Here we continue with the exposition of living resources in Quadrants 3 and 4. In the previous chapter, we introduced a single biomass scalar B and described its bioeconomic interactions. The natural extension of this is to distinguish different life stages both biologically and economically. The biological description will be in terms of numbers of individuals per stage. The size, weight, etc. of each stage is treated as a biological fact.

We will carry forward the same approach to harvesting, but consider the realistic possibility of directing effort toward certain life stages only. In addition to the life stage realism, this reveals the important opportunity to describe recruitment – that is, the number of individuals born into the population. There are two limiting extremes: (1) where recruitment depends on the number of mature individuals; and (2) where recruitment is effectively controlled by other things. Needless to say, there is a continuum, and one needs to know which regime is controlling and to manage accordingly.

The case of fish populations and management continues to be used to illustrate ideas.

POPULATION STRUCTURE

In the previous chapter, we examined the simple model with a single biological state – the scalar biomass B. Here we expand the description to include natural systems with greater biological complexity stemming from growth and development of specific organisms through different natural life stages. Hence, stage-structured descriptions.