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Fishery Bulletin 1 10(3) 
of the seafloor. Optical methods also allow researchers 
to collect length information from smaller individuals, 
but this advantage can be offset by potential inaccura- 
cies in species identification because these small indi- 
viduals are difficult to identify with optical methods. 
There are cost advantages of using the SDC over 
both the ROV and trawl methods because the initial 
investment in equipment is smaller. The stereo cameras 
allow scientists to accurately measure the height of 
individual fish off the seafloor and the opportunity to 
measure the length of a higher proportion of observed 
fish than does the ROV. These are both critical factors 
for acoustic surveys where it is important to know the 
size of fish that are observed acoustically in the water 
column. The major disadvantages of the SDC are the 
difficulties associated with identifying all fish to spe- 
cies and an inability to finely control the position of 
the cameras. 
For this analysis, we assumed that the distribution of 
height off bottom for each species was accurately repre- 
sented by the data collected with the SDC. Any behav- 
ioral reactions to this camera system (for example fish 
diving away from the camera as it approached) would 
have influenced our ability to perceive the height of fish 
off the bottom accurately. Errors in this measurement 
would have serious effects on the acoustic estimates 
of abundance for any species that reacted to the SDC. 
For example, if one rockfish species had a tendency to 
dive to the seafloor before coming into the view of the 
SDC, as has been observed with manned submersibles 
(Krieger and Ito, 1999), the species could be under- 
represented in the biomass estimate of fish from above 
the acoustic dead-zone. As the SDC is a relatively small 
vehicle without a motor that drifts at low speeds with 
the prevailing current (creating less noise), its potential 
for eliciting a reaction by fishes is probably less than 
that of the bottom trawl or ROV. During the analysis 
of the video from this study, we observed that reactions 
to the SDC by rockfish were minimal, consistent with 
a previous study with a SDC (Rooper et ah, 2010) and 
a study where a larger towed camera sled was used 
(Rooper et ah, 2007). Fish reactions to underwater ve- 
hicles have generally been found to vary with both the 
species examined (Krieger and Ito, 1999; Lorance and 
Trenkel, 2006; Ryer et ah, 2009) and the type of un- 
derwater vehicle used (Stoner et ah, 2008). This is an 
area where more research should be completed in order 
to gauge the ability of the SDC and other underwater 
vehicles to accurately measure the height of rockfishes 
off the seafloor. 
Conclusion 
Our overall recommendation for verification of target spe- 
cies in acoustic surveys in areas of patchy untrawlable 
habitat is that a combination of technically advanced 
stereo-optic equipment and more rugged bottom trawls 
be used where species identification is likely to be dif- 
ficult or where many species are found in the water 
column. In cases where the rockfish assemblage is domi- 
nated by one or two easily distinguishable species, the 
stereo-optic methods will be the least destructive way to 
obtain the basic information needed to conduct fisheries 
acoustic surveys. An important problem highlighted by 
this research is that species exclusively found in the 
acoustic dead zone (for example, yelloweye rockfish in 
this study) will not be able to be assessed acoustically. 
For these species, alternative methods such as bottom 
trawls, long-lines, or optical methods using line transect 
or area swept survey methods will be the only adequate 
means for estimating the abundance of these fish. There- 
fore, our results suggest that the selection of appropriate 
methods for target verification depends on the specific 
objectives, habitat types, and species complexes being 
assessed. 
Acknowledgments 
The authors thank the captain and crew of the NOAA 
research vessel Oscar Dyson and the FV Epic Explorer. 
Field assistance was provided by M. Wilkins, D. King, 
K. Stierhoff, and C. Conrath. We also thank M. Wilkins, 
A. DeRobertis, C. Wilson, D. Demer, and T. Weber for 
their reviews and conversations regarding this project. 
The manuscript was also improved by reviews from D. 
Somerton, N. Laman, G. Fleischer, M. Donnellan, and 
two anonymous reviewers. The project was partially 
funded by a grant from the North Pacific Research Board 
(contribution no. 344). 
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