Ryer et al.: Flatfish herding behavior in response to trawl sweeps 
147 
(x- 87.5, SE = 6.3, £ [34] = 3.28, P = 0.003). During long 
tows, accumulating catch can distort meshes and back 
up into the intermediate portion of the net, altering 
gear selectivity (Herrmann, 2005). However, catches in 
this study were small compared to net capacity, never 
filling the codend. Hence, we assume that differences 
in duration between day and night did not influence 
net performance or fish catchability in a manner that 
would bias our results. Similarly, during long tows 
proportionately more fish will tire and fall back into 
the net, particularly so for many roundfish species, 
which can swim for prolonged periods in front of the 
net (Main and Sangster, 1981). However, flatfish typi- 
cally swim for less than 1 minute in front of nets ( Ryer, 
2008), and thus this source of bias was also unlikely 
in our study. 
For our first analysis, we compared daytime and 
nighttime catches from the control nets only; where 
sweeps were in contact with the bottom along their 
entire length. Catch per unit of effort (CPUE: kg/min) 
was calculated for total catch (all species) as well as for 
six flatfish species: yellowfish sole ( Limanda aspera ); 
flathead sole ( Hippoglossoides elassodon); arrowtooth 
flounder ( Atheresthes stomias)\ rock sole ( Lepidopsetta 
spp. ); Alaska plaice ( Pleuronectes quadrituberculatus); 
and Pacific halibut. CPUE values were natural log (In) 
transformed and tested for day and night differences 
with t-tests (Sokal and Rohlf, 1969). Where variances 
were heteroscedastic, Satterthwaite’s adjusted degrees 
of freedom were used (Snedecor and Cochran, 1980). Be- 
cause CPUE was based upon weight, we also compared 
mean total length between daytime and nighttime hauls 
for each flatfish species. 
For our second analysis, we used the subset of samples 
from trawls where 25.4-cm disks were attached to el- 
evate sweeps of the experimental net to an approximate 
height of 10 cm (the distance between sediment surface 
and bottom of the sweep material). For this analysis, 
catch of the experimental net was compared to that of 
the paired control net (with bottom contact sweeps) by 
using a paired /-test (Sokal and Rohlf, 1969). Separate 
analyses were conducted for daytime (rc = 10 pairs) and 
nighttime (n = 5 pairs) hauls. Similar analysis was con- 
ducted for flatfish lengths. 
Laboratory experiments 
Northern rock sole and Pacific halibut were collected 
as age-0 juveniles by using a 2-m plumb-staff beam 
trawl from Chiniak Bay, Kodiak, Alaska. Fish were 
transported to the Hatfield Marine Science Center in 
Oregon and reared in 2. 2-m (diameter) circular tanks 
with flow-through seawater (28-35%o, 9°C [± 1°]) on a 
diet of krill and gelatinized food. After reaching age 1, 
fish were transferred to 3-m diameter pools for contin- 
ued growth. 
Simulated sweep exposure took place in an elongated 
tank (10.7x1.5x1.2 m) filled to a depth of 0.9 m. This 
tank was provided with flow-through seawater (28-35%e) 
and located in a light-proof room, allowing for control of 
illumination by an overhead bank of fluorescent lamps. 
The tank bottom was covered to a depth of 4 cm with 
sand, allowing flatfish to completely bury themselves. 
Details of this apparatus are presented elsewhere (Ryer 
and Barnett, 2006) and will only be described briefly 
here. By means of a moveable carriage a simulated 
sweep was propelled down the length of the tank. This 
sweep consisted of a piece of 5-cm diameter PVC pipe, 
painted green to resemble the actual sweep used in the 
field study. It could be positioned so that it ran down 
the tank in contact with the bottom, or elevated so that 
it was approximately 10 cm off the bottom. 
Fish were maintained on a 12/12 h photo period 
during all experiments, with lights turned on at 0700 
and off at 1900. At 1600 on the day before the trials, 
the length of the tank was subdivided into three equal 
3-m sections, by means of four removable partitions, 
of which two of these partitions prevented fish from 
moving to the extreme ends of the tank. Next, fish 
were introduced to each of the three main sections of 
the tank. This sectioning assured that fish would not 
aggregate in a single area of the tank. At 0800 on the 
day of trials, the footrope carriage was lowered into 
the tank, behind one of the end partitions and secured 
to its tracks. Then the lighting was either turned off 
(dark trials) or kept on (light trials), and fish were 
allowed 2 h acclimation before a trial. Illumination at 
the sand surface was measured once at the beginning 
of the study. For light trials, illumination was approxi- 
mately 1.5 pmol photons/m 2 /s (-125 lux), whereas, for 
dark trials illumination was <lxl0 -8 pmol photons/m 2/ s 
(~10 -6 lux). Both species used in this study have the 
same light thresholds ( 10 5 pmol photons/m 2 /s) for vi- 
sual discrimination of small motile prey (Hurst et al., 
2007), and we assumed they would see approaching 
footrope in the light trials, but not in the dark trials. 
Illumination was measured with a research radiometer 
(International Light Inc., Model IL1700, Peabody, MA) 
equipped with a 2 ji PAR (photosynthetically active ra- 
diation) sensor. Water supply to the tank was filtered 
through sand, making it unlikely that water clarity, 
and hence light levels, changed appreciably from day 
to day. At 1000 h, immediately before a trial, the parti- 
tions were removed; for dark trials red flashlights were 
used during this process, and care was taken to avoid 
shining the lights directly into the tank. Five minutes 
later the footrope carriage was pulled from one end of 
the tank to the other at a speed of 1.0 m/s (± 0.1 m/s), a 
speed roughly equal 3.6 km/h or 2 knots; flatfish trawls 
are commonly towed at 2-5 knots. Afterwards, the 
lights in the room, if turned off, were turned back on 
and rakes were used to herd fish back into each of the 
three main sections of the tank, after which the parti- 
tions were put back in place and the footrope carriage 
was removed from the tank. This entire process was 
repeated in the afternoon, using the opposite lighting 
from that of the morning: at 1200 h, a footrope carriage 
was lowered into the tank and lighting was adjusted; 
at 1400 h, partitions were removed and the footrope 
carriage was pulled. We assume that this alternation 
