EcoSim is presently being incorporated in Ecopath with EcoSim, Version 4.0. A testversion of EcoSim can be downloaded from www.fisheries.com. The integrated software will be available as a testversion in February 1997. Please contact v.christensen@cgnet.com for further information. Below are the abstracts of two papers describing EcoSim
Abstract
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.
Abstract (Submitted)
EcpSim II uses results from the Ecopath procedure for trophic mass balance analysis to define biomass dynamics models for predicting temporal change in exploited ecosystems. Key populations can be represented in further detail using delay-difference models to account for both biomass and numbers dynamics. A key problem revealed by linking the population and biomass dynamics models is in representation of population responses to changes in food supply; simple proportional growth and reproductive responses lead to unrealistic predictions of changes in mean body size with changes in fishing mortality. EcoSim II allows users to specify two alternative life history mechanisms to avoid such unrealistic predictions: animals may translate changes in feeding rate into changes in reproductive rather than growth rates, or they may translate changes in food availability into changes in foraging time that in turn affects predation risk. These options, along with model relationships for limits on prey availability caused by predation avoidance tactics, tend to cause strong compensatory responses in modeled populations. It is likely that such compensatory responses are responsible for our inability to find obvious correlations between interacting trophic components in fisheries time series data.