Walker et aL: Use of an underwater camera to monitor distribution and density of Placopecten magellanicus 
269 
Figure 5 
Boxplots of shell heights of sea scallops {Placopecten magellanicus) obtained from photo- 
graphs taken with an underwater camera of an autonomous underwater vehicle with the 
Long Island (LI) and New York Bight (NYB) areas. Surveys where <16 sea scallops were 
collected were not plotted. 
Discussion 
Automated underwater vehide as an image-produdng 
survey platform 
The AUV is an efficient platform that allows surveys 
from images (optical and acoustical simultaneously) 
over 15 km of seafloor on a single battery charge, and 
allows the noninvasive study of benthic organisms over 
any bed type, including rocky or uneven terrain that 
would be difficult or impossible for dredges. For sea 
scallops, we found that an altitude of 2.2 m allov/ed for 
the largest image area, while still maintaining visibil- 
ity and resolution to size scallops. Particulate matter 
in the water column drastically decreased visibility of 
the seafloor for altitudes over 4 m. Continual logging 
of geographic and environmental conditions allowed 
accurate sizing and enumeration of scallops after pro- 
cessing. The highly accurate navigation — typically a 
1-m drift over 1 km of trackline of the AUV — allowed 
precise repeatability of survey lines. Targets visible in 
overlapping side-scan sonar imagery exhibited horizon- 
tal offsets of less than 2 m — a finding that is consis- 
tent with numerous other AUV benthic and geomorphic 
survey studies (e.g. Patterson et ah, 2008; Forrest et 
aL, 2012; Rankey and Doolittle, 2012; Raineault et aL, 
2013). This navigational precision allowed for the re- 
occupation of survej’’ lines. 
A variety of survey designs were evaluated in our 
study. Although we believe designs that propagate in 
a continuous linear direction (e.g., in a stair-case pat- 
tern) have a use for surveying an extremely elongated 
bed of scallops, we did not find those designs suited 
this type of study or fully incorporated the strengths 
of the AUV. The boustrophedon survey design, or a 
more regular and approximately rectangular pattern 
design, was found to be most useful in simultaneously 
photographing the seafloor and acoustically mapping it. 
Surveys were designed to allow complete coverage of a 
rectangular survey site (-1.75 kmxO.3 km) with side- 
scan sonar. The use of the geo-referenced data of each 
image also made it possible to plot the precise location 
of each scallop and to evaluate the distribution of indi- 
viduals within the population (Trembanis et aL, 2012; 
Walker, 2013). 
Logistically, the AUV offers an effective and produc- 
tive platform for the collection of sea scallop images as 
part of a larger stock assessment effort. The ability to 
quickly deploy and retrieve the AUV from a support 
vessel allows the rapid acquisition of photographic and 
acoustic data that can be analyzed at sea during tran- 
sit time or after the completion of the cruise. Imaging 
the seafloor is a noninvasive way to survey the scallop 
population and gather data about the small-scale spa- 
tial structuring of the population, seafloor texture and 
morphological features, and water quality. Photogram- 
metric sizing of the scallops was rapid, requiring only 
a few seconds once a scallop had been located in an 
image. We found that the digital sizes agreed favorably 
with the measurements of dredged specimens from the 
survey sites. 
One of the major advantages of the AUV is the high 
volume of data that can be collected in a few hours, but 
this high volume also results in a significant challenge 
for data processing. However, we showed that with the 
aid of sizing software, a team of trained scallop annota- 
