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Appendix 1 
Following the approach of the Northeast Fisheries Sci- 
ence Center (NEFSC, 2>3 ) we used a likelihood approach 
to fitting the CASA model to sea scallop stock assessment 
data. The best estimates from the model minimized the 
combined negative log likelihood of all the data. Relevant 
details are described below. Appendix B10 in the NEFSC 
report (NEFSC 3 ) is a complete technical description of 
the CASA model for sea scallops. Appendix B12 in that 
same report (NEFSC 3 ) describes CASA model perfor- 
mance with simulated stock assessment data. 
Estimates of population abundance and survey size 
selectivity are available for each shell height and year 
as the CASA model is fitted. In a single year, for ex- 
ample, we calculated the number of sea scallops in the 
population that were available or selected by the video 
gear with the following equation: 
n h=Qh N h > (A1 > 
where N h = the predicted number of sea scallops in the 
population for shell height bin h; 
q h = the size-specific probability of detection 
(selectivity) in the video survey (on a scale 
of 0 to 1 and relative to the bin with maxi- 
mum probability of detection); and 
n h = the estimated number of sea scallops in the 
population that are available to the video 
survey gear. 
In the absence of measurement error, the predicted shell- 
height composition ji h for the survey is 
1=1 
where L = the number of shell-height bins in the model. 
If if is a row vector of length L containing the predicted 
proportions (before measurement errors) for each length 
group in the survey, then 
p = nE, (A3) 
where p the row vector of predicted proportions (includ- 
ing measurement errors). 
