Walker et al.: Use of an underwater camera to monitor distribution and density of Piacopecten magellanicus 
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Figure 1 
A map of the Mid-Atlantic Bight showing the sites where photographs of the sea bottom 
and sea scallops {Piacopecten magellanicus) were taken with an underwater camera from 
an autonomous underwater vehicle (AUV) at 22 survey sites in 2011. 
as a “lawn mower” pattern. Multiple equidistant tran- 
sects were run parallel to each other at a commanded 
even spacing that ranged between 2 to 40 m laterally. 
This method had the advantage of allowing 100% im- 
aging and side-scan sonar coverage depending on tran- 
sect spacing. Less frequently, our survey design con- 
sisted of equidistant oblique transects that propagated 
along only one of the transect axes in a slalom path. 
This design would be most useful for sampling an elon- 
gated bed of scallops. The third most used survey de- 
sign consisted of equidistant orthogonal transects that 
propagated along both transect axes (in the profile of 
a staircase). This design provided the largest extent of 
geographic coverage from a single battery charge. 
Equipment 
Our surface vessel was the FV Christian and Alexa, 
a 30-m eastern rig, commercial fishing ship with port 
and starboard New Bedford style 15-ft (4.57-m) scallop 
dredges. For comparison of the AUV imagery data, each 
survey site was dredged immediately after the AUV 
survey with the starboard scallop dredge by towing for 
15 minutes at 4.5 to 5.0 knots at every site along the 
initial AUV transect line. The dredges were fitted with 
4-inch (10.2-cm) interlocking rings to coincide with 
commercial fishing requirements, along with an 11- 
inch (27.9-cm) twine mesh top and turtle chains. Shell- 
height-frequency data were collected on the deck from 
the dredged contents by using standard survey meth- 
ods for sizing a randomly selected bushel of scallops. 
The photographic imaging platform used was a Tele- 
dyne Gavia AUV that has an operational depth limit 
of 500 m. The AUV was run in an imaging and sonar 
mapping configuration consisting (from front to back) 
of a nose cone camera, lithium ion battery module, 
GeoSwath phase measuring bathymetric sonar (500 
kHz) module, Kearfott T-24 inertial navigation sys- 
tem (INS) and Doppler velocity log (DVL), command 
module (900/1800 kHz Marine Sonic side-scan sonar), 
and a propulsion module. During a survey, the AUV 
can simultaneously optically image the seafloor, map 
the seafloor with side-scan sonar and phase measuring 
bathymetric sonar, log depth and altitude of the vehi- 
cle, and measure water temperature, dissolved oxygen 
saturation, turbidity, and salinity. 
The nose cone camera was a Point Grey Scorpion 
20SO research camera, equipped with a Sony ICX274 
Type 1/1.8” (8.923 mm diagonal) CCD camera, with a 
resolution of 800x600 pixels. This camera was config- 
ured to acquire images at a rate of 3.75 images per 
second. Illumination was provided by LED strobe array, 
positioned obliquely aft of the camera. The camera has 
a Fujinon DF6HA-1B 6-mm focal length lens and a hor- 
izontal viewing angle of 44.65° in salt water based on 
a salinity of 35 (PSU). Calibration results determined 
