Poussard et al.: Discriminating between high- and low-quality field depletion experiments 
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Dimension 1 
Figure 8 
Correspondence analysis for dimensions 1 and 3 for the data set from deple- 
tion experiments conducted during 1997-2011 to examine efficiency of 
hydraulic dredges for capturing clam species and to estimate stock density for 
populations of ocean quahogs (Arctica islandica) and Atlantic surfclams (Spi- 
sula solidissima) off the mid-Atlantic coast of the United States. Error terms 
Err1 (R1), Err2 (R2), Err3 (R38), and Err4 (R4) are supplementary variables. 
Estimates of the following characteristics are entered as quartiles: dredge 
efficiency (E); clam density (D); the k parameter (K), which is the negative 
binomial dispersion parameter; coefficient of variation (CV) of efficiency (C); 
CV of density (N); CV of the 2 parameter (P); effective area swept (S); overlap 
score (T); latitude (L); and depth (Z); only quartiles 1 and 4 are shown in the 
plot. Other variables that describe the experiments include species, the ocean 
quahog (O) and Atlantic surfclam (S); region, Long Island (LI) in New York, 
New Jersey (NJ), and the Delmarva Peninsula (DMV) of Delaware, Maryland, 
and Virginia; and dredge width. Dredge widths are 2.54, 3.30, and 3.81 m. 
Error estimates are entered as 1 (below the 80th percentile) or 2 (at or above 
the 80th percentile). The inner box demarcates the area with loading factors 
from —0.5 to 0.5 on both axes. 
surfclams that took place farther south 
(Fig. 1). The relationship is not associ- 
ated with dredge width, although effi- 
ciency and dredge width are significantly 
correlated for experiments with ocean 
quahogs (Fig. 4). These experiments took 
place in deeper water, on the average, but 
correlation and correspondence analysis 
agree that depth, per se, does not influ- 
ence outcomes. Edaphic factors may be 
examined as proxies for the influence of 
region, but little information is available 
to make a determination. 
Depth might be considered to be an 
effective variable that determines the 
success of a depletion experiment for 
hydraulic dredges because these dredges 
are operated by using an onboard water 
pump attached to the dredge by means 
of a large hose. The vessel is less maneu- 
verable in deeper water because of the 
increased amount of hose required to 
maintain an adequate scope while dredg- 
ing. Surprisingly, neither correlation 
analysis nor correspondence analysis 
offer any evidence for a significant cor- 
relation between depth and experimen- 
tal performance or the final efficiency 
estimate. Depth-related variables, in 
fact, fall orthogonally to experiment per- 
formance characteristics and estimates 
of the error terms Err2, Err3, and Err4 
in correspondence analysis. 
Conclusions 
When the 16 experiments (7 exper- 
iments with Atlantic surfclams and 
9 experiments with ocean quahogs) that 
had estimates of error terms Err2, Err3 
and Err4 that fall at or above the 80th 
percentile were removed from the field 
depletion data set, the mean efficiency 
largest dredge were conducted in later years, when the 
depletion experiment method was more consistent among 
experiments and produced higher efficiency estimates, 
yielding higher OS values. Accordingly, the improved per- 
formance cannot unequivocally be assigned to the largest 
size of the dredge used. 
Location of the depletion experiment might also have 
affected the efficiency estimate. Depletion experiments 
that targeted ocean quahogs off Long Island have higher 
efficiency estimates than experiments conducted farther 
south. The relationship is shown objectively (Fig. 1) and 
in correlation (Fig. 4). Results of the correlation analysis 
do not indicate a significant relationship between latitude 
and the efficiency estimate, but this finding accrues from 
the inclusion of high-efficiency experiments with Atlantic 
estimate increased from 0.635 to 0.694 for experiments 
that targeted Atlantic surfclams (Table 10). The median 
likewise rose substantially from 0.590 to 0.647 and 
the interquartile range, although remaining relatively 
unchanged in dimension, shifted to higher efficiency val- 
ues. The mean efficiency estimate for experiments that 
targeted ocean quahogs increased from 0.586 to 0.711, 
and the median rose from 0.629 to 0.667. The inter- 
quartile range was substantially reduced in dimension 
and shifted to higher efficiency values. The efficiency 
estimates for the data set after removal of experiments 
flagged by an error term indicate that experiments 
flagged by Err1 do not have efficiency estimates that are 
biased in either direction and do not meaningfully negate 
the trends established with the other 3 error terms. 
