Williams et al.: Use of stereo-camera systems in assessing rockfish abundance and pollock behavior 
359 
A raw image B processed image C 3D reconstruction 
Figure 5 
Images of walleye pollock (Theragra chalcogramma ) from a still-frame stereo camera in a midwater trawl and a 3-D 
reconstruction of fish in relation to the trawl net. (A) Fish lengths were measured by enlarging the image of a fish 
and indicating the position of the snout and tail (shown as U) in both right and left raw images (only the left image is 
shown above). (B) The chosen fish endpoints are overlaid on the image as lines. In addition to estimates of fish length, 
stereo-processing allows the position of fish in relation to the trawl to be estimated. Additional points in the images can 
be determined by finding corresponding left-right image pixel coordinates (B, shown as crosses). (C) Following stereo- 
triangulation, a three-dimensional plot shows the fish targets as arrows and trawl mesh knots as dots. 
sured, and thus the number of 
fish that can be measured is 
larger from the same length 
transect. Improvements in the 
quality of the still-frame im- 
ages and in the collection of 
calibration data from a target 
at the beginning of each tran- 
sect may allow more precise 
measurements to be taken 
in future studies. Given our 
inability with other survey 
gears to determine fish size 
and species composition in un- 
trawlable habitats, the use of 
stereo cameras holds promise 
for stock assessments of rock- 
fish and other species. Stereo- 
camera-based sampling could 
also be used broadly wher- 
ever gears other than bottom 
trawls are needed to obtain 
species- or size-composition 
information. 
Lengths of rockfish derived 
from the video-drop system 
were generally comparable to 
trawl catch-based size distri- 
butions for the species exam- 
180’0' 177°0'W 174°0'W 171°0'W 168°0'W 
Figure 6 
Map of study areas in the eastern Bering Sea showing the location of field 
tests of the drop stereo video cameras for sampling untrawlable areas (black 
square) in July 2008 and for sampling fish behavior in a trawl (circles) with 
a still-frame stereo camera in July 2007. 
