Jones et al.: Evaluation of rockfish abundance in untrawlable habitat 
333 
zone, (i.e., the near-bottom zone where the echo from 
the seafloor masks acoustic signals from organisms 
near the seafloor), a large portion of the population may 
go undetected (Ona and Mitson, 1996), particularly in 
areas where the bottom terrain is rough or variable. 
Besides the problem of resolving fish backscatter 
within the dead zone, scientists also must consider 
the problem of determining the species composition 
and size distribution of fishes that are detected in that 
zone. Starr et al. (1996) used a submersible in asso- 
ciation with acoustics to estimate rockfish distribution 
and abundance. Krieger (1992), and Krieger and Ito 
(1998) used visual surveys from manned submersibles 
to assess rockfish abundance in untrawlable areas and 
compared their numbers with those from trawl catches. 
For surveying in large areas, however, manned sub- 
mersibles are costly, labor-intensive, and inefficient. 
Williams et al. (2010) demonstrated the feasibility of 
using stereo-video drop (i.e., lowered) camera systems 
for assessing rockfish species and size in untrawlable 
areas. Ressler et al. (2009) and Rooper et al. (2010) suc- 
cessfully used underwater cameras and echo sounding 
systems to assess rockfish populations in rocky habitat. 
However, the species of interest in these studies were 
far enough above the bottom that assessment in the 
acoustic dead zone was not necessary. 
Rockfishes, of the genus Sebastes , constitute a large 
and diverse assemblage within North Pacific marine 
ecosystems and are important components of this re- 
gion’s commercial fisheries. Of the rockfish species in 
the GOA, Pacific ocean perch ( Sebastes alutus ), north- 
ern rockfish (S. polyspinis), and dusky rockfish (S. 
variabilis) are among the most abundant. They are the 
only rockfish species supporting commercial fisheries 
(aside from occasional directed fisheries for the demer- 
sal shelf rockfish complex in specific areas), and all 3 
species have experienced local depletions within the 
last decade (Hanselman et al., 2007). In our study on 
Snakehead Bank, dusky, northern, and harlequin (S. 
variegatus ) rockfishes were the most abundant species 
observed during our surveys and the species on which 
our analyses focused. Determination of precise popula- 
tion estimates for dusky, northern, and harlequin rock- 
fishes is challenging because these species aggregate in 
rocky, high-relief areas where it is difficult to conduct 
trawl surveys to estimate abundance. 
Dusky rockfish are managed as part of the pelagic 
shelf rockfish assemblage and are routinely caught by 
trawlers on the outer continental shelf at depths of 
100—150 m. Dusky rockfish also have been observed 
on banks and near gullies with hard, rocky habitats 
containing sponges and corals. Commercial catches of 
dusky rockfish are primarily located on banks near 
Yakutat in southeast Alaska and to the east and south 
of Kodiak Island, Alaska (Lunsford et al., 2009). 
Northern rockfish are presently managed as a single 
stock in the GOA (Heifetz et al., 2009). The preferred 
habitat of adult northern rockfish in the GOA appears 
to be hard, rocky, or uneven substrate on relatively 
shallow rises and banks on the outer continental shelf 
at depths of -75-150 m. One such rise south of Kodiak 
Island known as Snakehead Bank accounted for 46% of 
the northern rockfish catch during the 1990s (Clausen 
and Heifetz, 2002). Northern rockfish stocks on Snake- 
head Bank have been depleted, and the commercial 
fishery is nearly absent compared to past effort in this 
area (Heifetz et al., 2009). 
The primary objective of this work, which formed 
part of a larger study, was to use acoustic and com- 
plementary sampling tools to evaluate the feasibility 
of improving abundance estimates of rockfish species 
in an untrawlable habitat in the GOA. Other aspects 
of the larger study, comparing sampling technologies 
(Rooper et al., 2012 [this issue]) and investigating the 
use of acoustics for substrate classification (Weber 1 ), are 
reported elsewhere in this issue of Fishery Bulletin or 
otherwise available. We used a combination of acoustic 
backscatter measurements, video observations from a 
stereo-video drop camera (SDC), a remotely operated ve- 
hicle (ROV), and catch composition data from a modified 
bottom trawl to estimate abundances of rockfish species 
on Snakehead Bank. To establish whether or not rock- 
fishes are disproportionately abundant in untrawlable 
areas, we compared estimates of rockfish biomass for 
the dominant species on Snakehead Bank with those 
obtained from the AFSC biennial bottom trawl survey. 
Materials and methods 
This study was conducted during the period of 3-12 
October 2009 with 2 vessels at a relatively shallow 
bank, known locally as Snakehead Bank, located at 
the GOA shelf break about 74.1 km (40 nautical miles 
[nmi]) south of Kodiak Island (Fig. 1). The acoustic 
surveys and ROV deployments were conducted aboard 
the NOAA Ship Oscar Dyson. The SDC and bottom trawl 
were deployed from the FV Epic Explorer. This site was 
selected because of high historical catches of northern 
rockfish in the commercial fishery and AFSC bottom 
trawl survey (Clausen and Heifetz, 2002) and an abun- 
dance of rough substrate designated as untrawlable by 
the AFSC GOA bottom trawl survey (Martin 2 ). 
The Snakehead Bank survey initially consisted of 14 
parallel transects 9.3 km (5 nmi) long and spaced 2.2 
km (1.2 nmi) apart (Fig. 1). Several transects were ex- 
tended where significant backscatter continued beyond 
the original endpoints used during the first pass. A 
pass, defined as a complete survey of all transect lines, 
was attempted twice — once during daylight hours and 
again at night — on 4 consecutive days. The number and 
length of transects surveyed were similar within each 
pair of passes (day and night) but varied between pairs 
because deteriorating weather conditions made it impos- 
1 Weber, T. 2011. Unpubl. data. Center for Coastal and 
Ocean Mapping, Univ. New Hampshire, Durham, NH 03824. 
2 Martin, M. 2009. Personal commun. Alaska Fisheries 
Science Center, Seattle, WA 98115. 
