A population model that describes, using simple functions, how the population biomass changes from year to year (or, how biomass changes in equilibrium as a function of fishing mortality). The simplest production functions aggregate all of the biological characteristics of growth, natural mortality and reproduction into a simple, deterministic model using three or four parameters. Production models are primarily used in simpledata situations, where total catch and effort data are available but age-structured information are either unavailable or deemed to be less reliable (although some versions of production models allow the use of age structured data).