STRUCTURING DYNAMIC-MODELS OF EXPLOITED ECOSYSTEMS FROM TROPHIC MASS-BALANCE ASSESSMENTS

The linear equations that describe trophic fluxes in mass-balance, equ ilibrium assessments of ecosystems (such as in the ECOPATH approach) c an be re-expressed as differential equations defining trophic interact ions as dynamic relationships varying with biomasses and harvest regim es. Time patterns of biomass predicted by these differential equations , and equilibrium system responses under different exploitation regime s, are found by setting the differential equations equal to zero and s olving for biomasses at different levels of fishing mortality. Incorpo ration of our approach as the ECOSIM routine into the well-documented ECOPATH software will enable a wide range of potential users to conduc t fisheries policy analyses that explicitly account for ecosystem trop hic interactions, without requiring the users to engage in complex mod elling or information gathering much beyond that required for ECOPATH. While the ECOSIM predictions can be expected to fail under fishing re gimes very different from those leading, to the ECOPATH input data, EC OSIM will at least indicate likely directions of biomass change in var ious trophic groups under incremental experimental policies aimed at i mproving overall ecosystem management. That is, ECOSIM can be a valuab le tool for design of ecosystem-scale adaptive management experiments.