Poussard et al.: Discriminating between high- and low-quality field depletion experiments 275 
box, 2.4—4.0 m wide and constructed of evenly spaced steel 
bars, and is towed over a seabed (Meyer et al., 1981; 
Lambert and Goudreau, 1996). A manifold at the head of a 
dredge distributes high-pressure water from a shipboard 
pump through a connecting hose. The water is focused 
with a series of jets onto the seabed, liquefying the sedi- 
ment and unearthing the clams for capture by the dredge. 
Hydraulic dredges, widely used in clam fisheries (Parker, 
1971; Hauton et al., 2007; Sala et al., 2017), are necessary 
for the commercial capture of the ocean quahog and 
Atlantic surfclam because of the depth and distance from 
shore at which these species are found and the fact that 
the fishery depends on the high volume and low unit cost 
of its catch. Therefore, rapid and efficient capture methods 
are economically essential. 
A series of depletion experiments were conducted by the 
National Marine Fisheries Service (NMFS) between 1997 
and 2011 on commercial clam vessels targeting populations 
of Atlantic surfclams and ocean quahogs off the northeast- 
ern and mid-Atlantic coasts of the United 
States to estimate the efficiency of both 
commercial hydraulic dredges. The exact 
locations of these depletion experiments 
are specified in appendix 3 of NEFSC 
(2017a, 2017b) (Fig. 1). As is often the 
case, estimates of dredge efficiency varied 
widely between individual experiments 
(Velstad et al., 2000; Powell et al., 2007; 
Hennen et al., 2012; Wilberg et al., 2013; 
Morson et al., 2018). Very little is known 
about how environmental and sampling 
conditions influence the efficiency of a 
hydraulic dredge, factors that are likely 
the source of variation between experi- 
ments. Consequently, estimates of gear 
efficiency based on depletion experiments 
are a key source of uncertainty in the 
stock assessments for the Atlantic surf- 
clam and ocean quahog. 
The Patch model was developed to ana- 
lyze the results of depletion experiments 
to estimate the efficiency of capture of 
sedentary species, such as the Atlantic 
surfelam and ocean quahog (Rago 
et al., 2006). The Patch model has been 
rigorously tested in previous studies 
(Hennen et al., 2012) and has been used 
to inform stock assessments of species, 
such as the Atlantic surfclam, ocean qua- 
hog, monkfish (Lophius americanus) 
(NEFSC, 2010a), and Atlantic sea scal- 
lop (Placopecten magellanicus) (NEFSC, 
2010b; NMFS, 2011), that are commer- 
cially fished along the mid-Atlantic coast 
of the United States. Over 14 years, 50 
depletion experiments were carried out 
along the continental shelf off the coasts of 
Massachusetts, Long Island in New York, 
and New Jersey and off the Delmarva 
Peninsula in Delaware, Maryland, and Virginia to deter- 
mine the efficiency of hydraulic dredges used in the fisher- 
ies that target Atlantic surfclams and ocean quahogs. The 
Patch model provides estimates of capture efficiency and 
average density of organisms in the target area by tracking 
the relative reduction in catch for each dredge tow. Theo- 
retically, capture efficiency is a measurable characteristic 
of the gear as well as of the species (Hennen et al., 2012). 
In a field depletion experiment, the dredge is towed over 
the same ground repeatedly while covering the majority of 
a predetermined area. Depletion experiments are con- 
ducted in a rectangular area on average about 10 dredge 
widths wide (23-24 m) and about 400-1000 m long. A 
series of intersecting dredge tows are conducted across the 
selected area, with the path of the dredge tow beginning at 
one short end of the rectangle and finishing at the opposite 
end (Fig. 2). The tow paths are not parallel, rather the 
paths overlap and intersect at various points in the area by 
design in order to meet the requirements of the Patch 
Atlantic Ocean 
Figure 1 
Map of the locations of the 50 depletion experiments that targeted ocean qua- 
hogs (Arctica islandica) (circles) and Atlantic surfclams (Spisula solidissima) 
(squares) off the mid-Atlantic coast of the United States between 1997 and 
2011. White symbols indicate experiments with efficiency estimates under 0.4, 
gray symbols indicate experiments with efficiency estimates between 0.4 and 
0.7, and black symbols indicate experiments with efficiency estimates above 
0.7. Sources for Google Earth base map: data SIO, NOAA, U.S. Navy, NGA, 
GEBCO; image Landsat/Copernicus. 
