145 
Flatfish herding behavior in response 
to trawl sweeps: a comparison of diet responses 
to conventional sweeps and elevated sweeps 
Email address for contact author: cliff.ryer@noaa.gov 
1 Fisheries Behavioral Ecology Program 
Resource Assessment and Conservation Engineering Division 
Alaska Fisheries Science Center, NOAA Fisheries 
Hatfield Marine Science Center 
2030 Marine Science Drive 
Newport, Oregon 97365 
2 Resource Assessment and Conservation Engineering Division 
Alaska Fisheries Science Center, NOAA Fisheries 
7600 Sand Point Way 
Seattle, Washington 98115 
Abstract — Commercial bottom trawls 
often have sweeps to herd fish into 
the net. Elevation of the sweeps off 
the seafloor may reduce seafloor 
disturbance, but also reduce herd- 
ing 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) eleva- 
tion 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 sig- 
nificantly decreased catches during 
the day, but not at night. Laboratory 
experiments revealed northern rock 
sole ( Lepidopsetta polyxystra ) and 
Pacific halibut ( Hippoglossus stenol- 
epis ) were more likely to be herded 
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 labo- 
ratory 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. 
Manuscript submitted 8 June 2009. 
Manuscript accepted 15 December 2009. 
Fish. Bull. 108:145-154 (2010). 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Clifford H. Ryer 1 (contact author) 
Craig S. Rose 2 
Paul J. Iseri 1 
Trawl catches for many fish species 
exhibit diel patterns (Casey and Myers, 
1998). This is generally viewed as a 
product of two independent factors: 
availability and catchability of the 
fish species. Many gadids exhibit diel 
changes in availability associated with 
vertical migration (Beamish, 1965; 
Casey and Myers, 1998; Schabets- 
berger et al., 2000). Gadids aggregate 
close to the bottom during the day and 
are highly available to bottom trawls. 
At night, dispersal into the overly- 
ing water renders them less available. 
Interestingly, for many flatfish species 
the opposite pattern, higher catches 
at night, has been observed (Walsh, 
1991; Walsh and Hickey, 1993; Casey 
and Myers, 1998). Seasonal migra- 
tions will occasionally take flatfish 
into the water column (Metcalfe et al., 
1990; Nichol and Sommerton, 2009), 
as will the occasional exploitation of 
pelagic prey. However, under normal 
circumstances many flatfish species 
appear to remain on the bottom and 
are consistently available to trawl 
gear, day or night. As a consequence, 
greater flatfish catches at night are 
thought to be associated with higher 
catchability, that is, with a decreased 
ability to evade capture (Ryer, 2008). 
Video cameras mounted on trawls, 
as well as supplemental bag nets be- 
hind the main net, have documented 
extensive flatfish escapement beneath 
the footrope during the day (Main 
and Sangster, 1981; Walsh, 1988). For 
obvious technical reasons, behavior 
in front of the footrope, or sweeps, at 
night has not been observed in field 
studies, except with flash photogra- 
phy (Walsh and Hickey, 1993). How- 
ever, laboratory experiments indicate 
that northern rock sole ( Lepidopsetta 
polyxystra), Pacific halibut ( Hippo- 
glossus stenolepis), and English sole 
( Parophrys vetulus) are more likely 
to rise or hop into the water column 
during darkness, than to herd (Ryer 
and Barnett, 2006). By moving off the 
bottom, these fish remove themselves 
from the “zone of influence” of the 
ground gear, and as they cease swim- 
ming they pass over the footrope and 
into the net. This behavior potentially 
explains why flatfish are captured in 
greater numbers at night. 
This paradigm, i.e., higher flatfish 
catches at night, stems largely from a 
series of published studies (Main and 
Sangster, 1981; Walsh, 1988, 1991; 
Walsh and Hickey, 1993; Casey and 
Myers, 1998; and references therein), 
based on survey trawls. On survey 
trawls, the combined length of bridles 
and sweeps is typically minimized. In 
contrast, on commercial flatfish trawls 
lengthy sweeps are used to herd fish 
inward toward the net (Winger et 
