140 
Fishery Bulletin 108(2) 
Figure 4 
(A) Sonar and ( B ) video imagery of the seafloor after passage of the raised otter trawl sweeps. Video picture was 
taken as the seafloor sled passed over the location indicated on the sonar image. Otter trawl sweeps were raised 
with widely spaced disk clusters, which caused the parallel tracks seen in the sonar image and the flattened 
swath in the video image. 
sweeps themselves were brief puffs after contact with 
high spots on the seafloor. Areas covered by the modi- 
fied sweeps showed marks from the disk clusters that 
were approximately 10-cm wide separated by seafloor 
indistinguishable from unaffected areas (Fig. 4A). This 
disk cluster mark was approximately 5% of the 2-m 
interval between marks. This spacing is 
much shorter than the 9-m spacing on the 
cable because sweeps are sharply angled 
to their direction of movement (angle-of- 
attack). Images of such tracks from the 
video (Fig. 4B) showed a flattening of very 
low-profile surface textures. 
The use of 15-cm disks on the sweeps 
did not cause significant differences in 
catch rates (LogDif was not different 
from 0) for any of the six species, and 
only the pollock catch rate changed (12% 
increase, P=0.007) with the 20-cm disks 
(Fig. 5). Northern rock sole and flathead 
sole catches both decreased significantly 
(-11%, PcO.OOl, and -5%, P=0.02, respec- 
tively) when the 25-cm disks were used, 
whereas pollock catch increased again 
(+12%, P=0.03). Decreases for the other 
two flatfish were also observed — although 
not statistically significant at the 0.05 
level (P=0.08 for arrowtooth flounder and 
P=0.07 for yellowfin sole). A consistent 
decrease in the mean relative catch with 
increasing disk size for all of the flatfish 
species, although only significant for the 
largest disks, indicates that smaller ef- 
fects may have occurred for the smaller 
disks that could not be statistically de- 
1.30 -i 
Yellowfin 
sole 
Pacific cod Alaska 
pollock 
Figure 5 
Estimates of and 95% confidence intervals for the ratios of fish catches 
with the modified trawl sweeps raised to three different heights off the 
seafloor to fish catches with conventional sweeps for the four principal 
flatfish species (yellowfin sole [Limanda aspera ]; northern rock sole 
[Lepidopsetta polyxystra]\ flathead sole I Hippoglossoides elassodon J; 
arrowtooth flounder [Atheresthes stomias ]); and two principal gadid 
species: Pacific cod ( Gadus macrocephalus)\ and Alaska pollock ( Ther - 
agra chalcogramma ) taken in Bering Sea trawl fisheries. 
