150 
Fishery Bulletin 108(2) 
A Yellowfin sole B Flathead sole C Arrowtooth flounder 
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Figure 3 
Mean catch per unit of effort (CPUE) ±1 standard error (SE) for daytime and nighttime 
catches of each of six flatfish species from both control nets, where the sweep was in contact 
with the seafloor, as well as experimental nets where the sweep was elevated 10.2 cm off 
the seafloor: (A) yellowfin sole ( Limanda aspera); (B) flathead sole ( Hippoglossoides elas- 
sodon)\ (C) arrowtooth flounder ( Atlieresthes stomias); (D) rock sole ( Lepidopsetta spp.); 
(E) Alaska plaice ( Pleuronectes quadrituberculatus)\ and (F) Pacific halibut (Hippoglossus 
stenolepis). 
Table 2 
Statistics for comparison of total catch and catch of six individual species of flatfish between trawl nets equipped with control 
(bottom contact) and those equipped with elevated (10 cm off bottom) sweeps, from both day and night tows. 
Species 
Paired t-test statistic 
df 
P value 
Total catch 
Day 
2.11 
9 
0.064 
Night 
-0.22 
4 
0.834 
Yellowfin sole ( Limanda aspera ) 
Day 
1.84 
9 
0.099 
Night 
0.09 
4 
0.935 
Flathead sole (Hippoglossoides elassodon) 
Day 
2.33 
9 
0.045 
Night 
-0.78 
4 
0.481 
Arrowtooth flounder (Atheresthes stomias ) 
Day 
4.35 
9 
0.002 
Night 
-0.71 
4 
0.519 
Rock sole (Lepidopsetta spp.) 
Day 
5.42 
9 
<0.001 
Night 
0.23 
4 
0.830 
Alaska plaice (Pleuronectes quadrituberculatus) 
Day 
2.39 
9 
0.041 
Night 
-0.67 
4 
0.539 
Pacific halibut (Hippoglossus stenolepis) 
Day 
-0.59 
9 
0.753 
Night 
0.29 
4 
0.785 
daytime catches of yellowfin sole or Pacific halibut. In 
contrast to daytime results, elevated sweeps had no ef- 
fect upon nighttime catches for any species. Of the four 
species that were measured, fish lengths did not differ 
between tows with elevated sweeps and control tows, 
regardless of time of day (P>0.05 for each species, day 
and night). 
Laboratory experiment 
Overall, 28% of fish initiated herding behavior in 
response to simulated sweep disturbance. Herding was 
most prevalent in the light, and tended to be replaced 
by fish passing under the sweep, as well as hopping or 
rising off the bottom in the dark (Fig. 4). There was also 
