Jones et al. Evaluation of rockfish abundance in untrawlable habitat 
337 
A 350 
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Survey day 
(A) Mean rockfish backscatter (s A m 2 nmr 2 ) and (B) mean height 
Figure 3 
i 2 nmi~ 2 ) 
off bottom (m) of rockfish backscatter in the core area of surveys 
conducted on Snakehead Bank by survey day for each pair of passes 
(one during the day and one at night for each day of the surveys). 
Trawlability index 
Multibeam acoustic data collected with an 
ME70 echo sounder were processed to char- 
acterize parameters that could potentially 
be used as an index for trawlability (Weber 1 ). 
SDC and ROV images were used to verify 
substrate typing from these multibeam data. 
The trawlability index was mapped along 
with EK60 backscatter by using ARCMAP 
software, vers. 9.3.1 (ESRI, Redlands, Cal- 
ifornia) to determine the amount of area 
designated to each substrate type and the 
association between substrate type and fish 
backscatter. 
Results 
The core area of the acoustic survey com- 
prised parts of 7 transects (numbers 5-11, 
Fig. 1) totaling -59 km (32 nmi) for each of 
8 passes. Bottom depths ranged from 63 to 
233 m (mean 118 m) over all transects and 63 
to 171 m (mean 101 m) within the core area. 
Backscatter designation and height off bottom 
Most (63%) of the backscatter attributed to rockfishes on 
the bank was observed within the core area, primarily 
along the 3 eastern transects (numbers 8-10, Fig. 1). The 
variation in rockfish backscatter among passes was rela- 
tively low (CV=0.27, N- 8, Fig. 3A), and no significant 
difference was observed in mean rockfish backscatter 
between day and night passes (P=0.29, Fig. 3A). 
Counts of fishes off bottom, determined from deploy- 
ments with only one functional camera, were verified 
by comparing them with counts from the stereo-video 
camera deployments where both cameras functioned 
properly. With the single-camera deployments, -10% 
of dusky rockfish and 25% of northern rockfish were 
closer to the bottom than those same speicies observed 
with the stereo-cameras during the same delployments. 
No harlequin rockfish were seen during the deploy- 
ments from which single- and stereo-camera compari- 
sons were made. When the deployments during which 
images were collected from only one camera were not 
included in analysis, overall abundance estimates de- 
creased -40% for dusky rockfish and increased 350% 
for northern rockfish. These differences in abundance 
estimates resulted from a change in the relative species 
abundance: 83% of all dusky rockfish and 79% of all 
harlequin rockfish encountered on all SDC deployments 
were observed during the 2 single-camera deployments. 
Because of the relatively minor change in assignments 
of height off bottom and the large change in species 
composition and abundance that would result if these 
data were not included, estimates of height off bottom 
from single-camera deployments were included in our 
analyses. 
The mean height off bottom for backscatter attributed 
to rockfishes over all passes in the core area was 1.5 m 
(Fig. 3B). Height off bottom for rockfish backscatter was 
variable among passes (CV=0.47, N= 8) largely because 
the height off bottom of backscatter was greater on the 
last daytime pass of the survey than on other passes. 
Species composition 
Relatively similar species compositions of the major rock- 
fish species (dusky, northern, and harlequin rockfishes) 
were observed with the different sampling tools. For all 
sampling tools, the dusky rockfish was more abundant 
(40% of individuals for SDC, 51% for ROV, and 67%< for 
trawl) than all other species, and the harlequin rockfish 
was the second-most observed species (12% of individu- 
als for both SDC and ROV, and 28% for trawl). Aside 
from juvenile Pacific ocean perch observed with the ROV 
(12%), northern rockfish was the third-most abundant 
species (3% of individuals for ROV, 4% for trawl, and 
12% for SDC). The SDC observed the highest number 
of unidentified juvenile (22%) and adult (8%) rockfishes, 
and the ROV observed the largest number (10) of spe- 
cies identified (for full details, see Rooper et al., 2012). 
Stereo-video drop camera 
In total, 9 deployments of the SDC were conducted (Fig. 
1). More than 3 times as often as any other species, 
dusky rockfish were observed at heights >0.5 m off the 
bottom with the SDC (Fig. 4). Although dusky rockfish 
composed only -10% of all fishes identified at heights 
<0.5 m off the bottom (Fig. 4), 56% of all observed dusky 
rockfish were seen in this depth layer (Fig. 5). Surveyed 
with the SDC, unidentified juvenile rockfishes composed 
the largest group (43%) that was observed at heights 
<0.5 m off the bottom (Fig. 4). 
