At least two approaches have been used for population modelling in fisheries: macro
and micro approaches (Pethig & Tschirhart 2002; Eichner & Pethig 2006). According to
Pethig and Tschirhart , the macro approach (aggregate biomass models) uses populations
as basic units of analysis. Species are presented as (differential) equations containing their
own populations and the populations of other species as variables such as prey and predators (May et al. 1979; Flaaten 1988, 1990).