Jones et al : Evaluation of rockfish abundance in untrawlable habitat 
339 
□ Harlequin rockfish 
Ona and 1.0-m 0,5-m 
Mitson SDC ratio SDC ratio 
Estimation method 
Figure 6 
Total abundance values (measured in metric tons of observed fish) for 
dusky ( Sebastes variabilis), northern (S. polyspims ), and harlequin 
(S. variegatus ) rockfishes in the core area of the surveys conducted 
on Snakehead Bank and calculated with the Ona and Mitson (1996) 
dead zone correction, 1.0-m and 0.5-m SDC ratios, and with the mean 
of al! of these abundance estimation methods combined. 
The majority of all fish species were 
<0.5 m off the bottom according to SDC 
counts and ROV observations (Fig. 4). 
Although the 3 major species considered 
here composed <40% of all fishes ob- 
served <0.5 m off the bottom (Fig. 4), 
the majority of the observed individuals 
from these 3 species were encountered 
in this depth layer (Fig. 5). 
The abundance estimates determined 
by using the Ona and Mitson ( 1996 ) dead 
zone correction for fishes observed <1.0 m 
off the bottom resulted in an additional 
2082 t of dusky rockfish (43% of all fishes 
in that depth layer) for a total water-col- 
umn biomass of 2676 t. Harlequin rock- 
fish (13% of all fishes in that depth layer) 
were the second-most abundant species 
<1.0 m off the bottom, but their biomass 
amounted to only 79 t because of their 
small size. Biomass of northern rockfish 
<1.0 m off the bottom (8% of all fishes in 
that depth layer) was 217 t, based on the 
Ona and Mitson correction method, and 
total water-column biomass was 321 t 
(Fig. 6). 
The abundance estimate for rockfishes 
<1.0 m off the bottom determined using the approach 
of the 1.0-m SDC ratio resulted in an additional 1171 t 
of dusky rockfish (3.5 times the estimate for the 1.0- 
2.0- in depth layer) and 117 t of northern rockfish (1.1 
times the estimate for the 1.0-2.0-m depth layer). 
Combining all depth layers resulted in total water-col- 
umn estimates of 1765 t of dusky rockfish and 220 t of 
northern rockfish (Fig. 6). Because no harlequin rock- 
fish were observed >1.0 m off the bottom, it was not 
possible to estimate their biomass with this method. 
Abundance estimates determined with the 0.5-m 
SDC ratio and camera counts in the 0. 5-1.0 m depth 
layer resulted in 574 t of dusky rockfish (70% of all 
fishes in that depth layer), 90 t of northern rockfish 
(12% of all fishes in that layer), and 28 t of harlequin 
rockfish (11% of all fishes in that depth layer). For 
rockfishes encountered <0.5 m off the bottom, the fol- 
lowing estimates were calculated: an additional 1441 
t of dusky rockfish (2.5 times the estimate for the 0.5- 
1.0- m depth layer); 258 t of northern rockfish (2.9 times 
the estimate for the 0.5-1.0-m depth layer estimate); 
and 167 t of harlequin rockfish (6 times the estimate 
for the 0.5-1.0-m depth layer). Summing over all depth 
layers resulted in total water-column estimates of 2609 
t for dusky rockfish, 452 t for northern rockfish, and 
195 t for harlequin rockfish (Fig. 6). 
The total abundance estimates that resulted from 
these 3 approaches were within 34% of one another 
for dusky rockfish, 30% for northern rockfish, and 40% 
for harlequin rockfish (Fig. 6). Because no specific ap- 
proach to estimate biomass was clearly superior, the 
estimates were averaged, and an overall biomass for 
each species was calculated. The resulting mean bio- 
mass estimates were 2350 t for dusky rockfish, 331 t 
for northern rockfish, and 137 t for harlequin rockfish 
(Fig. 6). 
Backscatter attributed to bubbles 
Backscatter at numerous sites within our Snakehead 
Bank study area resembled rising bubble plumes. These 
backscatter patterns were visible at all 5 EK60 frequen- 
cies and often extended from the seafloor vertically 
through the lower half of the water column. An ROV 
deployment in the vicinity of these backscatter verified 
the presence of bubbles seeping from the seafloor (Fig. 7). 
Most of the backscatter attributed to bubbles (62%) 
in the entire survey was recorded within untrawlable 
areas. At the base of several areas of bubble backscatter, 
we observed aggregations that appeared to be fish based 
on echo morphology and frequency response. It was dif- 
ficult to classify backscatter as either bubbles or fishes. 
However, the total amount of backscatter attributed to 
bubbles was <7% of the backscatter attributed to rock- 
fishes. Additionally, most of the backscatter attributed to 
rockfishes (pass average: 78%) occurred in areas without 
bubble plumes. 
Rock formations, presumably calcium carbonate pave- 
ments, and bubbles emanating from the substrate of- 
ten co-occurred. Subsequent water collections near the 
bubble seeps verified methane levels in the water col- 
umn up to 40 times those of atmospheric equilibrium 
conditions at ambient temperature and salinity (Lilley 4 ). 
4 Lilley, M. 2010. Unpubl. data. School of Oceanography, 
LTniv. Washington, Seattle, WA 98195. 
