EconMult is adiscrete timesimulation model where the fleet activity is controlled by twovariables:Number of vessels (v) (within each fleet segment) andNumber of fishing daysd) (within each time interval).The biomass units (x) are exogenous variables. The model resolution is determined by four structural variables:Number of fleet groupsj),Number of targeted speciesn),Number of biomass unitsi) (which may be more than one within each targeted species) andPeriod length (time step in the simulation). The number of vessels and fishing days therefore are presented in fleet (columns)-targeted species (rows) matrices, while the biomass units is presented in a column vector (X):
Afishery is in EconMult defined as a unique fleet/targeted species combination. The total catch within each fishery may include all biomass units defined in the model. Each biomass unit vessel catch is computed byCobb–Douglasproduction function, applying two input variables:number of fishing daysd) andbiomass unitx). In the matrix below each column represents a fleet group and each row a targeted species so each element in the matrix is a fishery and gives the vessel catch of the biomass units represented. The biomass units represent all the targeted species. Each catch is represented a Cobb–Douglas production equation as shown in the vessel yield matrix (Y):
The corresponding fleet catches are
αβ andq are parameters, the first two known as output elasticities of effort and biomass respectively,q is often referred to asthe catchability coefficient. All the three parameters have the same dimension as the matrix above, e.g. the catchability coefficient:
