336 
Fishery Bulletin 1 10(3) 
because they were conducted in locations where fish 
aggregations were acoustically detected and therefore 
the level of catches would be biased high. Because our 
trawl deployments were not done at random locations, 
catch estimates from them could not be compared with 
results from regular bottom trawl surveys designed to 
provide estimates of rockfish density and biomass. 
Abundance estimation 
Fish abundance was estimated for the 3 most abundant 
rockfish species encountered in the core area covered 
in our study on Snakehead Bank: dusky, northern, 
and harlequin rockfishes. Abundance estimates above 
the acoustic dead zone were calculated for each species 
and depth layer. These estimates were then combined 
with abundance estimates from the acoustic dead zone, 
which were calculated by using 2 different methods 
(described later in this section), to obtain estimates of 
total species abundance. 
Length-frequency distributions and species compo- 
sitions were derived from SDC, ROV, and trawl de- 
ployments. Length-frequency distributions, backscat- 
ter measurements, species compositions, and a target 
strength (TS) regression (described later in this sec- 
tion) were used to estimate the total number of fish in 
1-cm length bins, by following Simmonds and MacLen- 
nan (2005). Length-weight relationships obtained from 
catch data for each species, from AFSC bottom trawl 
surveys conducted in the summer in the GOA, were 
used to estimate a biomass for each species and depth 
layer above the acoustic dead zone. 
It was not possible to obtain an estimate of rockfish 
TS during this study, and no published estimates for 
the primary species encountered are available. There- 
fore, the regression described for generic physoclist 
fishes, TS = 201og 10 L-67.5, where L is fork length (cm) 
(Foote, 1987), was used as an approximation. Stan- 
ley et al. (2000) used this TS relationship for widow 
rockfish (S. entomelas ) because it was shown to also 
agree with several studies on deepwater redfish (S. 
mentella). Rooper et al. (2010) also used the same 
TS regression for a combination of Sebastes species 
in the Bering Sea. Furthermore, Kang and Hwang 
(2003) examined ex situ TS of Korean rockfish (S. 
schlegelii ) and obtained a similar relationship of TS = 
201og 10 L - 67.7. 
Biomass in the 0.7-m acoustic dead zone was cal- 
culated by 2 methods to account for the binning of 
the video observations in 0.5-m increments. The first 
method used the correction proposed by Ona and Mit- 
son (1996). This correction extrapolates backscatter 
to the dead zone from a designated zone above the 
dead zone. The resulting backscatter within the dead 
zone was apportioned to species based on species com- 
position data from the SDC. The second method for 
calculating abundance in the dead zone used 2 com- 
binations of depth layers and species ratios from SDC 
counts ( i . e . , “1.0-m SDC ratio” and “0.5-m SDC ra- 
tio”, Fig. 2, B and C). This method, where a constant, 
weight-specific TS across species and size classes is 
assumed, used the ratio of species relative abundance 
from SDC counts in adjacent depth layers to extrapo- 
late abundance from a depth layer above the dead zone 
to a layer within the dead zone with the 
following equation: 
A 
B 
c 
2.0 m 
EK60 
' 
2.0 m 
EK60 
2.0 m 
EK60 
EK60 
EK60 
EK60 
1.0 m 
1 .0 m 
1.0 m 
0.5 m 
bottom 
Ona and 
Mitson 
correction 
1 ,0-m 
SDC 
ratio 
EK60 
bottom 
bottom 
OTET-m 
SDC ratio 
Figure 2 
Diagram of depth layers used in calculations of abundances for the 
dead zone in 2 methods: (A) Ona and Mitson (1996) dead zone correc- 
tion, as well as (B) extrapolation of abundance from the depth layer 
of 1. 0-2.0 m to the depth layer of 0-1.0 m using the ratio of fishes 
observed in counts from images collected with the stereo-video drop 
camera (SDC) and (C) extrapolation of abundance from the depth 
layer of 0. 5-1.0 m to the depth layer of 0-0.5 m by using the ratio of 
fishes observed in SDC counts. EK60 refers to abundance estimation 
by using backscatter collected with a Simrad EK60 scientific echo 
sounder. A dead zone is a near-bottom area where the echo from the 
seafloor masks acoustic signals from organisms near the seafloor. 
- «V C I+1J ) x A z+1 j, 
where A, ■ = the abundance in metric tons 
of species j in depth layer 2 ; 
C ZJ = the relative abundance of spe- 
cies j in depth layer 2 (from 
the camera data); 
C z+1 J = the relative abundance of spe- 
cies j in the depth layer 2 + 1 
(also from the camera data); 
and 
A z+1 j = the abundance in metric tons 
(derived from acoustic mea- 
surements) of species j in 
depth layer 2 + 1 . 
For the “1.0-m SDC ratio,” 2 represents 
the depth layer of 0-1.0 m and 2+1 repre- 
sents the depth layer of 1. 0-2.0 m. For the 
“0.5-m SDC ratio,” 2 represents the depth 
layer of 0-0.5 in and 2 + 1 represents the 
depth layer of 0. 5-1.0 m. Abundance esti- 
mates for all depth layers were combined 
for total biomass values by species and 
method. 
