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Fishery Bulletin 108(2) 
response displayed, or interactions with light level or 
sweep height (P>0.05 for all). Examination of Figure 4 
could lead one to conclude that age-3 halibut behaved 
somewhat differently than the other species and age 
groups. However, the number of age-3 halibut tested 
(;z=15) was small compared to each of the other species 
and age groups (n >50 for each), and as a consequence, 
had little influence upon our statistical model. We pooled 
data across species and collapsed response categories 
down to those fish that herded in contrast to those that 
did not (pass under, hop, and rise combined), so as to 
render the data into a form most similar to our at-sea 
trawl-catch experiments. Again, ambient light (light 
or dark) mediated the influence of sweep height upon 
behavioral response (G tl j = 5.75, P=0.017). In Figure 5 
we have simplified this relationship by graphing the 
percentage of fish herding under the two light and sweep 
height treatments. In addition to a conspicuous decrease 
in herding in the dark, elevation of the sweep decreased 
herding in the light but had little influence in the dark- 
ness — results consistent with those observed in the at- 
sea experiment. 
Discussion 
Ambient illumination controls many aspects of fish 
behavior, from feeding and habitat use (Janssen, 1978; 
Helfman and Schultz, 1984; Ryer and Olla, 1999; De 
Robertis et ah, 2003; Petrie and Ryer, 2006) to social and 
antipredator behavior (Shaw, 1961; Ryer and Olla, 1998). 
Similarly, light has a pervasive influence upon interac- 
tions between fish and trawls. In this study, field data 
were largely consistent with our principal hypothesis; 
that trawls configured with sweeps that are in contact 
with the seafloor would catch more flatfish during the 
60 r 
50 - 
CD 
40 
0 
.c 
E 30 ' 
c 
0 
9 20 - 
(D 
CL 
10 - 
0 L - 1 
Light Dark 
Figure 5 
Percentage of fish that herded in response to simulated 
trawl sweep disturbance under both light and dark con- 
ditions, with the sweep both in contact (control) and 
elevated 10 cm off the bottom. Data were pooled across 
species and age classes. 
day than during the night. This pattern was observed 
for four out of six flatfish species examined: flathead sole, 
arrowtooth flounder, rock sole, and Alaska plaice. Herd- 
ing, as seen in both roundfish and flatfish, is an ordered 
behavioral response in which fish move away from an 
approaching threat, i.e., the doors, sweeps, bridles, and 
wings of the net. Through either continuous swimming, 
or sudden swimming bursts, interspersed with rests on 
the bottom (Winger et al., 1999, 2004), fish then funnel 
to the center of the gear, where they concentrate before 
tiring and falling back into net. Several studies have 
demonstrated that both roundfish (Olla et al., 2000; Ryer 
and Olla, 2000) and flatfish (Ryer and Barnett, 2006) 
lose the ability to orient themselves in relation to gear 
and initiate herding when ambient light falls below the 
threshold for visual perception of the gear (Kim and 
Wardle, 1998a, 1998b). 
Given the brief evolutionary time during which fish 
have interacted with towed fishing gear, approximate- 
ly 100 years, it is unlikely that specific gear avoid- 
ance behavior has evolved. Rather, we consider it most 
parsimonious to assume gear avoidance is rooted in 
antipredator behavior. Although flatfish may initially 
erupt from the seafloor upon being disturbed by trawl 
ground-gear, as when attacked by a predator, subse- 
quent herding behavior is consistent with “distance 
keeping” behavior, during which the fish attempts to 
maintain a safe distance between itself and a slowly 
pursuing predator. Scuba and skin divers who have at- 
tempted to follow fish along the seafloor are certainly 
familiar with this behavior. For flatfish, movement in 
the vertical dimension also plays a critical role during 
herding. It has been observed that flatfish remain close 
to the bottom during herding, usually less than half a 
body length (Ryer, 2008). Staying close to the bottom 
reduces drag, lessening thrust requirements to achieve 
a given speed — the ground effect (Videler, 1993; Gib- 
son, 2005). Rising off the bottom makes flatfish more 
conspicuous, and due to the location of a flatfish’s eyes, 
also interferes with visual tracking of a pursuing preda- 
tor, in this case, the trawl ground-gear. Although they 
herd close to the bottom in the light, Pacific halibut 
and northern rock sole respond differently to ground- 
gear in the darkness, as demonstrated by laboratory 
experiments (Ryer and Barnett, 2006). Unable to see, 
the fish respond to contact with the ground-gear ini- 
tially by hopping or swimming upward and away from 
the bottom. Similarly, in this study the percentage of 
fish moving off the bottom increased from 4% in the 
light to 21% in darkness, for all species and bar heights 
combined. Moving off the bottom in darkness probably 
functions as an antipredator tactic, making the flatfish 
more difficult to follow and may simply be the flatfish 
version of the Mauthner-cell triggered (lateral line) 
startle response (Eaton and Hackett, 1984). 
Our second hypothesis, that elevation of sweeps off 
the bottom, 10 cm in this case, would decrease catch 
during daylight, but not at night, was also partial- 
ly supported by our field experiment. Again, four of 
six flatfish species examined displayed the predicted 
Control 
Elevated 
