Commercial bottom trawls often have sweeps to herd fish into
the net. Elevation of the sweeps off the seaf loor may reduce seafloor disturbance, but also reduce herding
effectiveness. In both field and laboratory experiments, we examined the behavior of flatfish in response to sweeps. We tested the hypotheses that 1) sweeps are more effective at
herding flatfish during the day than at night, when fish are unable to see approaching gear, and that 2) elevation
of sweeps off the seafloor reduces herding during the day, but not at night. In sea trials, day catches were greater than night catches for four out of six flatfish species examined. The elevation of sweeps 10 cm significantly
decreased catches during the day, but not at night. Laboratory experiments revealed northern rock sole (Lepidopsetta polyxystra) and Pacific halibut (Hippoglossus stenolepis) were more likely to be herded\ud
by the sweep in the light, whereas in the dark they tended to pass under or over the sweep. In the light, elevation
of the sweep reduced herding, and more fish passed under the sweep. In contrast, in the dark, sweep elevation
had little effect upon the number of fish that exhibited herding behavior. The results of both field and laboratory
experiments were consistent with the premise that vision is the principle sensory input that controls fish behavior and orientation to trawl gear, and gear performance will differ
between conditions where flatfish can see, in contrast to where they cannot see, the approaching gear
