### 4.1.4 Variables and units

The Birth-rate

In practice, the determination of birth-rates may be quite complicated, one would need information on, for example:

1. the ages between which females can lay eggs;
2. the rate of breeding of the fish;
3. the mortality rate of the young fish;
4. the proportion of females in the population;
5. the gestation cycle;

and possibly many other things as well.

The Death-rate

As you might expect, the determination of death-rates will also be quite complicated, one would need information on, for example:

1. the natural life cycle;
2. the death-rate due to predators;
3. the death-rate due to disease;
4. the relative death-rates of males and females in the population as a whole;

and possibly other things as well.

We are now in a position to be able to build our simple model.