Fish Population Dynamic and Stock Assessment

4.1.2. Types of Mortality:

Natural mortality: the removal of fish from the stock due to causes not associated with fishing. Such causes can include disease, competition, cannibalism, old age, predation, pollution or any other natural factor that causes the death of fish. In fisheries population dynamics, natural mortality is denoted by (M).

Fishing mortality: the removal of fish from the stock due to fishing activities using any fishing gear. It is denoted by (F) in fisheries models.

Total Instantaneous Mortality

The removal of fish from the stock due to natural causes (M) and fishing activities (Z).

(M) and (F) are additive instantaneous rates that sum up to (Z), the instantaneous total mortality coefficient; that is, Z=M+F. These rates are usually calculated on an annual basis.