Walker et al.: Use of an underwater camera to monitor distribution and density of Placopecten magellanicus 
271 
Meat weight (g) 
Meat weight (g) 
Figure 7 
Histograms of calculated biomass based on meat weight 
(g) of sea scallops (Placopecten magellanicus) collected 
in 2011 from (A) the New York Bight (NYB) and (B) 
Long Island (LI) areas. 
the examination of a much smaller random subset may 
yield sufficiently precise density values and substantial 
savings in time and effort. 
Sea scallop stock assessment 
The results of the inshore surveys showed that the LI 
region had an overall density of 0.077 scallops per m^, 
which agrees with the density of 0.061 scallops per m^ 
reported by Rudders and DuPaul'^ for dredge-based 
survey of the LI region in 2011. The NYB inshore sites 
were only slightly less populated (0.013 scallops per 
m^) in comparison both with the density of 0.015 scal- 
lops per m^ reported by Rudders and DupauP for the 
deeper NYB waters and with the population density in 
the LI region in general. The higher population level 
of the LI region has been hypothesized by Law (2007) 
to be due to seeding from the Georges Bank area. Con- 
versely, the lower NYB region densities could be ex- 
plained by the interruption of scallop larvae transport 
from the LI region caused by the influx of freshwater 
from the Hudson River (Law, 2007). Additionally, the 
Hudson Shelf Valley forms a natural bathymetric di- 
vide between the two regions. The two scallop popula- 
tions were also different in their shell height distribu- 
tions. The LI population was skewed toward smaller 
shell heights in contrast with the more symmetrical 
distribution of the NYB population. Both regions had 
very few recruit-size class scallops (<1.5%) and were 
found overall to possess large-size scallop populations. 
We also noted that the largest scallop populations oc- 
curred around the 9°C ocean water isotherm, which 
corresponds well with the optimal scallop growth tem- 
perature of 10°C reported by Posgay (1953). 
Dredging effort and sea scallop density 
The combined optical and acoustic AUV method used 
in this study was found to be an efficient way to fur- 
ther use commonly collected side-scan data to quantify 
dredging effort. This method could be used to assess 
the effects of dredging effort on other benthic organ- 
isms, particularly on common bycatch species in the 
scallop fishery. As would be expected, a direct compari- 
son of dredging effort with scallop density revealed 
that fishermen concentrated dredging in only the most 
populated sites, and that size-selective dredges had a 
noticeable impact on the shell height distribution of 
the remaining scallop population (see Fig. 5). 
The effects of dredging on the substrate of the sea- 
floor have been investigated in multiple studies (Har- 
getts and Bridger®; Caddy, 1973; Krost, 1990; Hall- 
Spencer and Moore, 2000; Jenkins et al. 2001; NRC, 
2002; Lucchetti and Sala, 2012; Krumholz and Bren- 
nan, 2015). These studies have found that substrate 
texture and fishing effort are the leading variables in 
the preservation of trawl marks. Finer grained sedi- 
ment (muddy sediment versus sandy bottom) allows 
the gear to penetrate further into the substrate due 
to lower mechanical resistance between the substrate 
and the gear (Krost, 1990). Researchers have reported 
dredge marks remaining for up to 1.5 years on con- 
tinually fished substrate (Hall-Spencer and Moore, 
2000). In the absence of dredging, disturbed bed con- 
touring effects last for significantly longer periods of 
time; Hall-Spencer and Moore (2000) reported dredge 
scars can remain for up to 2.5 years without further 
fishing efforts and Bernhard (reported by Krost, 1990) 
reported a single trawl scar remaining for up to 5 years 
® Margetts, A. R., and J. P. Bridger. 1971. The effect of a 
beam trawl on the sea bed. ICES Council Meeting (C.M.) 
Documents 1971/B:8, 9 p. [Available at website.] 
