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

Since its development in the early 1980s, the mass-balance approach incorporated in the Ecopath software has been widely used for constructing food-web models of marine and other ecosystems. Generalizations on the structure and functioning of such ecosystems, relevant to the issue of fisheries impacts, have been developed and these have affected the evolution of the Ecopath approach. Thus, the description of the average state of an ecosystem, using Ecopath proper, now serves to parametrize systems of coupled difference and differential equations, which are used to depict changes in biomasses and trophic interactions in time (Ecosim) and space (Ecospace). The outcomes of these simulations can then be used to modify the initial parametrization, and the simulations are rerun until external validation is achieved. This reconceptualization of the Ecopath approach as an iterative process, which helps address issues of structural uncertainty, does not increase its input requirements markedly. Rather, it has become possible, through a Bayesian resampling routine, to explicitly consider the numerical uncertainty associated with these inputs. We present the key features of the reconceptualized approach, and two indices based thereon for quantifying the ecosystem impacts of fisheries. We conclude with a brief discussion of its limitations, both present and intrinsic.