Total Mortality (Z)

Total mortality Z is the instantaneous rate at which fish die from all causes, the sum of natural mortality and fishing mortality: Z = M + F. It is estimated from the decline of a cohort across ages or from a catch curve, where the slope of log abundance against age gives Z directly. Splitting Z into M and F is central to assessment because F is the part managers control: with M assumed from life-history relationships, F follows as F = Z minus M. Annual survival is exp(minus Z), so a Z of 0.7 leaves about half the cohort alive each year.