Rose et al.: Effective herding of flatfish by cables with minimal seafloor contact 
143 
though light levels affect the herding process (Ryer and 
Barnett, 2006). The effects of light on flatfish herding 
are analyzed and reported in a separate paper (Ryer 
et al., 2010). 
Although not the focus of this study, an unexpected 
result was the increase in pollock catches that occurred 
with two of the sweep modifications. Pollock herd differ- 
ently from flatfish, reacting to stimuli at much greater 
distances (Rose, 1996). The forward sections of the 
most modern pollock trawls have “meshes” that are 
more than 25-m long. Although large groups of pol- 
lock could easily swim through such meshes, they still 
avoid the netting and are eventually herded into parts 
of the net that physically restrain them. These nets 
would not work if pollock herded only at short ranges. 
Separation of the sweeps from the seafloor, or the disk 
clusters themselves, could have increased visibility of 
the sweeps, which may have enhanced pollock herding. 
Both factors would be reduced at the smallest disks, 
where herding improvement was not detected. 
Sonar observations of the elevated sweeps showed 
that their interaction with the seafloor was radically 
changed. The continuous sediment clouds produced 
along the entire length of the unmodified sweeps were, 
for the modified sweeps, reduced to isolated clouds be- 
hind each disk, indicating substantial reductions in the 
area of direct contact. Therefore, any effects based on 
direct contact, as well as resuspension of sediments, 
should have been greatly reduced. The sonar images 
of the seafloor after passage of the sweep showed that 
the contact area of the disks was approximately 5% of 
the total swept area. Seafloor texture between the disk 
tracks was indistinguishable from unaffected areas, 
but areas covered by conventional sweeps showed slight 
smoothing. The seafloor directly contacted by the disks 
was uniformly smoothed. Although the texture change 
due to conventional sweeps appeared slight, the resus- 
pension observed during fishing indicated some distur- 
bance of the bottom and we believe that the substantial 
reduction of contact due to using the disks more than 
compensates for any increased disturbance to the small 
area directly under the disks. 
In another recent study (Guyonnet et al., 2008), the 
concept of slightly raising trawl sweeps, therein called 
“legs,” was also applied to reduce their impact on the 
seafloor. Instead of disk clusters, Guyonnet et al. used 
neutrally buoyant sweep material that was weighted 
only by dangling chains attached every 50 cm. They 
also found no significant effects on catch composition 
or size selectivity for target animals. They found that 
damage to benthic animals was reduced with the al- 
ternative gear. 
Our results alone, although promising, do not address 
the full potential of sweep modifications to reduce the 
effects on the seafloor of trawling for Bering Sea flat- 
fish. Although creating several centimeters of separation 
between the sweeps and the seafloor greatly reduces the 
potential for damage to infauna and small epifauna, 
it does not prevent contact with seafloor features and 
animals larger than that spacing. The vulnerability of 
ecosystem features to trawling operations is a function 
of the amount of damage caused by each trawl exposure 
(e.g., the proportion of a particular species in the path 
of a trawl that dies due to trawl contact) and the fre- 
quency and coverage of the trawling effort. An analysis 
of such factors for the Bering Sea shelf highlighted 
structure-forming animals as the seafloor feature most 
vulnerable to trawling. 2 The structure-forming animals 
of the eastern Bering Sea shelf are generally small and 
flexible; therefore it is quite conceivable that creating 
a space below the sweeps could also reduce damage to 
these animals. That potential is being examined by 
the authors in a subsequent study that will focus on 
how these sweep modifications change damage rates to 
structure-forming animals of the Bering Sea shelf. 
Successful gear modifications for reducing trawling 
effects on seafloor habitats would add a habitat pro- 
tection option in addition to area closures and gear 
switching. Closures of areas to trawling can move fish- 
ing effort from productive grounds, and therefore can 
increase the total effort required or concentrate fishing 
and its effects in the remaining fishing grounds (Fu- 
jioka, 2006). The list of alternative gear for harvesting 
these flatfish is quite limited and none are without 
some negative effects on habitat. With beam trawling, 
herding sweeps are not used to concentrate fish into the 
path of the capture device. Therefore, the entire area 
from which fish are collected is swept with the capture 
net itself. Studies to reduce the effects of beam trawls 
on habitat have focused on other stimuli to move fish 
from the seafloor into the net (van Marlen et al., 2005). 
The capture process for demersal seines is similar in 
many ways to that of Alaska otter trawls with long 
sweeps — weighted cables are pulled across the seafloor 
to herd fish into the path of a capture net. Demersal en- 
tangling nets depend on natural movements of the fish 
to bring them to the gear, and therefore they are effec- 
tive only during periods when fish are actively moving. 
They are still unlikely to produce catch rates similar 
to those produced with trawls unless vast fleets of nets 
are deployed. Such extensive net deployments would 
exacerbate the most notable problem with demersal 
entangling nets — ghost fishing of derelict and lost gear. 
Finally, although longline fishing is the foundation for 
one of the most successful commercial flatfish fisheries 
(Pacific halibut), most flatfish species are not of the size 
and do not have a predatory diet that make longlines 
particularly effective. 
Implementing the trawl gear modifications described 
here would require some adaptations in equipment and 
handling methods for fishermen. The volume of the 
elevating devices would require additional space on 
deployment reels or net drums, thus requiring either 
that sweep lengths be shortened to fit onto the reels 
or larger reels be installed on vessels. The disks would 
2 Final environmental impact statement for essential fish 
habitat identification and conservation in Alaska. April 
2005 [online], http://www.fakr.noaa.gov/habitat/seis/efheis. 
htm. 
