Williams et al.: Use of stereo-camera systems in assessing rockfish abundance and pollock behavior 
361 
Figure 10 
Orientiation of walleye pollock ( Theragra clialcogramma) within a midwater 
trawl. A still-frame stereo camera was used to determine (A) the distribution 
of individual fish tilt angle (deviation of snout-tail axis from the horizontal), 
and (B) the yaw (angle of fish heading in the horizontal plane). Most fish 
were oriented horizontally and facing toward the trawl opening. Image data 
were collected in the eastern Bering Sea in July 2007 as part of a study of 
fish behavior within the trawl. 
as a quantitative comparison of 
trawl- and stereo-camera-derived 
size estimates, they demonstrate 
the similarities in information 
supplied by the two methods and 
the potential for stereo cameras to 
overcome some problems with the 
catchability of juvenile fish. 
The stereo camera was very use- 
ful for studying behavior of pollock 
in the trawl. The data show the 
possibility of performing a length- 
based analysis of behavior which 
will directly contribute to studies 
of gear selectivity and future de- 
signs of scientific trawl gear. Al- 
though a postsurvey calibration 
was not performed with the still- 
frame system, the cameras were 
securely fastened in the housings 
and were not removed during the 
entire data collection, thus main- 
taining intercamera spacing and 
angles. The agreement between 
the catch-based length measure- 
ments and the stereo-derived 
lengths provides direct validation 
of the stereo-derived measurements. The low sampling 
frequency of 1 frame per 5 s ensured minimal influ- 
ence of the artificial lighting from the cameras on 
behavior because the fish photographed had not been 
previously exposed to the light source. 
Recent development of high-resolution digital im- 
aging devices and an increased access to custom de- 
signed, freely available software tools have made ste- 
reo-camera methods easy to implement by research 
groups without direct expertise in the subject. The 
camera calibration toolbox (Bouguet, 2008) provided 
the basis for software development. Although the cur- 
rent analysis approach is still fairly time intensive, 
the volumes of data analyzed were not very large. In a 
routine application of stereo-video cameras in untraw- 
lable areas, additional levels of automated processing 
would likely be required because the quantity of video 
footage would substantial. For some aspects of the 
analysis, such as the matching of targets on the stereo 
cameras and the extraction of fish lengths, automation 
may be attainable, whereas automating more difficult 
tasks of isolating and identifying fish targets may not 
be feasible. 
Stereo photography will continue to be developed as 
survey tools are developed for monitoring fish stocks 
and thereby improving the quality of stock assessments 
of fishery resources in Alaska. Some challenges remain; 
for instance, the challenge of institutionalizing image- 
based sampling as a routine survey method for untraw- 
lable habitats. As a method of studying fish behavior 
in trawls, stereo cameras provide promising results by 
allowing three-dimensional reconstructions of the trawl 
environment. 
Acknowledgments 
The development and deployment of the camera and 
winch system was possible only through the assistance 
of G. McMurrin, G. Mundell, B. Lauth, G. Hoff, and 
especially S. McEntire. T. Cosgrove, K. Sjong, L. Mavar, 
and M. Booth of the FV Vesteraalen were instrumental 
in conducting the field tests. Calibration and deploy- 
ment of the still-frame camera system were significantly 
aided by D. Jones and A. McCarthy. This manuscript 
was reviewed by D. Somerton, G. Hoff, R. Lauth, and 
M. Wilkins. 
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