Rose et al.: Effective herding of flatfish by cables with minimal seafloor contact 
141 
Figure 6 
Size compositions for flatfish and gadid species taken during tests of 
whether raised trawl sweeps reduce herding of fish. Yellowfin sole (Limanda 
aspera); northern rock sole ( Lepidopsetta polyxystra) ; flathead sole (Hippo- 
glossoides elassodon)\ arrowtooth flounder ( Atheresthes stomias); Pacific cod 
( Gadus macrocephalus ); and Alaska pollock ( Theragra chaleogramma). 
tected in our experiment. Pacific cod 
catches did not change significantly 
with any of the modifications. 
For evaluating the likelihood of 
substantial losses of catch, the confi- 
dence intervals provide more informa- 
tion than the basic significance tests 
alone. For example, the lower confi- 
dence bounds for the effects of 20-cm 
disks on flatfish catches leave only a 
2.5% (1 of 40) probability that catch 
losses would exceed 4-6%. Correspond- 
ing “worst case” losses for the 15-cm 
disks were even smaller. Similarly, al- 
though none of the Pacific cod catch 
results passed the threshold of a 95% 
two-tailed probability of being differ- 
ent from no change, all three of the 
confidence intervals were almost en- 
tirely above a value of 1. Therefore, a 
trawler could implement one of these 
modifications with little expectation of 
catching fewer Pacific cod and with a 
reasonable chance of slight increases 
in Pacific cod catch. 
The size composition of each spe- 
cies from the unmodified nets (Fig. 6) 
showed truncation at the lower end of 
the size distribution, owing to use of 
large mesh in the body of the net (20 cm, stretch mea- 
sure), intermediates (14 cm) and codends (15 cm) that 
release smaller fish. Although the proportions varied 
somewhat between experiments, each study encoun- 
tered a representative range of sizes available to the 
commercial fishery. 
ANOVA tests for differences in catch effects between 
major size classes (thirds or quartiles of control size 
frequencies) revealed no significant differences for any 
of the flatfish species (Fig. 7). One significant difference 
(P=0.04) was detected for pollock in sweeps with the 
smallest disks (15 cm), attributable to a lower catch 
rate of the smallest pollock. Confidence intervals were 
included in Figure 7 to aid comparisons between size 
groups within species and sweep modification classes. 
Confidence intervals were wider for the largest and 
smallest categories because few individuals from these 
ranges were encountered in some tows, increasing vari- 
ability, whereas all tows had substantial numbers of 
fish in the central ranges. 
Discussion 
Flatfish can be effectively herded by trawl sweeps 
and with greatly reduced seafloor contact. Signifi- 
cant catch reductions, averaging 5% for flathead sole 
and 11% for rock sole, were only detected when 25-cm 
disks were installed that raised the sweeps 10 cm 
above the substrate at the ends of each 9-m section. No 
detectable catch reductions occurred during tests with 
smaller clearances (5 and 7.5 cm). Confidence intervals 
indicated only a 2.5% probability of catch reductions 
greater that 5% with 7.5-cm clearances. Nor did sweeps 
with such clearances appear to change size selectivity 
significantly. 
Flatfish exhibit predator avoidance behaviors that 
allow them to be effectively herded by the sweeps. In 
contrast to roundfish, flatfish cease movement when a 
predator is detected and only flee upon very close ap- 
proach (Ryer, 2008). Therefore, observed flatfish reac- 
tions to trawl gear (Main and Sangster, 1981; Rose, 
1996; Ryer and Barnett, 2006) mostly occur at horizon- 
tal ranges of much less than 1 m. However, because con- 
ventional fishing gear has either continuous or closely 
spaced contact with the seafloor, there has been little or 
no information to assess the role of gear contact or prox- 
imity to the seafloor in either initiating or sustaining 
the flight behaviors that result in herding. Given the 
cryptic behaviors of flatfish, we could not assume that 
stimuli several centimeters above the seafloor would be 
as effective as those that would directly contact flatfish 
on the seafloor. The current results demonstrate that 
flatfish do respond with effective herding behaviors to 
sweep cables displaced from the seafloor by 5 to 10 cm. 
Even the largest of the flatfish encountered here would 
not have contacted the raised sweeps if they remained 
resting on the seafloor. At the highest clearance (10 
cm), slightly reduced catches indicated that the flight 
response began to break down and some of the flat- 
fish were not herded as well as with the conventional 
sweeps. Winger et al. (2004) found that flatfish size 
