136 
Effective herding of flatfish by cables 
with minimal seafloor contact 
Carwyn F. Hammond 1 
Email address for contact author: craig.rose@noaa.gov 
1 NOAA, National Marine Fisheries Service 
Alaska Fisheries Science Center, Conservation Engineering Program 
7600 Sand Point Way NE 
Seattle, Washington 98115 
2 Best Use Cooperative 
4241 21 st Avenue West, Suite 302 
Seattle, Washington 98199 
Abstract — Otter trawls are very 
effective at capturing flatfish, but 
they can affect the seafloor ecosys- 
tems where they are used. Alaska 
flatfish trawlers have very long 
cables (called sweeps) between doors 
and net to herd fish into the path 
of the trawl. These sweeps, which 
ride on and can disturb the seafloor, 
account for most of the area affected 
by these trawls and hence a large pro- 
portion of the potential for damage 
to seafloor organisms. We examined 
modifications to otter trawls, such 
that disk clusters were installed at 
9-m intervals to raise trawl sweeps 
small distances above the seafloor, 
greatly reducing the area of direct 
seafloor contact. A critical consider- 
ation was whether flatfish would still 
be herded effectively by these sweeps. 
We compared conventional and modi- 
fied sweeps using a twin trawl system 
and analyzed the volume and com- 
position of the resulting catches. We 
tested sweeps raised 5, 7.5, and 10 
cm and observed no significant losses 
of flatfish catch until sweeps were 
raised 10 cm, and those losses were 
relatively small (5-10%). No size com- 
position changes were detected in the 
flatfish catches. Alaska pollock ( Ther - 
agra chalcogramma ) were captured 
at higher rates with two versions of 
the modified sweeps. Sonar observa- 
tions of the sweeps in operation and 
the seafloor after passage confirmed 
that the area of direct seafloor contact 
was greatly reduced by the modified 
sweeps. 
Manuscript submitted 16 January 2009. 
Manuscript accepted 13 November 2009. 
Fish. Bull. 108:136-144 (2010). 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Craig S. Rose (contact author ) 1 
John R. Gauvin 2 
Otter trawling is one of the most 
effective methods for capturing com- 
mercial quantities of flatfish and is 
the principal method for flatfish har- 
vest in Alaska waters. However, trawl 
fisheries have received increasing 
attention for their potential to affect 
seafloor habitats. Changes to seafloor 
ecosystems resulting from the pas- 
sage of trawl gear have been have 
described in a wide range of studies 
(Barnes and Thomas, 2005; Lokke- 
borg, 2005). These include changes 
to infaunal (Tuck et al., 1998) and 
epifaunal (Kaiser et al., 1998; Prena 
et al., 1999; McConnaughey et al., 
2000) communities, as well as indi- 
rect effects from changes to seafloor 
structure and resuspension of sedi- 
ments (Churchill, 1989). The most 
common response to mitigate these 
problems has been closures of sen- 
sitive areas to trawling. When such 
areas have rough, rocky substrates, 
regulations requiring that trawl foot- 
rope cross-sections be below a certain 
size have been used to discourage 
fishing in these areas; the smaller 
footropes make nets more vulnerable 
to damage (Hannah, 2003; Bellman 
et al., 2005). 
Alaskan commercial flatfish fisher- 
ies, among the largest in the world, 
are pursued almost exclusively with 
demersal otter trawls. (The excep- 
tion is the fishery for Pacific halibut 
| Hippogossus stenolepis ], a large, pi- 
scivorous species that is harvested 
by longlines.) These otter trawls gen- 
erally use very long cables, herein 
called “sweeps,” that skim the sea- 
floor ahead and to both sides of the 
trawl net. In Alaska flatfish fisher- 
ies, the fishermen have used progres- 
sively longer sweeps to increase the 
width of their gear and, hence, the 
area from which flatfish are captured. 
These sweeps now account for the 
overwhelming majority of the seafloor 
area swept by these trawlers to cap- 
ture flatfish. Although these sweeps 
greatly increase flatfish catches, they 
also account for most of the negative 
effects of trawling on the seafloor. 
Although some reviews (Kaiser et 
al., 2007) have recommended devel- 
opment of modified fishing gear to 
reduce the effects of trawling on sea- 
floor communities, studies that test 
such gear are just beginning to be 
published. He (2007) reviewed such 
efforts for all mobile fishing gears. A 
substantial effort in Europe focused 
on modifications for beam trawling 
(van Marlen et al., 2005). Guyonnet 
et al. (2008) described tests of modi- 
fied gear that reduce the contact of 
the cables between trawl doors and 
nets with the seafloor. Although 
their tests were accomplished with 
different modifications to gear 
(dangling chain sections attached 
to neutrally bouyant rope) and in a 
very different fishery, their concept 
is very similar to the modifications 
we tested. 
Ryer (2008) has described flat- 
fish behaviors that are important to 
