Structuring dynamic models of exploited ecosystems from trophic mass-balance assessments

The linear equations that describe trophic fluxes in mass-balance, equilibrium assessments of ecosystems (such as in the ECOPATH approach) can be re-expressed as differential equations defining trophic interactions as dynamic relationships varying with biomasses and harvest regimes. Time patterns of biomass predicted by these differential equations, and equilibrium system responses under different exploitation regimes, are found by setting the differential equations equal to zero and solving for biomasses at different levels of fishing mortality. Incorporation of our approach as the ECOSIM routine into the well-documented ECOPATH software will enable a wide range of potential users to conduct fisheries policy analyses that explicitly account for ecosystem trophic interactions, without requiring the users to engage in complex modelling or information gathering much beyond that required for ECOPATH. While the ECOSIM predictions can be expected to fail under fishing regimes very different from those leading to the ECOPATH input data, ECOSIM will at least indicate likely directions of biomass change in various trophic groups under incremental experimental policies aimed at improving overall ecosystem management. That is, ECOSIM can be a valuable tool for design of ecosystem-scale adaptive management experiments