Mortality includes all factors reducing abundance of a closed population. In fish models, mortality is typically modeled as an instantaneous rate Z. Thus, abundance decreases by a constant fraction in each instant of time:
The solution to eqn  at time t is
given initial abundance N0 at t = 0. The instantaneous mortality rate Z carries units inverse to those of t; for example, when t is expressed in years, Z has units yr—1 The annual proportion dying is 1 — exp(—Z).
If the population is structured by age, eqns  and  apply to each portion of the population, so that
where Na,t is abundance at age and time, and Zat is annual mortality rate at age.
The total mortality rate Z (age-specific or otherwise) is often partitioned into components, usually into natural mortality rate M and fishing mortality rate F, assumed noncompensatory. A property of instantaneous rates is that mortality from various sources is additive:
This allows the fishing mortality rate, a primary focus of fishery management, to be considered separately from other sources of mortality.
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