352 
Abstract — We describe the applica- 
tion of two types of stereo camera 
systems in fisheries research, includ- 
ing the design, calibration, analysis 
techniques, and precision of the data 
obtained with these systems. The first 
is a stereo video system deployed by 
using a quick-responding winch with a 
live feed to provide species- and size- 
composition data adequate to produce 
acoustically based biomass estimates 
of rockfish. This system was tested on 
the eastern Bering Sea slope where 
rockfish were measured. Rockfish 
sizes were similar to those sampled 
with a bottom trawl and the relative 
error in multiple measurements of the 
same rockfish in multiple still-frame 
images was small. Measurement 
errors of up to 5 . 5 % were found on a 
calibration target of known size. The 
second system consisted of a pair of 
still-image digital cameras mounted 
inside a midwater trawl. Processing 
of the stereo images allowed fish 
length, fish orientation in relation 
to the camera platform, and rela- 
tive distance of the fish to the trawl 
netting to be determined. The video 
system was useful for surveying fish 
in Alaska, but it could also be used 
broadly in other situations where it 
is difficult to obtain species-compo- 
sition or size-composition informa- 
tion. Likewise, the still-image system 
could be used for fisheries research 
to obtain data on size, position, and 
orientation of fish. 
Manuscript submitted 21 January 2010. 
Manuscript accepted 27 May 2010. 
Fish. Bull. 108:352-362 (2010). 
The views and opinions expressed 
or implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Use of stereo camera systems 
for assessment of rockfish abundance 
in untrawiable areas and for recording 
pollock behavior during midwater trawls 
Kresimir Williams (contact author) 
Christopher N. Rooper 
Rick Towler 
Email address for contact author: Kresimir.Williams@noaa.gov 
Alaska Fisheries Science Center 
National Marine Fisheries Service 
National Oceanic and Atmospheric Administration 
7600 Sand Point Way NE 
Seattle, Washington 98115 
For modern fisheries stock assess- 
ments, fisheries-independent data 
are necessary to estimate population 
abundances and population trends. 
For most marine species, fisheries- 
independent abundance estimates are 
primarily obtained from large-scale 
multispecies bottom trawl surveys 
(e.g., Gunderson and Sample, 1980) 
and from acoustic surveys of pelagic 
fish stocks (e.g., Karp and Walters, 
1994). Although acoustic backscatter 
is used to measure fish abundance, 
midwater trawl samples are needed 
to determine the size and species 
composition of acoustically sampled 
fish populations. Both of these survey 
methods require physical sampling 
of trawl catches and such sampling 
can result in unrepresentative data 
in several ways. 
Bottom-trawl surveys are limited 
to the areas they can sample be- 
cause many research trawls are not 
constructed to efficiently fish over a 
rough or rugose seafloor. Thus, sur- 
veys with bottom trawls may not be 
appropriate for some species with 
affinities for untrawiable habitat or 
in survey areas where significant 
patches of untrawiable ground can be 
found (Zimmermann, 2003; Cordue, 
2007). In Alaska, semipelagic species 
such as northern rockfish ( Sebastes 
polyspinis) and Pacific ocean perch (S. 
alutus) are an important part of the 
commercial catch, but they also show 
some affinity for untrawiable areas 
(Clausen and Heifetz, 2002; Rooper 
et al., 2007). 
In addition, inferences from spe- 
cies- and size-composition data ob- 
tained from trawl catches can be 
biased on account of trawl selectiv- 
ity. Trawls are generally designed 
to capture larger, market-size fish, 
and their design for this selected 
size results in the under-retention 
of juvenile size classes. In acoustic 
surveys of walleye pollock (Theragra 
chalcogramma ), biases in midwater 
trawl catches directly translate into 
biases in abundance estimates for 
areas where large and small fish are 
found (Godo et al., 1998). Selective 
retention of fish is a consequence of 
size and species-dependent fish be- 
havior during the trawling process. 
Observation of fish reactions to trawl 
gear is critical to understanding the 
behavioral mechanisms responsible 
for trawl selectivity and to develop 
future trawl gear for research. 
Here, we describe the use of stereo 
photography to sample rockfish in un- 
trawlable habitats using a drop unit 
with a stereo video camera (hereaf- 
ter termed “video-drop” camera), and 
to study fish behavior in midwater 
trawls using a trawl-mounted pair of 
still-frame stereo cameras (hereafter, 
termed “still-frame” camera). Stereo 
cameras have been successfully used 
to measure fish in controlled aquacul- 
ture settings (Ruff et al., 1995; Har- 
vey et al., 2003) and in open water 
