274 
National Marine 
Fisheries Service 
NOAA 
OF on Ve 
Fishery Bulletin 
@ established in 1881 ~~ 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Abstract—Between 1997 and 2011, 
the National Marine Fisheries Service 
conducted 50 depletion experiments to 
examine efficiency of survey gear for 
capturing clam species and to estimate 
stock density for populations of Atlantic 
surfclams (Spisula solidissima) and 
ocean quahogs (Arctica islandica) by 
using commercial hydraulic dredges. 
The Patch model was formulated to 
estimate gear efficiency and organism 
density from the depletion experiment 
data. The range of efficiencies estimated 
is substantial, leading to uncertainty 
in the application of these estimates in 
stock assessment. Known values of 4 
measures of experimental performance 
for each of the 50 depletion experiments 
conducted in the field were compared 
with values of those same character- 
istics from 9000 simulated depletion 
experiments, values that were assumed 
to represent a suite of conditions that 
might also occur in the corresponding 
field experiment. These comparisons 
allow analyses of the quality of field 
experiments that would otherwise not 
be possible and provide evidence for 
weighting the results of field exper- 
iments beyond traditional measures 
of uncertainty. The characteristics of 
performance were used to identify a 
subset of field experiments that were 
more likely to have produced inaccurate 
estimates of gear efficiency, potentially 
introducing bias and, as a result, lower- 
ing the efficiency estimates in the entire 
depletion data set. 
Manuscript submitted 31 March 2021. 
Manuscript accepted 4 November 2021. 
Fish. Bull. 119:274-293 (2021). 
Online publication date: 24 November 2021. 
doi: 10.7755/FB.119.4.7 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Discriminating between high- and 
low-quality field depletion experiments 
through simulation analysis 
Leanne M. Poussard (contact author)! 
Eric N. Powell' 
Daniel R. Hennen? 
Email address for contact author: leanne1poussard@gmail.com 
" Gulf Coast Research Laboratory 
University of Southern Mississippi 
703 East Beach Drive 
Ocean Springs, Mississippi 39564 
? Northeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
166 Water Street 
Woods Hole, Massachusetts 02543 
The implementation of a definitive 
measure of dredge efficiency for 
analyzing shellfish survey data sub- 
stantially improves the estimation 
of abundance. Commonly, depletion 
experiments are used to estimate 
gear efficiency and population den- 
sity for sessile and sedentary species 
in a target area (Leslie and Davis, 
1939; Skalski et al., 1983; Lasta and 
Iribarne, 1997; Gedamke et al., 2005; 
Wilberg et al., 2013). These experi- 
ments are conducted by dredging over 
the same area and tracking the decline 
in catch of a target organism per 
dredge tow. Other dredge efficiency 
quantification methods, such as use 
of an underwater camera in between 
dredge tows, have also been used (Chai 
et al., 1992; Ragnarsson and Thora- 
rinsdottir, 2002; Morson et al., 2018). 
For a range of dry dredges, estimates 
exist for their efficiency for catching 
epibenthic animals, including oyster 
(Powell et al., 2007; Morson et al., 
2018), clam (Pezzuto et al., 2010), 
crab (Vglstad et al., 2000; Wilberg 
et al., 2013), and scallop (Lasta and 
Iribarne, 1997; Beukers-Stewart and 
Beukers-Stewart’) species. By compar- 
ison, highly efficient hydraulic dredges 
are the primary gear type used to cap- 
ture infaunal clam species (Da Ros et al., 
2003; Hauton et al., 2007; Moschino 
et al., 2003; Gilkinson et al., 2005; 
Meseck et al., 2014). Hydraulic dredges 
increase the catchability of the target 
organism by using water pressure to liq- 
uefy the sediment (Da Ros et al., 2003; 
Gilkinson et al., 2003; Hauton et al., 
2007; Meseck et al., 2014). 
The ocean quahog (Arctica islandica) 
and the Atlantic surfclam (Spisula 
solidissima) support substantial fisher- 
ies that operate on the continental shelf 
of the northeastern United States and 
are captured exclusively by using 
hydraulic dredges because of the depths 
at which they are found. A typical 
hydraulic dredge is a large rectangular 
1 Beukers-Stewart, B. D., and J. S. Beukers- 
Stewart. 2009. Principles for the manage- 
ment of inshore scallop fisheries around 
the United Kingdom. Report to Natural 
England, Scottish Natural Heritage and 
Countryside Council for Wales. Univ. York, 
Mar. Ecosyst. Manage. Rep. 1, 58 p. [Avail- 
able from website.] 
