Two measures of overall fishing mortality

In age-structured models fishing mortality is usually described by the vector of F-at-age. There is a need for single number (scalar) measures of the overall fishing mor-tality, in order to (1) express the overall effect of exploitation suffered by the popula-tion, and (2) for comparison and correlation with measures of fishing effort. These two purposes are incompatible, since the 'effect of exploitation ' measure should change when the exploitation pattern is changed systematically (as for example by a mesh change), whereas the effort ' measure should not. It is suggested that the ratio of catch (in number)-per-recruit to population (in number)-per-recruit is a suitable measure of the effect of exploitation, whilst average fishing mortality weighted by catch-at-age (on a per-recruit basis) is a suitable meas-ure of F on the exploited phase, for correlation with effort. Since both measures are calculated on a cohort (per-recruit) basis, both are independent of fluctuations of year-class strength, which corrupt the usual average of F weighted by actual popula-tion number