The parameters used for each stock are for the fished population. The fished population is defined as those age groups making a significant contribution to the fishery.
An age group was judged to be making a significant contribution if it was at least 10% recruited to the fishery. Thus, the
estimates relate to that part of the stock that is available to the fishery.
In most cases this equates to fish which are age 3 or age 4 and older.
In the case of fishing mortality (F), this approach averages the F experienced by young fish which are only partially vulnerable to the gear with the F on older fish which are fully vulnerable, i.e. fully-recruited to the fishery.
A weighted averaging method is used, i.e. the F at each age is multiplied by the numbers of fish at each age before summing over age groups and dividing by the total number of fish in the fished population.
Thus, variation in the number of recruitshas an important influence on the weighted average F.
Nonetheless, this is a good F to use for comparing trends over long time series of data or among stocks because it takes into account all the differences resulting from changes or differences in fishing gears or fishing behaviours.