Ryer et al.: Flatfish herding behavior in response to trawl sweeps 
149 
LU 
LU 
C Arrowtooth flounder 
LU 
q_ E Pacific halibut 
Figure 2 
Mean catch per unit of effort (CPUE) ±1 standard error (SE) for daytime and nighttime 
hauls for each of six flatfish species from control nets where the sweep was in contact with 
the seafloor: (A) yellowfin sole (Limanda aspera); (B) flathead sole ( Hippoglossoides elas- 
sodon)\ (C) arrowtooth flounder (Atheresthes stomias ); (D) rock sole (Lepidopsetta spp.); 
(E) Alaska plaice ( Pleuronectes quadrituberculatus); and (F) Pacific halibut ( Hippoglossus 
stenolepis ). 
Table 1 
Statistics for a comparison of day and night trawl catches, by total catch, and catch of six individual species of flatfish. For both 
day and night tows, trawl nets were equipped with control sweeps (that had contact with the bottom). Where needed, Satterth- 
waite’s adjusted degrees of freedom were used to mitigate for nonhomogeneity of variance. 
Species 
t-test statistic 
df 
P value 
Total catch 
4.85 
31.3 
<0.001 
Yellowfin sole (Limanda aspera ) 
1.71 
30.6 
0.097 
Flathead sole (Hippoglossoides elassodon ) 
-7.44 
34 
<0.001 
Arrowtooth flounder (Atheresthes stomias) 
-3.26 
34 
0.003 
Rock sole (Lepidopsetta spp.) 
-2.38 
29.3 
0.024 
Alaska plaice (Pleuronectes quadrituberculatus) 
-3.74 
26.4 
0.001 
Pacific halibut (Hippoglossus stenolepis) 
1.58 
34 
0.126 
larger catches was also exhibited by four out of six 
flatfish species examined (Table 1, Fig. 2). Flathead 
sole, arrowtooth flounder, rock sole, and Alaska plaice 
were all characterized by higher CPUE during the day. 
Yellowfin sole and Pacific halibut exhibited no signifi- 
cant differences in catch between day and night. Of the 
four species for which fish total length was measured 
in catch subsamples (i.e., yellowfin sole, flathead sole, 
arrowtooth flounder, and rock sole), fish tended to be 
slightly larger at night. This was only statistically sig- 
nificant for yellowfin sole (f [24 4] =3.93, P=0.001), where 
fish averaged 1 cm longer during the night (x=32.8 cm, 
SE = 0.2) than during the day (x=31.8 cm, SE = 0.1), and 
rock sole (f [32] =2.91, P=0.006), where fish averaged 0.9 
cm longer during the night (*=33.3 cm, SE = 0.2) than 
during the day (5;=32.4 cm, SE = 0.2). 
The effect of elevating sweeps 10 cm off the bottom 
differed, depending upon whether tows were made dur- 
ing the day or night (Fig. 3). During the day, total catch 
tended to decrease when sweeps were elevated (Table 
2, elevated: 5c=93.4, SE = 8.7; control: 3c=100.6, SE = 9.6). 
However, during the night, elevation of sweeps had little 
influence upon catch (elevated: x=55.1, SE = 6.8; control: 
x=53.1, SE = 6.1). This same pattern was evident for four 
out of six flatfish species examined. Species for which 
daytime elevation of sweeps decreased catch included 
flathead sole, arrowtooth flounder, rock sole, and Alaska 
plaice. Sweep configuration had no significant effect on 
