Walker et al.: Use of an underwater camera to monitor distribution and density of Placopecten magellanicus 
267 
tilde set point along the trackline. As part of the image 
analysis and review process, annotators classified the 
quality of each image on the basis of whether the im- 
age was out of focus, too dark, or the water was too tur- 
bid for scallops to be recognized. More than 95% of all 
the images v/ere of sufficient quality for the annotators 
to recognize, count, and size the scallops. It is worth 
noting that many towed camera systems have only a 
fractional portion of the images annotated, whereas we 
were able to annotate all of our images. Additionally, 
the stability of the AUV platform ensured that fewer 
images (<5%) were removed with altitude or image 
quality filtering. 
Biomass 
Because our project methods were based on seabed im- 
ages, we used an empirical relationship from the litera- 
ture to estimate the meat weight of scallops for com- 
parative purposes. This parameter has been shown to 
vary on the basis of a number of geographical and en- 
vironmental factors and decreases with depth (Hennen 
and Hart, 2012). The equation chosen from Rothschild 
et al. (2009) is based on meat-weight measurements 
of sea scallops dredged from within the Mid-Atlantic 
Bight (the study area) and therefore was not further 
corrected for latitude and longitude. The meat-weight 
biomass was calculated for each scallop by using the 
photogrammetrically measured shell height of each 
scallop and the depth recorded for each image with the 
following equation: 
W (Eq. 2) 
where = meat-weight biomass of the sea scallop in 
grams; 
1 ^ 5 = shell height of the sea scallop in 
millimeters; 
d - depth of the sea scallop in meters. 
Fishing effort 
Commercial fishing dredges create distinctive pat- 
terns of sediment disturbance on the seafloor, and 
these dredge scars are visible with side scan sonar 
(Dickson et al., 1978; NRC, 2002; Lucchetti and Sala, 
2012; Krumholz and Brennan, 2015). For each survey, 
the dredge marks were determined from the side-scan 
data collected by the AUV at the same time that the 
photos were gathered. SonarV/iz 5 (Chesapeake Tech- 
nology Inc., Mountain View, CA) was used to process 
the acoustic data collected by the 900 kHz side-scan 
sonar and gridded at a 0.25x0.25 m horizontal resolu- 
tion for inspection. Each dredge scar was then manu- 
ally traced in each side-scan mosaic by using the So- 
narWiz digitizing tool. The track length was then mul- 
tiplied by the measured width of the dredge scar (4.57 
m) to determine the total area of the dredge scar. The 
area dredged was compared with the area acoustically 
surveyed (survey track length multiplied by a 20-m 
swath width) to give a ratio that represented a mea- 
sure of recent fishing effort. For one site, we ran the 
AUV both before and after the dredge and verified 
that our dredge track was visible in the side-scan im- 
agery. In most of the sonar mosaic images there were 
many dredge marks visible such that our sampling 
mark was only a small fraction of the total estimated 
dredge area. 
Results 
Over the 10-day cruise in July 2011, we completed 22 
surveys with the AUV and covered 257 km of track 
line for a combined surveyed area of 490,000 m^. In 
all, 203,000 images of the seafloor were produced, from 
which annotators identified and digitally sized 15,252 
sea scallops. 
Scallop density 
The New York Bight (NYB) region was surveyed over 
14 discrete sites and sea scallops were identified from 
images collected from 12 of those sites (i.e., 2 survey 
sites had no scallops). Overall, 276,000 of seafloor 
were surveyed (optically and acoustically) in the NYB, 
and densities for each survey ranged from 0 to 0.039 
scallops per (Table 1). The area-weighted mean 
scallop density for the region was 0.012 scallops per 
m^. The 6 sites that had scallop densities «0.01 scal- 
lops per were the shallowest surveys (20.1-35.5 m) 
and also coincided with the warmest near bottom tem- 
peratures (10.0-12.7°C). We also observed that these 
sites typically had dense sand dollar populations. 
The histogram in Figure 4A shows the shell-height 
frequency for all of the photogrammetrically sized sea 
scallops within the NYB. Taken together, 1.5% (48) of 
the 3,213 sized scallops fell into the recruit size class 
(<70 mm), and 13.8% were of a size larger than that of 
recruits but smaller than the 4" dredge rings (>70 mm 
and <101.6 mm). The harvestable size class accounted 
for the remaining 84.7%, which results in an exploit- 
able (harvestable) scallop density of 0.010 scallops per 
for NYB. The mean shell height for the NYB region 
was found to be 121 mm. These results indicate that 
the NYB had a size population dominated by large har- 
vestable scallops. 
The Long Island (LI) region was surveyed at 8 
distinct sites, and an area of over 156,000 of sea- 
floor was photographed in the region. The sea scal- 
lop density was 0.077 scallops per m^, and there was 
a large variability in densities ranging from 0.01 to 
0.20 scallops per (Table 1). As found in the NYB 
region, the denser scallop populations were found at 
near bed temperatures of 8-9°C, whereas the warmer 
(>10°C), shallower survey sites had the lowest popula- 
tion densities. 
The distribution of shell heights in the LI region is 
shov/n in Figure 4B. Of the 12,039 scallops that were 
sized, 61.2% (7,368) were classified as harvestable and 
