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Fishery Bulletin 108(2) 
and arrowtooth flounder ( Atheresthes stomias ). Towing 
continued through both day and night periods, reflect- 
ing commercial practice. All of the tows were in areas 
with bottom substrates composed of mixtures of sand 
and mud (McConnaughey and Smith, 2000). 
The trawls were identical two-seam nets with 200- 
mm mesh in the forward portions and equipped with 
130-mm codends. The mouth opening of each net was 
much wider (25 m) than high (3 m). Similar nets in a 
single trawl configuration are used to target flatfish on 
the eastern Bering Sea shelf. The distances between 
each of the doors and the central clump were monitored 
for equality with acoustic measurement systems and 
were each approximately 80 m. Both nets were equipped 
with sensors that indicated the direction of water flow 
in relation to the center of the headrope. The three 
towing cables were adjusted to keep that flow perpen- 
dicular to the headropes of both nets and to keep their 
door-clump openings equal, assuring comparable fishing 
characteristics for both fishing systems. 
The sweeps were 180-m long and were composed of 
5-cm (2-inch) diameter combination rope constructed of 
steel cable covered with polyethylene fiber. This is the 
most common sweep material currently used in U.S. 
Bering Sea flatfish fisheries. Sweeps used on vessels to 
target flatfish on the eastern Bering Sea shelf are 200 
to 450 m long (C. Rose, unpubl. data). The sweeps of 
the two adjacent trawls had to be about half as long as 
those used with commercial single trawls, because the 
entire twin trawl system is similar in width to a con- 
ventional single trawl. The shorter sweep lengths were 
necessary to assure that the angle of the test sweeps 
to the direction of towing was similar to that common 
in the fishery. In this field study, clusters of disks (disk 
clusters) were attached over the experimental sweeps 
at 9-m (30-ft) intervals (Fig. 2). The disks were either 
15, 20, or 25 cm (6, 8, or 10 inch) in diameter attached 
to 5-cm (2 inch) diameter sweeps, creating nominal 
clearance between the cables and the seafloor of 5, 7.5, 
and 10 cm (2, 3, and 4 inch), respectively. Nominal 
clearances are those immediately adjacent to a disk 
when the disk is resting on a hard surface. The press- 
ing of disks into the seafloor and the sagging of sweeps 
between elevating devices would affect actual clear- 
ances. For stability, disk clusters were approximately 
the same length as their diameter. These disk clusters 
were fixed in position with a combination of clamps 
and rope seizings, which were run through the sweep 
cable to prevent the clusters from sliding along the 
cable. Disk clusters were installed on the aft 90 m of 
the modified sweeps. Halfway through each experiment, 
the sweeps were switched between the two trawl nets, 
but each trawl net remained in place. 
Catches from each trawl were kept separate through- 
out the sampling process. As catches entered the sam- 
pling area, they passed across a motion-compensated 
flow scale which provided a total catch weight. All indi- 
viduals of four flatfish species (yellowfin sole, northern 
rock sole, flathead sole, and arrowtooth flounder) and 
two gadids (Pacific cod [Gadus macrocephalus ] and 
Alaska pollock [ Theragra chalcogramma ]) were sorted 
into separate holding bins. These are the principal 
flatfish and gadid species harvested from the eastern 
Bering Sea shelf. Fish from each bin were then run 
across a second flow scale to measure the weight of each 
of these species. During the sorting of catch from each 
trawl, 50-150 fish of each species were sampled and 
their fork lengths were measured to 1-cm intervals to 
determine their size composition. These length samples 
were periodically taken from the catch as it passed 
through the sorting area to avoid bias in case fish size 
varied between parts of each catch. Because of their 
large size, limited holding space and handling 
requirements precluded adequate length sampling 
of Pacific cod. 
Tows were planned to last 2 hours, unless catch 
sensors indicated substantial catches (>8 met- 
ric tons [t] per net) before that time. Actual tow 
durations ranged from 33 to 150 minutes. We 
eliminated hauls where debris (e.g., crab pots) 
was large enough to clog the net, or where gear 
components became entangled, because such con- 
ditions could influence gear performance and the 
size and composition of the resulting catch. Tow 
locations were selected in order to encounter com- 
mercial concentrations of the major flatfish species 
of the eastern Bering Sea shelf. Environmental 
parameters at the trawl, including depth, tempera- 
ture and light level, were sampled throughout the 
experiment with a Mk9 logger (Wildlife Comput- 
ers, Redmond, WA) mounted at the center of the 
trawl’s headrope. 
We used a high-resolution, rapid-update sonar 
(SoundMetrics DIDSON, Dual-frequency IDen- 
tification SONar, Lake Forest Park, WA) to ob- 
serve how the sweep modifications affected sea- 
Schematic diagram of a cluster of disks (disk cluster) attached 
to trawl sweeps to raise the sweeps above the seafloor to 
test whether this gear modification reduces flatfish herding. 
Rubber disks (A, 20 cm-diameter, and B, 15 cm-diameter) 
were installed over the sweep cable, between clamps (D) that 
fix their location on the cable. Steel washers (C) prevented 
rubber disks from passing over clamps. Ropes seized over and 
tucked through cable (E) blocked clamps from shifting. 
