Fish Population Dynamic and Stock Assessment

4.1.4. Estimation of total instantaneous mortality (Z)

Mortality parameters are extremely difficult to estimate precisely in aquatic animals. This is because of mortality rate, the rate at which the animals are removed from the population by death, is a parameter of population and not of individuals. To estimate mortality rate, it is therefore necessary to examine a group of aquatic animals.

Fishery scientists divide the instantaneous total mortality rate (Z) of a population into two parameters i.e., instantaneous fishing mortality (F) and instantaneous natural mortality (M). An advantage of total instantaneous mortality is that they can be added or subtracted.

Thus, we have

Z = F + M

Where, Z is total instantaneous mortality coefficient

F is instantaneous fishing mortality coefficient and

M is instantaneous natural mortality coefficient

In unexploited stock, F=0 then, Z=M. This means total mortality will have the same value when there is no fishing mortality.