Strategic Modelling and Business Dynamics: A Feedback Systems Approach

1.4. Model of a Natural Fishery

I have taken the situation and factors outlined above and used them to create a simple fisheries model (though the scaling I use is different from Fish Banks and there are no competing fishing companies). Figure 1.5 shows the fish population and regeneration. For now there are no ships or fishermen, they appear later. So what you see is a natural fishery, free from human intervention.^[] The fish population or fish stock, shown as a rectangle, accumulates the inflow of new fish per year (here the inflow is defined as births minus deaths). Initially, there are 200 fish in the sea and the maximum fishery size is assumed to be 4000 fish. Incidentally, the initial value and maximum size can be re-scaled to be more realistic without changing the resulting dynamics. For example, a fishery starting with a biomass of 20 thousand tons of a given species and an assumed maximum fishery size of 400 thousand tons would generate equivalent results.

^[] The diagram was created in the popular iThink language (isee systems, 2006). The symbols are pretty much standard for all system dynamics models, though there are differences of detail between the main alternative modelling software packages. Here, in Chapter 1, I briefly explain each symbol when I first refer to it in the text. Later, in Chapter 3, there is a more formal introduction to modelling symbols and equations, with a fully documented example.

Figure 1.6. Net regeneration as a non-linear function of fish ...