Rooper et al.: Estimating species and size composition of rockfishes to verify targets in acoustic surveys 
325 
rockfish grouped into the “other species” 
category were observed exclusively in un- 
trawlable areas, with the exception of one 
redbanded rockfish (Fig. 7). This division 
resulted in a significantly higher propor- 
tion of the “other species” group being 
found in untrawlable areas than in traw- 
lable areas (P<0.0001, £=-40. 09, n = 12). 
Together, the differences in both vertical 
(height off the seafloor) and spatial (traw- 
lable versus untrawlable habitat) distribu- 
tions of the rockfish, resulted in a complex 
picture of the verification of fish species 
potentially observed in acoustic data dur- 
ing the survey of Snakehead Bank (Fig. 
8). Rockfishes within the acoustic dead 
zone (<1 m) over trawlable areas were 
dominated by dusky rockfish and north- 
ern rockfish (Fig. 8). In the untrawlable 
areas, the acoustic dead zone contained 
dusky, harlequin, and northern rockfishes 
in greatest abundance. Fish in the water 
column (>1 m off bottom) that were likely 
to be observed by using the vessel acous- 
tics comprised mostly dusky and northern 
rockfish in both trawlable and untrawlable 
areas, although as shown in Figure 7, the 
higher proportion of these two species was 
observed in untrawlable areas. 
Figure 3 
Mean (and standard error) fork length (cm) for dominant rockfish spe- 
cies observed with the 3 gear types ( ROV=remotely operated vehicle, 
Trawl = bottom trawl, and SDC = stereo drop camera) at the Snakehead 
Bank, Alaska, in 2009. Sample sizes for length measurements are the 
same as those shown in Figure 2. 
Northern rockfish Dusky rockfish 
( Sebastes poly spin! s) ( Sebastes variabilis) 
Figure 4 
Mean (and standard error) fork length (cm) for rockfish species observed 
and measured in both trawlable and untrawlable regions of the Snake- 
head Bank, Alaska in 2009. Data from the remotely operated vehicle 
and stereo drop camera are combined. « = number of fish measured 
for each species. 
Data analysis, processing time, and cost 
The ROV required both the highest level 
of expertise and the longest time to deploy 
(Table 4). The bottom trawl required the 
least amount of time to deploy, retrieve, 
and process samples (Table 4). The level of 
expertise required to deploy and retrieve 
the gear was high, but other tasks asso- 
ciated with the bottom trawl required 
moderate expertise. The level of exper- 
tise required to deploy and retrieve the 
SDC was also high, although it could be 
done in relatively short time. The level of 
expertise to process the SDC video footage 
into data required for acoustic surveys was 
also high, and the time required to col- 
lect and process one sample (1 h of video) 
was large (7 h). Once the ROV video was 
collected, processing it into data required 
for verification of target species in the 
acoustic surveys was comparable to that 
required with the SDC, although more 
time was necessary to measure the lengths of fish with 
the lasers than with the stereo cameras. The initial costs 
of purchasing the ROV and constructing the bottom 
trawl were quite high. The SDC was the cheapest of the 
3 equipments to purchase and construct. The cost per 
unit of area surveyed during this project was cheapest 
with the bottom trawl and most expensive with the ROV. 
Discussion 
In this study, the rockfish species observed in the 
water column were similar between trawlable and 
untrawlable areas, which is encouraging for the poten- 
tial to assess the biomass of these species acoustically 
in both types of habitats. However, clear differences 
