Jones et al Evaluation of rockfish abundance in untrawlable habitat 
335 
on the bank , indicated rockfishes located on top of the 
relatively shallow bank (<150 m) within ~5 m of the 
bottom and represented the dominant species observed 
by SDC and ROV and captured by trawl. Backscatter 
in the other category, hereafter referred to as deep 
rockfishes, indicated rockfishes located at depths >150 
m and generally >10 m off the bottom over the bank 
flanks. The deep rockfish backscatter over the bank 
flanks was attributed to Pacific ocean perch because 
that was the only species observed in an SDC deploy- 
ment in that vicinity. 
Several areas on the bank contained backscatter 
that resembled bubble plumes rising from the seafloor. 
Such backscatter was characterized by comparing the 
frequency response relative to 38 kHz. The expected 
volume backscattering strength from rockfishes at 18, 
70, 120, and 200 kHz is within 5 dB of the volume 
backscattering strength at 38 kHz (De Robertis et 
ah, 2010). Any backscatter resembling bubble plumes 
with a frequency response that differed from the re- 
sponse at 38 kHz by more than 5 dB was classified 
as bubbles; otherwise, backscatter was classified as 
rockfishes on the bank or as deep rockfishes over the 
flanks. 
Differences in mean rockfish backscatter for all 8 
passes were evaluated with ANOVA. Tests were per- 
formed on natural log-transformed data because of un- 
equal variances in the raw data. Differences in rockfish 
backscatter within pass pairs (between night and day) 
were evaluated with a paired T-test. All tests were con- 
sidered significant at an alpha level of 0.05. 
The mean height above the seafloor of the seafloor, 
or height off bottom (m), for backscatter attributed to 
rockfishes on the bank was calculated for each pass 
with the following formula: 
Mean height off bottom = X(s Ai x hf) / X s Aj , 
where s Ai = the nautical area scattering coefficient 
(MacLennan et ah, 2002) in each bin with 
a resolution of 185x0.5 m (except in the bin 
closest to the bottom, which was 0.25 m 
high and offset from the bottom by an addi- 
tional 0.25 m); and 
h, = the height off bottom of each respective bin. 
For rockfish backscatter and height off bottom, each 
pass was considered a sample unit because data for 
adjacent transects were not independent. In addition, 
transects differed in length, and, if transects were 
used as sample units, the contribution of the shorter 
transects would be disproportionate compared to the 
contributions of other transects because shorter tran- 
sects would receive the same weight as longer ones. 
Because of these conditions, estimates of sampling 
variance were expressed as coefficients of variance 
(CV) with passes as the sample unit, rather than as 
standard deviations derived from transects as the 
sample units, and, for that reason, we do not show 
error bars in our figures. 
Stereo-video drop camera 
The SDC (for a full description, see Williams et al., 
2010) was used to identify and count fish species. Paired 
still images from 2 video cameras were used to estimate 
fish length and height off bottom. All SDC deployments 
were conducted in locations where fish aggregations 
were identified acoustically. The SDC was maintained 
at a constant height off bottom by using a live video feed 
to the surface. The paired cameras were oriented at 30° 
off horizontal (forward and slightly down), allowing the 
field of view to extend vertically from the seafloor to ~3 
m off bottom. The horizontal field of view surveyed by 
the cameras (W) was ~2.4 m. The distance the SDC cov- 
ered along the seafloor (L) was approximated by using 
the GPS on the Epic Explorer. The area swept during 
each SDC deployment was calculated as WxL and the 
catch per unit of effort (CPUE) for each species was cal- 
culated as the number of fish observed per area swept. 
All fishes observed in a camera deployment were 
counted and identified to species when possible. Height 
off bottom was measured from the seafloor to 2.0 m off 
the bottom and grouped in 0.5-m increments. Height 
off bottom was estimated from a single camera for 2 
deployments because a malfunction of one of the cam- 
eras did not allow stereo measurements of fish length 
or height off bottom. Height off bottom was compared 
for single- and stereo-camera counts from deployments 
where both cameras functioned properly. 
Remotely operated vehicle 
A Phantom DS4 ROV (Deep Ocean Engineering, Inc., 
San Jose, California) was used to collect data to verify 
substrate type, identify species, measure length of domi- 
nant rockfishes, and determine species-substrate rela- 
tionships (for a full description, see Rooper et al., 2012). 
All measurements were made with a pair of parallel 
lasers 20 cm apart and a third laser that crossed each 
parallel laser at specified distances from the cameras. 
Height off bottom for fishes observed in ROV deploy- 
ments was estimated as either on the bottom, up to 2.0 m 
off the bottom, or >2.0 m off the bottom. The ROV was 
not maintained with a constant field of view above the 
seafloor; therefore, we did not calculate the area swept 
and a CPUE for this survey tool. 
Modified bottom trawl 
Trawl deployments were conducted to collect rockfish 
specimens for species and size composition for compari- 
son with SDC data (for a full description, see Rooper 
et al., 2012). The trawl was a modified 4-seam Poly- 
Nor’Eastern bottom trawl similar to those trawls used 
by the AFSC in the GOA bottom trawl survey (Stauffer, 
2004). The major modifications to the net were heavier 
netting material in the belly of the net, a footrope with 
tire gear through the center, and continuous roller 
gear through the sweeps. Estimates of rockfish densi- 
ties were not calculated from these trawl deployments 
