where the variables are; "H", referring to catch (harvest) over a given period of time (e.g. a year); "E", the fishing effort over the given period; "X", the fish stock biomass at the beginning of the period (or the average biomass), and the parameter "q" represents the catchability of the stock. Assuming the catch to equal the net natural growth in the population over the same period (formula_2), the equilibrium catch is a function of the long term fishing effort "E":