Ecopath, Ecosim, and Ecospace as tools for evaluating ecosystem impact of fisheries

Since its development in the early 1980s, the mass-balance approach incorpo rated in the Ecopath software has been widely used for constructing food-we b models of marine and other ecosystems. Generalizations on the structure a nd functioning of such ecosystems, relevant to the issue of fisheries impac ts, have been developed and these have affected the evolution of the Ecopat h approach. Thus, the description of the average state of an ecosystem, usi ng Ecopath proper, now serves to parametrize systems of coupled difference and differential equations, which are used to depict changes in biomasses a nd trophic interactions in time (Ecosim) and space (Ecospace). The outcomes of these simulations can then be used to modify the initial parametrizatio n, and the simulations are rerun until external validation is achieved. Thi s reconceptualization of the Ecopath approach as an iterative process, whic h helps address issues of structural uncertainty, does not increase its inp ut requirements markedly. Rather, it has become possible, through a Bayesia n resampling routine, to explicitly consider the numerical uncertainty asso ciated with these inputs. We present the kev features of the reconceptualiz ed approach, and two indices based thereon for quantifying the ecosystem im pacts of fisheries. We conclude with a brief discussion of its limitations, both present and intrinsic. (C) 2000 International Council for the Explora tion of the Sea.