244 
Fishery Bulletin 108(2) 
Table 5 
Results from the catch-at-size-analysis (CASA) model for Mid-Atlantic Bight sea scallops (Placopecten magellanicus) and four 
model configurations. The “no measurement error” model configuration does not accommodate shell-height measurement errors. 
Other model configurations accommodate bias and imprecise measurement errors in various combinations as shown in the table. 
Lower negative log likelihood (NLL) values indicate better model fit. Coefficients of variation (CV) shown in parenthesis are 
asymptotic variances calculated by the delta method. For ease of comparison, the “no measurement error” configuration NLL 
values were subtracted from corresponding NLL statistics for all three configurations. The lowest NLL, biomass or fishing mor- 
tality estimates in each row are printed in boldface. 
Variable or estimate 
No 
measurement 
error 
Bias only 
Imprecision 
only 
Imprecision 
and 
bias 
Bias and precision (mm) assumed in modeling 
Standard deviation — video survey 
0.0 
0.0 
6.1 
6.1 
Bias — video survey 
0.0 
-4.5 
0.0 
-4.5 
Standard deviation — dredge survey 
0.0 
0.0 
1.7 
1.7 
Bias — survey 
0.0 
-0.6 
0.0 
-0.6 
Negative log likelihood (NLL) 
Total 
0.00 
20.92 
-14.62 
-1.16 
Commercial fishery shell-height data 
0.00 
4.99 
-0.34 
2.06 
Dredge survey shell-height data 
0.00 
-4.14 
-10.66 
-6.97 
Video survey shell-height data 
0.00 
19.45 
-3.00 
4.59 
Mean biomass and fishing mortality during 2004-06 
Fishing mortality (y-1) 
0.45 
0.41 
0.46 
0.42 
(8%) 
(7%) 
(8%) 
(8%) 
Biomass (t meats) 
81,211 
84,650 
80,844 
83,602 
(5%) 
(5%) 
(5%) 
(5%) 
fishing mortality estimates to assumptions about shell- 
height measurement errors. 
In principal, measurement-error parameters could be 
estimated directly in stock assessment models without 
resorting to experiments. Measurement-error param- 
eters in the CASA model were estimated in the NEFSC 
study, 2 but the estimates proved to be unstable (NEF- 
SC 3 ). Without at least one source of accurate body-size 
data, there may be too little information about mea- 
surement errors to estimate parameters. In addition, 
there may be strong correlations between estimated 
measurement errors and estimates of other factors that 
affect interpretation of body-size data, such as survey 
and fishery selectivity, natural mortality, and recruit- 
ment variability. 
Acknowledgements 
We thank F. Serchuk (Northeast Fisheries Science 
Center, Woods Hole, MA), S. Correia (Massachusetts 
Division of Marine Fisheries, New Bedford, MA), C. 
O’Keefe and C. Adams (SMAST, New Bedford, MA), 
and five anonymous reviewers for useful technical and 
editorial suggestions. We are grateful for support from 
the School of Marine Science and Technology, the Mas- 
sachusetts Division of Marine Fisheries, and NOAA 
awards: NA04NMF4720332, NA05NMF4721131, and 
NA06NMF4720097. We are grateful to the crews and 
scientific staff who collected and measured sea scallops 
in NEFSC and SMAST surveys. Live sea scallops used in 
the experiments were provided by commercial sea scallop 
vessels from New Bedford and Fairhaven, MA. 
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