Williams et al.: Use of stereo-camera systems in assessing rockfish abundance and pollock behavior 
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fish length = distance ([X h , Y h , Z h \, [X v Y v Z t ]) 
Method for determining fish length measurements by using stereo images. The three-dimen- 
sional coordinates of the fish head and tail (X h ,Y h ,Z h ; X t ,Y t ,Z t > were determined by stereo- 
triangulation and by using the image-based coordinates from the image pairs (i.e., LX h ,LY h ; 
RX h ,RY h ). Fish length was estimated as the Euclidian distance between the three-dimensional 
points of the head and tail. 
Results 
Calibration 
The estimates of distance of the fish to the trawl deter- 
mined with the second calibration of the drop-video 
system were significantly larger and more variable 
than those from the first measurement set across all 
three intervals (Fig. 7). Differences between the mean 
measurements and known values in the second set 
ranged from 6.6% to 8.2%. However, the 95% confidence 
intervals for both sets included the actual values for 
the intervals in all cases, and the coefficients of varia- 
tion for the measurements ranged to 5.5% of the mean 
value, indicating that the length measurements were 
reasonably precise. A similar procedure was also per- 
formed with the still-frame system, but only a single 
set of validation measurements was made before the 
start of field operations. The results of this set closely 
matched that of the first set made with the video cam- 
eras (Fig. 7). 
Fish lengths determined with the video-drop system 
The adult rockfish observed in the video were northern 
rockfish (96.94%), unidentified adult rockfish ( Sebastes 
spp., 0.98%), adult Pacific ocean perch (0.49%), and 
dusky rockfish (S. ciliatus, 1.60%), whereas most of the 
juveniles that were identified to species were Pacific 
ocean perch (Rooper et al. in press). Some of the juve- 
nile rockfishes observed in the video were too small to 
identify to species. Individuals of each species group 
were randomly chosen to be measured in proportion with 
their abundance. Up to 200 randomly selected individual 
rockfish were measured in each transect, resulting in 
a total of 1489 length measurements. Rockfish were 
measured by using fork length only if both the tip of 
their snout and the end of the tail were plainly visible 
in both still images. If the randomly chosen rockfish 
could not be measured, the next available rockfish of the 
same species group that was deemed measureable was 
chosen. In a few cases, where the occurrence of a spe- 
cies group was very small (<5 individuals in a transect), 
none were measured. 
A random sample of 20 rockfish that were observed in 
successive still frames of both video cameras was used 
to determine measurement precision and to estimate 
distance of the fish from the camera. These fish were 
measured in up to four consecutive frames and their 
estimated length were compared by using linear regres- 
sion (Fig. 8). The percent difference between successive 
length measurements was not significantly related to 
the average fish length (P=0.28); in other words, there 
was no length-related bias in the measurements. The 
length data were also tested for a relationship with dis- 
tance from the camera by using linear regression. There 
was no bias in the measurements of fish for distance 
from the camera (P=0.29). The standard deviation of 
