233 
Abstract — Body-size measurement 
errors are usually ignored in stock 
assessments, but may be important 
when body-size data (e.g., from visual 
surveys) are imprecise. We used 
experiments and models to quantify 
measurement errors and their effects 
on assessment models for sea scallops 
( Placopecten magellanieus). Errors in 
size data obscured modes from strong 
year classes and increased frequency 
and size of the largest and smallest 
sizes, potentially biasing growth, mor- 
tality, and biomass estimates. Model- 
ing techniques for errors in age data 
proved useful for errors in size data. 
In terms of a goodness of model fit 
to the assessment data, it was more 
important to accommodate variance 
than bias. Models that accommodated 
size errors fitted size data substan- 
tially better. We recommend experi- 
mental quantification of errors along 
with a modeling approach that accom- 
modates measurement errors because 
a direct algebraic approach was not 
robust and because error parameters 
were difficult to estimate in our 
assessment model. The importance 
of measurement errors depends on 
many factors and should be evaluated 
on a case by case basis. 
Manuscript submitted 22 June 2009. 
Manuscript accepted 25 January 2010. 
Fish. Bull. 108:233-247 (2010). 
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. 
Measurement errors in body size 
of sea scallops ( Placopecten magellanieus ) 
and their effect on stock assessment models 
Larry D. Jacobson (contact author ) 1 Deborah Hart 1 
Kevin D. E. Stokesbury 2 
Melissa A. Allard 2 
Antonie Chute 1 
Bradley P. Harris 2 
Tom Jaffarian 2 
Michael C. Marino II 2 
Jacob I. Nogueira 2 
Paul Rago 1 
Email address for contact author: Larry.Jacobson@noaa.gov 
1 NOAA Fisheries 
Northeast Fisheries Science Center 
166 Water Street 
Woods Hole, Massachusetts 02543-1026 
2 Department of Fisheries Oceanography 
School for Marine Science and Technology 
University of Massachusetts School of Marine Sciences 
838 South Rodney French Boulevard 
New Bedford, Massachusetts 02744-1221 
Two fishery-independent surveys are 
important for monitoring Atlantic 
sea scallop (Placopecten magellani- 
cus) abundance and biomass levels 
off the northeastern coast of the 
United States because they provide 
abundance, body size, 1 meat weight 
(weight of marketable adductor mus- 
cles), and other data (NEFSC 2 ’ 3 ). The 
National Marine Fisheries Service, 
Northeast Fisheries Science Center 
(NEFSC) sea scallop dredge survey 
has been conducted annually since 
1977 (Serchuk et ah, 1979; Serchuk 
and Wigley, 1986). In addition, an 
underwater video survey for sea scal- 
lops and other benthic organisms has 
been conducted annually since 2003 
(Stokesbury, 2002; Stokesbury et al., 
2004) by the University of Massachu- 
setts Dartmouth, School for Marine 
Science and Technology (SMAST). The 
dredge and video surveys are carried 
out across the range of sea scallops in 
U.S. waters. 
In this analysis, we used sea scal- 
lops to draw attention to errors in 
body-size data when the data are 
used in a length-structured stock 
assessment model. The topic of mea- 
surement errors in body-size data 
has received relatively little atten- 
tion, although Heery and Berkson 
(2009) evaluated effects of systematic 
errors (biased sampling) in fishery 
size-composition data used in an age- 
structured model. Our work was mo- 
tivated by questions that arose from 
examining video survey shell-height 
data in sea scallop stock assessments 
(NEFSC 2 ’ 3 ). Our experimental and 
analytical results may be important 
and useful in other situations where 
body-size data are imprecise. Body- 
1 Shell height (SH, the distance in mm 
between the umbo and shell margin) 
is the body size measurement for sea 
scallops. 
2 NEFSC (Northeast Fisheries Science 
Center). 2004. Stock assessment for 
Atlantic sea scallops. In 39th north- 
east regional stock assessment workshop 
(39th SAW) assessment summary report 
and assessment report. Northeast Fish- 
eries Science Center, National Marine 
Fisheries Service, Woods Hole Labora- 
tory, 166 Water St., Woods Hole, MA 
02543. Ref. Doc. 04-10, p. 87-211. 
3 NEFSC (Northeast Fisheries Science 
Center). 2007. Stock assessment for 
Atlantic sea scallops. In 45th northeast 
regional stock assessment workshop (45th 
SAW) assessment summary report and 
assessment report. Northeast Fisheries 
Science Center, National Marine Fisher- 
ies Service, Woods Hole Laboratory, 166 
Water St., Woods Hole, MA 02543. Ref. 
Doc. 07-16, p. 139-370. 
