Ecopath is mass-balance modeling approach that is widely used for incorporating ecosystem considerations into fisheriesscience. Up to now, users of Ecopath software who are constructing a model of a given area must carefully adjust input biomass,diets, and other parameters until the Ecopath parameterization is mass-balanced, a slow process leading to non-unique solutions.We present a new computer-automated iterative technique for mass-balancing Ecopath models which has the advantages of (1)reducing the lengthy process of and opportunity for encoding errors of the manual approach; (2) standardizing results for the sameset of starting conditions; and (3) allowing exploration of alternative solutions, with consideration of the estimated confidenceof each input parameter. Users can select random and/or gradient descent model perturbation of biomass and/or diet parameters,specify an objective (cost) function for optimization of the search, and modify decision logic, including simulated annealing. Anobjective function is defined to help target mass-balance solutions with minimum change to original input parameters. A MonteCarlo mode allows exploration of sensitivity to different starting conditions and random perturbations. The new procedure isimplemented in the current version of the freely available Ecopath with Ecosim software (http:// www.ecopath.org).
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