The following description is taken from CTC (1988).

The calculation of incidental mortalities associated with size limit restrictions depends critically upon the estimation of the proportion of each stock that is vulnerable (PV) in a particular fishery by age. Available data are not sufficient to permit estimation of stock-specific PVs. Therefore, age-size distributions for large fishing areas were calculated from available data. CWT recoveries turned out to be the best source of this type of age-length data. This is because these data belong to a large (and easily available) data set that can be identified accurately as to age and catch location. A description of the procedure used to estimate the proportion vulnerable by age follows.