We have considered the case when C is constant.
Let us now assume that C is not constant
C therefore varies.
The modelling question is C varies with respect to what?? What affects the fishing rate?
Thus
and
How do we combine these two variables. ? We use our rule that we keep it simple. The simplest forms are: C = k( f+P) or k(f-P) or kfP or kf/P or kP/f
Our new model is