Mortality (Natural M)

Natural mortality M is the instantaneous rate at which fish die from causes other than fishing: predation, disease, starvation, senescence, and spawning stress. It is expressed per year as the M in the total-mortality decomposition Z = M + F, and it is one of the hardest parameters to estimate directly. Assessments often borrow M from empirical relationships with longevity, growth rate, or temperature, such as a value near M = 4.3 divided by maximum age, or from tagging and predation studies. M scales heavily with size, so juvenile M typically exceeds adult M. A biased M propagates straight into estimated stock size and reference points.