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Fishery Bulletin 1 10(3) 
Figure 7 
Images captured during our study on Snakehead Bank: (A) several rockfish species above carbon- 
ate pavement as seen from the remotely operated vehicle (ROV); (B) mixed rockfishes in untrawlable 
habitat as seen from the ROV; (C) dusky rockfishes at various heights off the bottom as seen from the 
stereo-video drop camera and (D) bubbles emanating from the substrate (with inset of bubble close up) 
as seen from the ROV. 
Underwater observations by ROV of areas with hard 
substrate and bubble plumes in the northwest corner 
of our study region confirmed that rockfishes were also 
present in these areas. Underwater video also showed 
numerous species of rockfishes taking refuge in rocky 
crevices and under carbonate ledges (Fig. 7). 
Discussion 
Several sampling tools were used during an acoustic 
survey to assess the species and abundance of rock- 
fishes on a predominantly untrawlable bank in the GOA. 
Each tool has advantages and limitations, but, when 
used together, they can give a more complete picture of 
habitat and species abundances. Acoustic surveys are 
excellent means for enumerating midwater organisms 
of known target strength. However, species that are 
strongly bottom-oriented are difficult to assess with 
sonar because of the acoustic dead zone. In addition, this 
problem is exacerbated in high-relief or sloped terrain 
where rockfishes are abundant because the upper extent 
of the dead zone is determined by where the acoustic 
beam first encounters the seafloor within the beam 
footprint (i.e., the shallowest point within the beam). 
Furthermore, acoustic sampling alone is often insuffi- 
cient to differentiate between species if multiple species 
are aggregated or have similar frequency-response or 
backscattering characteristics. In areas of rough terrain, 
or for species that are bottom-oriented or aggregated 
densely, video images can provide a better mechanism 
to quantify relative species abundance. 
Differences observed in the amounts of the rockfish 
species between the other 3 sampling tools (SDC, ROV, 
and modified bottom trawl) could be partly explained 
by the deployment procedures for the different tools. 
The SDC was lowered to the seafloor and drifted along 
transects at a consistent height off bottom without al- 
tering the camera angle. Because we surveyed in this 
manner, the SDC sampling effort remained constant 
for the different depth layers and was viewable up to 
about 2.0 m off the bottom. The ROV, because it was 
