P=0.607). Mean total catch (CPUE) 
in terms of weight (kg/min) was greater during the 
day than at night (Table 1, day: x=100.6 kg, SE = 9.61; 
night: x=53.07 kg, SE = 6.14). This pattern of diurnally 
