Jones et al.: Evaluation of rockfish abundance in untrawlable habitat 
341 
powered, did not drift with the Oscar Dyson and was 
capable of observing specific objects of interest and 
identifying more fishes to species. The added control 
capability allowed greater flexibility to observe and 
identify fishes or features of particular interest, such 
as bubble plumes. Indeed, if not for the precise control 
of the ROV, the presence of bubble seeps would not 
have been confirmed. However, when specific objects 
are investigated, standardization of the viewing field 
and effort becomes more difficult. Bottom trawl surveys 
allow for complete species identification and length mea- 
surement of captured individuals but do not facilitate 
allocating catch to specific depth layers. Other aspects 
of the sampling procedures, such as time to deploy 
equipment and process samples, difficulty of operation, 
and cost, have been considered by Rooper et al. (2012). 
Generally, northern and harlequin rockfishes ob- 
served with the SDC and ROV were smaller than the 
rockfishes of those species observed with the bottom 
trawl, indicating size selectivity in the bottom trawl 
surveys (Rooper et al., 2012). Although the mesh size 
of the net may allow escape of juveniles and smaller 
adults, some of the difference in the estimated size 
distributions between the video and trawl equipment 
also could be a result of different reactions to the gear 
by juveniles compared to reactions by adults. Darting 
into cracks and crevices, juvenile rockfishes appeared to 
react differently to the ROV (and to a lesser degree to 
the SDC) than did adults. In contrast, most near-bottom 
adult rockfishes on the bank did not appear startled 
or exhibit obvious avoidance behavior to the ROV or 
SDC, although there was the potential for avoidance 
or attraction of adult fish to the ROV or SDC outside 
a camera’s field of view (Stoner et al., 2008). If this 
hiding behavior of juveniles also occurs in response to 
an approaching trawl and adults show less avoidance 
behavior, a disproportionate capture of larger fishes 
may occur. 
Despite a locally patchy distribution, abundance in 
the core area of our survey did not change significantly 
between passes, indicating that fishes were relatively 
stable in their geographic distribution over the limited 
duration of this study. Although most of the backscatter 
was located in habitat designated as untrawlable, dusky 
and northern rockfishes also were observed in trawlable 
areas. Juvenile rockfishes were much more prevalent 
in untrawlable areas than in trawlable areas, and the 
harlequin rockfish, which is smaller than the dusky and 
northern rockfishes, was not seen at all in the trawlable 
areas. This finding is likely a result of the shelter re- 
quirements of juvenile rockfishes and agrees with the 
observations of Krieger (1992) on unidentified, small 
(<25 cm fork length) rockfishes in southeast Alaska. 
The AFSC GOA bottom trawl survey is conducted 
biennially to assess the distribution and abundance 
of the principal groundfish species (von Szalay et. al., 
2010). Snakehead Bank lies primarily within the Ko- 
diak International North Pacific Fisheries Commis- 
sion (INPFC) statistical area. Results from the AFSC 
bottom trawl survey conducted in 2009 indicate that 
94% of dusky rockfish observed in the Kodiak INPFC 
statistical area were found in the depth stratum of 
100-200 m that covers an area of 43,333 km 2 (12,634 
nmi 2 ) (von Szalay et al., 2010). The density estimate for 
dusky rockfish was 8.8 kg/ha in the depth stratum of 
100-200 m from the 2009 bottom trawl survey in the 
Kodiak INPFC statistical area. Our estimate for dusky 
rockfish from surveys on Snakehead Bank was 167.1 kg/ 
ha, almost 19 times the value of the estimate from the 
2009 bottom trawl survey in the Kodiak INPFC statisti- 
cal area. The difference between our density estimates 
for Snakehead Bank and the AFSC estimates from the 
2009 bottom trawl survey for the entire statistical area 
is likely attributable to rockfishes being observed pre- 
dominantly within untrawlable habitat on Snakehead 
Bank. About 3% of the substrate in the depth stratum 
of 100-200 m within the Kodiak INPFC statistical area 
has been designated as untrawlable by the AFSC for its 
bottom trawl surveys. It is important to note that the 
designation of trawlability in the AFSC bottom trawl 
survey does not necessarily equate to our multibeam 
trawlability index because, unlike our index, the bottom 
trawl survey’s designation is applied to a grid consisting 
of cells of predefined size. When the higher densities of 
dusky rockfish from Snakehead Bank were applied to 
the untrawlable portion of the Kodiak INPFC statisti- 
cal area in the depth stratum of 100-200 m, the total 
abundance within that stratum increased by nearly 
60% from 38,000 t to 60,000 t. Similar patterns existed 
for the other 2 rockfish species. The results from the 
2009 bottom trawl survey indicated that the majority 
of northern (54%) and harlequin (97%) rockfishes were 
observed in the depth stratum of 100-200 m in the 
Kodiak INPFC statistical area. The density estimate 
for northern rockfish on Snakehead Bank was 5 times 
the estimate from the 2009 bottom trawl survey (depth 
stratum: 100-200 m), and the estimate for Snakehead 
Bank harlequin rockfish was nearly 60 times greater. 
The high rockfish abundances on Snakehead Bank 
indicate that a substantial quantity of fishes could be 
overlooked when trawl catches from trawlable areas 
are extrapolated to larger areas containing untrawlable 
habitat. Methods for near-bottom measurement are 
vital to determine accurate estimates of abundance 
for bottom-oriented species, but quantification becomes 
particularly difficult when fishes are in complex habi- 
tat inaccessible to both sonar and trawls. For the most 
accurate population assessments, adjustments must be 
made that account for the bottom-oriented proportion of 
the stock residing in these complex habitats. 
In our study, 2 methods were applied for estimating 
near-bottom abundance in complex habitat, one of which 
is applied in 2 different combinations of depth layers. 
All of the estimation methods use counts from video 
images to partition backscatter to species and depth 
layers. The Ona and Mitson (1996) correction essen- 
tially calculates the portion of the water column that 
lies within the dead zone and extrapolates the amount 
of backscatter in a specified area above the dead zone 
into that unknown area. This method assumes similar 
