Ryer et al.: Flatfish herding behavior in response to trawl sweeps 
153 
catch pattern. Arguably, our analysis is based upon 
a small set of paired tows, particularly at night (n = 5 
pairs). Taken alone, these at-sea trials might not be 
convincing. However, these results were mirrored by 
our laboratory experiments, where the elevation of 
sweeps decreased herding to a greater extent in the 
light, compared to darkness. The elevation of sweeps 
had several consequences, all of which were likely to 
have influenced flatfish behavior. First, because most 
flatfish react to ground gear at a very short distance, 
often only after being struck, the likelihood that fish 
would simply not react and be passed over by sweeps 
was probably increased by sweep elevation. Further, 
part of the visual stimulus to herd that is associated 
with ground gear is the sand and mud cloud that is 
kicked up by the gear. This visual stimulus would be 
absent or greatly diminished by sweep elevation, fur- 
ther decreasing the likelihood of flatfish response. Our 
laboratory experiments with rock sole exhibited a pat- 
tern of response nearly identical to that seen in the 
field and indicated that passage under or over the gear 
was probably responsible for the decline in herding as- 
sociated with sweep elevation during the day; in the 
light, fish passing beneath the sweep increased by 24% 
when the sweep was elevated. Lastly, even when herd- 
ing is initiated, it must be maintained. Flatfish will 
sometimes dive under ground gear when they perceive 
a gap between the gear and the bottom — a trait that 
has been used to reduce flatfish bycatch (DeAlteris et 
al., 1997). Sweep elevation probably facilitated such 
escape. Unfortunately, our laboratory data were of little 
aid in evaluating this possibility. Because of the physi- 
cal limitations of our apparatus, we characterized only 
the initial behavioral response of fish — not prolonged 
behavioral sequences that would characterize such 
deliberate escape tactics. 
Our field data indicate that Pacific halibut could have 
a different pattern of availability or catchability, com- 
pared to that of the other flatfish species we examined. 
By virtue of size, Pacific halibut stand apart from most 
other flatfish. Beyond three or four years of age, their 
size likely renders them immune to most predators. This 
may make them more likely to venture from the bottom, 
as may their piscivorous diet. Consequently, they may 
be more likely than other species to rise off the bottom 
and swim back over sweeps. If so, it follows that most 
of the fish captured are those directly in the path of 
the net, excluding the area swept by the sweeps. Our 
trawling operations tended to produce larger, albeit not 
significant, Pacific halibut catches at night — a trend re- 
ported by commercial fishermen as well. It may be that 
with their greater speed and endurance, many halibut 
escape trawls during the day, but at night cannot see 
the gear to coordinate their escape. In contrast to the 
halibut results, the nonsignificant differences for yel- 
lowfin sole were similar in direction and magnitude to 
the significant differences detected for the other small 
flatfishes. This finding opens the possibility that these 
flatfishes had similar reactions, but our experiment just 
did not have the statistical power to detect them. 
Diel patterns of catch in trawl fisheries and surveys 
reflect not only patterns in fish availability, but gear- 
specific behavioral influences upon catchability that 
are directly controlled by ambient illumination. Results 
of our laboratory experiments, along with earlier ex- 
periments (Ryer and Barnett, 2006), indicate that trawl 
footropes are likely to be more efficient at displacing 
flatfish from the bottom and rapidly transitioning them 
to the net under conditions of darkness (Ryer, 2008). In 
contrast, sweeps are probably more effective at herding 
flatfish inwards to the path of the net under daylight 
conditions. This disparity is probably responsible for 
the observed pattern of higher flatfish catches at night 
with survey nets, where bridles and sweeps are kept to 
minimal length, as compared to higher daytime catch- 
es with commercial flatfish nets and lengthy sweeps . 
These differences, as explained by the results of this 
work, highlight the importance of fish behavior for fish 
capture technology. 
Acknowledgments 
We wish to thank C. Hammond and J. Gauvin for assis- 
tance with the at-sea portion of this project, as well as 
the captain and crew of the FV Cape Horn. M. Ottmar, 
and S. Haines assisted with animal husbandry and labo- 
ratory experiments. A. Stoner, M. Davis, B. Laurel, and 
T. Hurst provided helpful comments and discussion of 
ideas explored in this research, and R. Hannah and W. 
Wakefield provided helpful critiques of an early draft of 
this manuscript. C. Sweitzer assisted with manuscript 
preparation. 
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