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Fishery Bulletin 1 10(3) 
Figure 1 
Map of study area showing the Snakehead Bank southwest of Kodiak 
Island, Alaska (indicated by black outline in inset). Deployment 
locations of remotely operated vehicle (triangles, n = 4 deployments), 
stereo drop camera (squares, n = 8 deployments), and bottom trawl 
(filled circles, n = 6 deployments). Acoustic transect lines and depth 
contours (m) are also shown. Trawlable areas are shown in light 
gray and untrawlable areas are shown in dark gray. 
seafloor. Some size classes of the population 
may occur exclusively near the bottom (<1 m), 
where they cannot be acoustically differenti- 
ated from the seafloor (Ona and Mitson, 1996; 
Rooper et ah, 2010). Therefore, the ability to 
estimate the distance of fishes off the seafloor 
is important in determining which species 
and size classes are acoustically observable. 
We evaluated the ability of gear types 
to discriminate species and size composi- 
tions of fish for the purpose of determin- 
ing the best methods for target verification 
for acoustic surveys for rockfishes in un- 
trawlable habitats. We compared the body 
lengths and species diversity of rockfishes 
from a modified bottom trawl with two op- 
tical methods — a remotely operated vehicle 
(ROV) and a stereo drop camera (SDC). For 
the SDC, the vertical distributions among 
species were compared. The proportion of 
rockfish species inhabiting trawlable and 
untrawlable areas was compared. We also 
compared the time and cost to employ each 
survey method in order to make recommen- 
dations for efficient and cost-effective methods 
for target verification in acoustic surveys. 
Materials and methods 
The research was conducted southwest of Kodiak 
Island, Alaska, at an offshore bank locally known 
as the “Snakehead Bank” from 3 to 12 October 
2009 (Fig. 1). The continental shelf of the Gulf 
of Alaska near Kodiak has been shaped by past gla- 
cial and seismic activity and generally comprises 
sedimentary bedrock covered with glacially deposited 
sediments overlying most of the shelf (Hampton, 1983). 
Much of the shelf south of Kodiak Island is a series of 
flat underwater banks with deep troughs carved by 
glaciers that separate adjacent flat banks. The Snake- 
head Bank is a relatively small (-210 km 2 ), shallow 
bank on the outer continental shelf that protrudes 
from the shelf and abuts the continental slope. At its 
shallowest point, the bank rises to within -65 m of 
the surface and deeper water (>150 m) is found on 
the continental shelf to the north. Much deeper water 
(200-2000 m) is located on the continental slope to the 
south and east. The depths of the Snakehead Bank 
are inhabited by a distinct assemblage of continental 
shelf rockfishes that typically extends to about 180 
m depth (Rooper, 2008). The Snakehead Bank has 
long been a productive area for commercial rockfish 
fisheries (Clausen and Heifetz, 2002; Hanselman et 
ah, 2007), and Gulf of Alaska bottom trawl survey 
tows conducted at the Snakehead Bank often have 
high catches of northern rockfish and dusky rockfish 
(e.g., von Szalay et ah, 2010). 
The research was conducted aboard two vessels, the 
NOAA ship Oscar Dyson and a chartered commercial 
fishing vessel, the FV Epic Explorer. The Oscar Dyson is 
a 64-m length overall stern trawler equipped for fisher- 
ies and oceanographic research. The Epic Explorer is a 
39.6-m house-forward stern trawler active in commer- 
cial fisheries in Alaska. Both vessels were present in 
the study area simultaneously. Researchers aboard the 
Oscar Dyson collected acoustic data using Simrad EK60 
scientific echosounders operating at five frequencies 
and a Simrad ME70 multibeam echosounder (Simrad, 
Horten, Norway 1 )- The ROV was also deployed from the 
Oscar Dyson. The modified bottom trawl and stereo drop 
camera were deployed from the Epic Explorer. During 
the survey with the Oscar Dyson, acoustic data were 
collected on a grid of parallel transects (Fig. 1). Eight 
individual passes of the parallel tracks were carried out 
(4 were completed during nighttime hours and 4 during 
daytime hours). From these acoustic data, researchers 
aboard the Oscar Dyson identified areas of fish aggre- 
gation and directed the deployment of the ROV, bottom 
trawl, and SDC to verify the species and length compo- 
sitions of acoustic targets at those locations. Then the 
acoustic survey data were used to estimate abundance 
of fish species identified by the target verification meth- 
J Mention of trade names or commercial companies is for iden- 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
