146 
Fishery Bulletin 108(2) 
al., 1999, 2004). On some modern flatfish trawls these 
sweeps may be up to 400 m in length, and as much 
as 90% of the seafloor is subject to the action of gear 
which is designed to affect capture by manipulating 
flatfish swimming behavior. But for the very reason that 
footropes are more efficient in the dark, sweeps may 
be less efficient. If flatfish, unable to see the approach- 
ing sweep, rise or hop into the water column, rather 
than herding as happens during the day, they will pass 
over the sweep and be lost to the catch. This situation 
raises the possibility that flatfish trawls that rely upon 
sweep herding may capture more flatfish during the day 
than during the night — a pattern not seen with survey 
trawls, which have minimal sweeps. 
In this study we investigated the performance of 
trawls equipped with sweeps under day and night con- 
ditions, using a combination of manipulative at-sea and 
laboratory procedures. For our at-sea experiment, we 
used a data set acquired during a series of cruises in 
the eastern Bering Sea, the goal of which was to evalu- 
ate sweeps designed to reduce damage to benthic habi- 
tat (Rose et al., 2010). In brief, trawling was conducted 
with sweeps that were elevated, to various degrees, off 
the seafloor to evaluate the trade-off between reductions 
in habitat disturbance and decreased flatfish herding 
efficiency. Here we test hypotheses related to our prin- 
ciple premise: flatfish behavior initiated by ground-gear 
is principally controlled by ambient light levels. More 
specifically, first we test the hypothesis that trawls 
configured with control (commercial type) sweeps in 
contact with the bottom, will catch more flatfish dur- 
ing the day than during the night. Following from this, 
we test a second related hypothesis: the elevation of 
sweeps off the bottom will have differential effects, 
day as opposed to night. During the day, elevation will 
reduce sweep efficiency, resulting in lower flatfish catch. 
During the night, because sweeps are already rela- 
tively ineffective, elevation of the sweeps will have no 
influence upon their efficiency, as reflected by flatfish 
catch. Lastly, we conducted comparable experiments 
under both light and dark conditions, using simulated 
ground-gear in the laboratory where behavior could be 
quantified, to ascertain whether the proposed effects 
of elevated sweeps on catch are directly attributable to 
ambient-light-mediated differences in flatfish behavior 
in relation to ground gear. 
Methods 
At-sea experiments 
Tows of paired trawls (control and elevated sweeps) 
were conducted during September 2007 in the eastern 
Bering Sea onboard the FV Cape Horn. Details of gear 
and onboard procedures can be found in Rose et al. 
(2010). Briefly, the Cape Horn is a 47-m trawler proces- 
sor, configured so as to allow twin trawling, i.e., fishing 
with two identical nets side-by-side. Each net had a 
set of independent 180-m sweeps, being spread by one 
otter board on each side of the vessel, and connected 
in the middle by a towed weight (clump). The sweeps 
were composed of 5-cm diameter combination rope, con- 
structed of steel cable and covered by polyethylene fiber. 
Modifying the sweeps on one net, while keeping all other 
trawl characteristics consistent, allowed the difference 
between the two catches to reflect the effect of the modi- 
fication. In this field study, disk clusters were attached 
to the experimental sweeps at 9-m intervals. The disks 
were either 15, 20, or 25 cm in diameter. This created 
a nominal spacing between the sweeps and the seafloor 
of 5, 7.5, and 10 cm, respectively. Test tows were made 
with modified sweeps on one net and unmodified sweeps 
on the other. Halfway through each experiment, the 
modified sweeps and unmodified sweeps were switched 
(left to right, right to left). 
Catches from each trawl were kept separate until the 
entire catch had been sampled. As catches entered the 
sampling area, they were passed across a motion-com- 
pensated flow scale to determine total catch weight. The 
five or six most abundant species were then completely 
sorted into holding bins. Fish from each bin were then 
run across a second flow scale to measure the weight of 
each of those species. To estimate the weight of other 
species, samples of the unsorted catch were taken at 
intervals, sorted, and weighed by species. The com- 
position of these samples was then expanded to the 
weight of the entire catch by calculating the fraction 
of the sample weight to the total catch weight. For the 
species cited in this paper, Pacific halibut and Alaska 
plaice catches were estimated from the samples and 
all other species were fully weighed on the second flow 
scale. During the sorting phase, samples of 50-150 fish 
of each species were drawn and measured to determine 
their length composition. Length samples were taken 
from throughout the catch as it passed through the 
sorting area and the length of each individual in the 
sample was measured 
Sixty-one paired hauls were made over depths rang- 
ing from 70 to 117 m. Ambient light on the bottom is 
greatly influenced by water depth. To minimize poten- 
tial depth effects upon ambient light, we limited our 
analysis to hauls where depth was between 79 and 
94 m: a 15-m range. In addition, we eliminated hauls 
where large debris (crab pots, etc.) were encountered, 
or where gear components became entangled, assuming 
that such conditions would influence gear performance 
and catch. After examining in situ light measurements 
(Wildlife Computers, MK9 light meter, Redmond, WA) 
we further eliminated daytime hauls where light levels 
fell below l.OxlO -4 pmol photons/m 2 /s, and nighttime 
hauls exceeding 1.0xl0~ 5 pmol photons/m 2 /s. This step 
eliminated hauls made around dusk or dawn and set 
a clear differentiation between daytime and nighttime 
light. In the resulting data set (36 hauls), mean tow 
depth did not differ between nighttime and daytime 
tows (day: n= 7, mean [x] = 82 m, standard error [SE] = 1; 
night: n =19, x=84 m, SE = 1; t (34 ]=1.54, P=0.133). Tow 
durations ranged from 33 to 150 min, being somewhat 
longer at night (x=115.8, SE = 5.9) than during the day 
