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Figure 2 
Distributions of ocean quahogs (Arctica islandica) and Atlantic surfclams (Spisula solidissima) from the analysis 
done with simulation of depletion experiments. Four types of distribution are shown: (A) a biased clam distribution 
with the highest densities in half the area, (B) a distribution with clams in a triangle wedge from southwest to 
northeast across the area, (C) a distribution with clams in even vertical bands, and (D) a clam distribution that is not 
patchy and is relatively uniform. The colored straight lines indicate paths of dredge tows passing through the area. 
Colors denote the amount of overlap (number of hits) of the dredge paths. Black dots indicate individual clams. Data 
used in the simulations are from depletion experiments conducted during 1997—2011 for populations of ocean quahogs 
and Atlantic surfclams off the mid-Atlantic coast of the United States. 
model. The experiment requires the assumptions that all 
catches are random samples and that no organisms are 
transported into or out of the study site during the experi- 
ment (Leslie and Davis, 1939). The catch and the incremen- 
tal dredge positions are recorded for each dredge tow. Over 
the course of the depletion experiment, catch per tow 
decreases; this rate of decline is proportional to the effi- 
ciency of the dredge (Hennen et al., 2012). If the rate of 
decline is steep, the dredge is highly efficient, and if it is 
shallow, the dredge may not be efficient. 
Field depletion experiments can take hours to complete 
and require much effort on the part of scientists and crew 
on the ship. Therefore, it is important to know if experi- 
ments produce reliable efficiency estimates for the gear 
used and to evaluate poor performance that might be 
avoided in future endeavors. Poussard et al. (2021) used 
simulation analysis to determine best practices for deple- 
tion experiments and evaluated which range of experi- 
ment protocols were most likely to provide high-quality 
estimates of capture efficiency. Here, we report results 
of the use of the simulation data to identify the quality 
of field depletion experiments. The depletion experiment 
data set analyzed, obtained from NMFS (Hennen?), is 
unique at the time of this writing. No other data set from 
depletion experiments is this expansive. 
Analyzing data from the NMFS hydraulic dredge 
depletion experiments provides an improved understand- 
ing of the sources of uncertainty in efficiency estimates, 
including the characteristics of experimental protocol 
and environmental factors affecting gear efficiency. In our 
study, first the characteristics of these experiments (e.g., 
dredge size, depth, and geographic region) were summa- 
rized, and then results of simulated depletion experi- 
ments were compared with those of the field depletion 
? Hennen, D. 2019. Personal commun. Northeast Fish. Sci. 
Cent., Natl. Mar. Fish. Serv., NOAA, 166 Water St., Woods 
Hole, MA 02543. 
