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Fishery Bulletin 108(3) 
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video - 1 st measurement set 
video - 2nd measurement set 
still-image calibration 
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Distance measured (m) 
Figure 7 
Results of measuring known distances on the check- 
erboard (see Fig. 2) during the first and second cali- 
brations of the drop stereo video-camera system and a 
still-frame stereo-camera system used for estimating 
fish length and studying behavior. Calibration is a 
necessary step in the use of stereo cameras to allow 
measurements to be made from images. Values repre- 
sent the percent deviation in measurements in relation 
to known values, including 95% confidence intervals 
based on five measurements. 
-20 -16 -12 -8 -4 0 4 8 12 16 20 
Percent difference from mean 
Figure 8 
Frequency of percent differences from multiple length 
measurements of n = 20 rockfish and the mean length 
of the fish. Individual fish were measured multiple 
times from a series of images extracted from video 
taken by a drop stereo-video camera. Measurements 
from each image were then compared to the mean 
measurement for that individual to estimate poten- 
tial measurement error. Rockfish observations were 
collected at Zhemchug Ridges, eastern Bering Sea 
in July 2008. 
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Figure 9 
Comparison between length frequencies of walleye pollock ( Tlieragra chal- 
cogramma ) estimated from (A) the images from a still-frame stereo camera 
(« = 360) within a midwater trawl, and length frequencies obtained from (B) 
fish captured in the codencl and directly measured (n = 1260). The smaller 
camera-based sample results are similar to the direct measurements in 
their overall size distribution, but there was less definition for larger fish 
(>20 cm). Data were pooled from three trawl samples taken in the eastern 
Bering Sea in July 2007. 
ined. Northern rockfish lengths 
from stereo-video images taken 
along transects at Zhemchug Ridg- 
es ranged from 9 to 41 cm (mean 
length = 30.0 cm). In three bot- 
tom-trawl surveys near the Aleu- 
tian Islands, Clausen and Heifetz 
(2002) found that the mean size 
of northern rockfish was 29.9 cm 
and ranged from 15 to 38 cm. Ju- 
venile Pacific ocean perch lengths 
(2.6 cm to 25.0 cm) were similar 
but ranged to smaller sizes than 
those found for the Aleutian Is- 
lands (8.3cm to 24.9 cm; Boldt and 
Rooper, 2009). Lengths of juvenile 
Pacific ocean perch obtained from 
stereo video were also similar to 
those in three experimental tows 
in the Zhemchug Ridges area in 
2004 and 2007 (juveniles ranged 
from 10 cm to 25 cm). However, 
these lengths were measured 
from fish captured during bottom 
trawl hauls, where the incidence 
of smaller fish may have been due 
to reduced catchability of smaller 
individuals. Although these ob- 
servations are not meant to serve 
