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Fishery Bulletin 1 10(3) 
The acoustic dead zone is the area near the seafloor 
where fish targets cannot be resolved from the seafloor 
echo. At the Snakehead Bank, it was found to be depth 
dependent but generally extended to 0.7 m above the 
seafloor (Jones et al., 2012 [this issue]). Therefore, we 
calculated the proportion of each rockfish species that 
was observed in the acoustic dead zone (<1 m off the 
seafloor) and compared this proportion to a random 
vertical distribution of fish using a chi-squared statistic. 
This analysis was conducted only for fish whose height 
off the seafloor was measured with the SDC and was 
used to test the hypothesis that rockfish were randomly 
sorting themselves into heights off the seafloor, regard- 
less of species. 
The distribution of rockfish species between traw- 
lable and untrawlable areas was also compared to a 
random distribution over the two habitats by using a 
chi-squared test. Additionally, the proportion of each 
of the major rockfish species and a combined “other” 
species group that occurred in untrawlable habitat was 
calculated along transects and compared to determine 
whether individual species were found in significantly 
different proportions in either trawlable or untrawlable 
habitats. For these analyses the replicates were tran- 
sects where the species (or species group) occurred and 
where both trawlable and untrawlable areas occurred 
along that transect. Thus, the distribution of a rockfish 
species was tested as to whether it was found predomi- 
nantly within trawlable or untrawlable habitat along 
a transect. The proportion data were arcsin square- 
root-transformed before the t-test to improve normality. 
To produce a target verification map of backscatter 
from fish targets for acoustic analysis, we then assumed 
that the height of rockfish off the seafloor would have 
been the same for the fish observed in the ROV and cap- 
tured in the bottom trawl (where this aspect of rockfish 
distribution was not measured) as was observed with 
the SDC. The proportions of each rockfish species <1 m 
off bottom and >1 m off bottom from the SDC were thus 
applied to the fish observed by the ROV and captured 
by the bottom trawl. The resulting proportions were 
shown graphically across the area of the acoustic survey 
where target verification transects and bottom trawl 
tows were conducted in order to show the spatial distri- 
bution of fish species, as well as their vertical distribu- 
tion as either within or above the acoustic dead zone. 
Finally, the amount of time needed to deploy and re- 
trieve each gear type and process the data to completion 
was estimated. The amount of time for each task was 
summed by each gear type for comparisons. The ap- 
proximate cost for building, deploying, and maintaining 
each of the gear types was also compared. 
Results 
Classification of substrate 
The most common seafloor substrates observed in the 
ROV and SDC video data from the Snakehead Bank were 
combinations of cobble, pebble, and sand. These 3 sub- 
strates comprised the primary substrate in 70.7% of the 
total seafloor area observed in the ROV videos and 89.8% 
of the seafloor observed in the SDC videos. However, 
23.6% of these otherwise trawlable substrates observed 
in the ROV videos and 71.7% of these substrates in SDC 
videos were judged to be untrawlable because of the 
presence of large boulders or rocks. In total, 46.0% of 
the substrate observed by the ROV was designated as 
untrawlable, whereas 74.6% of the substrate observed 
by the SDC was designated as untrawlable. The untraw- 
lable observations came predominantly from the eastern 
half of the study area. Acoustic data confirmed that the 
eastern half of the study area was mostly untrawlable 
and the western half of the bank was predominantly 
trawlable (Fig. 1; Weber et al. 2 ). However, some patches 
of trawlable ground occurred at transects in the area 
designated as predominantly untrawlable and vice versa. 
Identification of fish 
The ROV was deployed at four locations, the bottom 
trawl was deployed at six locations, and the SDC was 
deployed at eight locations where acoustic backscatter 
attributed to fish was observed near the seafloor and 
in the water column (Fig. 1). During two of the SDC 
deployments only a single camera collected images and 
during one deployment at a trawlable location, no rock- 
fish were observed. At 5 of the SDC sites, the bottom 
trawl was deployed at the same location immediately 
after SDC deployment. One of the ROV deployments was 
at the same location as that of a SDC deployment and 
two of the ROV deployments were at the same location 
as that of a bottom trawl (Fig. 1). However, all of the 
target verification deployments used in this analysis 
occurred between depths of 65 and 150 m on the top of 
the Snakehead Bank, and all were conducted within a 
210-km 2 area. 
Twelve different species of rockfishes were identified 
at the Snakehead Bank study area. Nine species were 
identified by using the ROV, 9 with the bottom trawl, 
and 7 with the SDC. Six species were observed in com- 
mon by all 3 gear types. The most common rockfish cap- 
tured in the bottom trawl and recorded by the ROV and 
SDC was dusky rockfish (Table 1). These were followed 
by harlequin rockfish (S. variegatus), northern rockfish, 
and Pacific ocean perch. Analysis of variance revealed 
there were no significant differences in the number of 
species observed among the three gear types (P=0.31, 
F=1.27, n = 16 deployments). The total numbers of fish 
observed were almost equal for the ROV and SDC (1251 
and 1176, respectively). The number of fish captured 
by the bottom trawl (6993) was much higher. The total 
amount of seafloor observed by the optical methods was 
2 Weber, T., C. N. Rooper, J. L. Butler, D. T. Jones, and C. D. 
Wilson. 2012. Seabed classification for trawlability using 
the Simrad ME70 multibeam echosounder on Snakehead 
Bank in the Gulf of Alaska. In review. 
