Ryer et al.: Flatfish herding behavior in response to trawl sweeps 
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A Age-3 Pacific halibut — light 
Age-3 Pacific halibut — dark 
B Age-2 Pacific halibut — light 
Age-2 Pacific halibut — dark 
C Age-1 Pacific halibut — light 
50 
40 
30 
20 
10 
0 
Age-1 Pacific halibut — dark 
D Age-2 Northrock sole — light Age-2 Northrock sole — dark 
Figure 4 
Behavioral response of flatfish, under light and dark conditions, with the simulated sweep both 
in contact (control) and elevated 10 cm off the bottom: (A) age-3 Pacific halibut ( Hippoglossus 
stenolepis)\ (B) age-2 Pacific halibut; (C) age-1 Pacific halibut; and (D) age-2 northern rock sole 
( Lepidopsetta polyxystra). “Pass under” represents fish that either did not react to the sweep, 
or reacted late, such that they passed under the sweep as it progressed down the tank. “Hop” 
characterized fish that reacted to the sweep with one or two body undulations, but almost 
immediately pass over the sweep. “Rise” characterized fish in which the initial jump off the 
bottom was followed by sustained swimming in an upward direction, such that the distance 
between fish and bottom continuously increased as the fish swam. “Herd” characterized fish 
which, after reacting to the gear, swam along in front of the sweep, close to the bottom, typically 
maintaining a distance of less than one body length between themselves and the bottom. 
a tendency for herding in the light to decrease when 
the sweep was elevated. These observations are sup- 
ported by results of log-linear model analysis, in which 
ambient light (light, dark) mediated the influence of 
sweep height upon behavioral response (G [3 | = 9.96, 
P=0.019). All three age classes of Pacific halibut, and 
northern rock sole, behaved comparably; there were 
no significant effects of species or age on the type of 
