Rose et al. : Effective herding of flatfish by cables with minimal seafloor contact 
137 
their capture by trawls. Flatfish generally react to 
approaching objects at much closer ranges (<1 m) than 
do roundfish and remain very close to the seafloor 
when avoiding such objects. To target these behaviors, 
towing cables (angled toward the trawl net across the 
sea floor) are used in both demersal seines and otter 
trawls to herd flatfish into the path of a capture net. 
Flatfish avoid the approaching cable by continuous or 
burst-and-pause swimming, both of which move them 
gradually into the path of the capture device. Conven- 
tional sweep cables have equal diameters throughout, 
and no structures to interrupt their contact with the 
seafloor. Flere, we test whether effective herding re- 
sponses could be stimulated if such cables were raised 
a short distance above the seafloor. 
Like most flatfish fisheries, those in Alaska operate 
on seafloors consisting of unconsolidated mixtures of 
sand and mud. The potential for reducing damage to 
the physical and biological features of these habitats 
by raising sweeps a short distance off the bottom is 
dependent on the presence of low vertical relief or flex- 
ible structures of the bottom relief. This modification 
would likely not prevent damage to high relief and rigid 
or fragile features more common on rockier substrates. 
For the modifications tested here to be effective, their 
effects on both catch rates and seafloor features need 
to be examined. 
To develop practical modifications for the trawl sys- 
tems used in Alaska’s flatfish fisheries, we convened a 
series of meetings with trawler captains and gear man- 
ufacturers. For initial study, they recommended raising 
the sweeps slightly above the seafloor, allowing small 
and flexible animals and other habitat structure to pass 
beneath. In the current study we examine the proposed 
change, focusing on determining which adjustments 
maintain catch rates and on using direct observations 
to demonstrate reduced seafloor contact. 
Methods 
To test the effect of the modified sweeps on their ability 
to herd flatfish, we used a twin trawl system (Fig. 1). A 
twin trawl system tows two separate trawls, including 
sweeps, simultaneously on parallel, adjacent tracks. 
Close proximity and simultaneous operation assure that 
both nets encounter very similar compositions of fish 
species at similar abundances. Therefore differences in 
catch are principally due to differences in the capture 
effectiveness of the two trawls. The only difference 
between the trawls in this experiment was the use of 
the elevating disks on the sweeps of the trawls. 
Twin trawl tests 
Field experiments were conducted during September 
2006 in the eastern Bering Sea onboard the FV Cape 
Horn. The Cape Horn is a 47-m trawler processor, 
active in the mixed groundfish fisheries of the Bering 
Sea. This vessel was equipped for a twin trawling 
system, with an extra winch and towing cable. The 
sweeps and trawls were towed with conventional trawl 
doors on each side and a weight (clump) in the middle. 
Both doors and the clump were towed from three sepa- 
rate cables that were adjusted so that both sides fished 
evenly. Towing sites were selected to provide com- 
mercial catch rates of a mixture of the four principal 
flatfish species of the Bering Sea shelf: yellowfin sole 
( Limanda aspera); northern rock sole (Lepidopsetta 
polyxystra); flathead sole (Hippoglossoides elassodon ); 
