ABSTRACT
Formal fishery stock assessments are generally based upon mathematical models of population production processes and of the population dynamics of the stock being fished. Model design, or model selection, is complex because the decisions made when putting a model together will depend on what is already known and the use to which the model is to be put. A model can be physical, verbal, graphical, or mathematical, however, the particular form chosen for a model imposes limits on what it can describe. There are many types of mathematical models. They can be characterized as descriptive, explanatory, realistic, idealistic, general, or particular; they can also be deterministic, stochastic, continuous, and discrete. Sometimes they can be combinations of some or all of these things. Whether a model is discrete or continuous, and deterministic or stochastic, is a matter of model structure and clearly influences what can be modelled.
