Rooper et al : Estimating species and Size composition of rockfishes to verify targets in acoustic surveys 
321 
in front of the ROV and consistently out to 3 m in front 
of the SDC. In situ identification of fish with the optical 
methods was more difficult than with the bottom trawl 
and resulted in some fish that could not be positively 
identified to species. Many of these were smaller rock- 
fish (<150 mm) that could not be positively identified 
to species with the ROV and SDC. Double counting of 
individual fish was assumed not to be an issue for the 
SDC because the camera drifted with the current in a 
relatively uniform direction and generally passed by fish 
as they were observed. For this vehicle, only fish that 
appeared in front of the camera were counted. In some 
cases during ROV deployments the vehicle was stopped 
so that an individual fish could be identified, and this 
brief pause could have resulted in double counting as 
fish milled around the stationary vehicle. An attempt 
was made to minimize double counting of individual 
fish in these cases by not counting fish that moved into 
the frame while the ROV was stationary; however, some 
double counting of fish probably occurred during these 
occasional stationary moments during ROV deployments. 
A single experienced observer identified the fish to spe- 
cies for both the SDC and ROV, and the habitat where 
each fish was observed was classified as either t.rawlable 
or untrawlable. 
The Canadian grid projection (Wakefield and Genin, 
1987) calculated with the 3-Beam software system was 
used to estimate fish length with the ROV. This limited 
the ability of the ROV to measure fish that were not in 
the same plane as the seafloor (i.e., above the seafloor). 
Additionally, the height of individual rockfish off the 
seafloor could not be measured. 
For the bottom trawl, each catch was sorted to species 
and weighed. A random subsample of up to approxi- 
mately 150 fish from each rockfish species identified 
in the catch was dissected to determine sex, and indi- 
vidual fork lengths were measured to the nearest cen- 
timeter. Because the bottom trawl integrates the catch 
spatially in both the vertical and horizontal planes, the 
height above the seafloor could also not be estimated for 
fish captured with the bottom trawl. 
For the SDC, fish lengths were measured by using 
stereo triangulation functions supplied with the camera 
calibration software package (Bouguet, 2008) and the 
protocols identified in Williams et al. (2010). Images 
were extracted from the two video feeds at 1-s inter- 
vals, as with the calibration video. The videos from 
each camera were synchronized at the beginning and 
end by using the LED synchronization light. Length 
measurements were obtained by identifying the pixel 
coordinates of corresponding pixel locations (i.e., fish 
snout and fork of tail) in the left and right still frames 
of the camera. These points were used to solve for the 
3-dimensional coordinates of the points in the images 
by triangulation, and by using the calibration-derived 
parameters. Once the 3-dimensional coordinates of the 
fish snout and tail were obtained, the length was mea- 
sured as the simple Euclidian distance between the 
points in real space. This measurement method under- 
estimated length for fish whose bodies were curved. 
However, fish in the video and still camera rarely ex- 
hibited body curvature and the few individuals that did 
were excluded. All individual fish that could possibly be 
measured or a random sample of 200 fish per species 
(where more than 200 were possible) were measured for 
each deployment of the SDC. 
For each fish that was measured with the SDC, the 
distance of the fish off bottom when it was first observed 
was also measured. These distances were then sum- 
marized into 0.5-m bins for each species. Because the 
SDC was deployed -1 to 2 m off the seafloor, the vertical 
field of view was approximately 2 m off the seafloor and 
rarely extended above 3 m off the seafloor. This obvi- 
ously limited the observed fish height off bottom. 
Data analysis 
Species diversity among the ROV, bottom trawl, and 
SDC samples was determined by examining the number 
of species observed with the 3 verification methods. The 
total number of species observed was compared among 
gear types by using analysis of variance (ANOVA) with 
video transects and bottom trawl hauls as replicates. 
The proportion of fish that were unidentified on each 
transect was also tested by using ANOVA to compare 
the ability of each of the gear types to allow identifica- 
tion of observed rockfishes to species. The proportion of 
unidentified fish by transect was the dependent variable 
for comparisons among the categorical variable of gear 
type. The proportion data were arcsin square-root-trans- 
formed before the tests to best approximate normality. 
Statistical significance for all tests was determined at 
a<0.05. 
The fish-length distributions for major species were 
compared among gear types by using pairwise Kol- 
mogorov-Smirnov tests to determine whether the length 
distributions from different gear types could have been 
drawn from the same sample. Fish-length composition 
was compared by using ANOVA to test for significant 
differences in mean length within major species for 
the 3 gear types. Owing to small overall sample sizes, 
individual fish lengths were used as replicates in this 
analysis and were combined across transects. The mean 
length of two rockfish species (northern rockfish and 
dusky rockfish) that occurred in both trawlable and 
untrawlable habitat were also compared to determine 
if fish were smaller in one habitat than in the other. 
The percentage of rockfish that could be measured 
out of the total number of rockfish observed per tran- 
sect for the major species was also calculated for each 
gear type. We used a f-test to determine whether the 
proportion of rockfish that could be measured was sig- 
nificantly different between the ROV and the SDC. 
For this analysis, the overall proportions of rockfish 
measured on each transect were used as the replicates. 
The proportion data were arcsin square-root-trans- 
formed before the t-test to improve normality. We did 
not consider this comparison for the rockfish captured 
in bottom trawl hauls because all the fish captured in 
the trawl could potentially be measured. 
