ABSTRACT: Exploited ecosystems are characterised by exports of secondary production from each fished trophic level that reduce the energy available for upper levels at the ecosystem scale, thus impinging on overall secondary production. Depletion in secondary production is proposed here as a proxy for quantifying the ecosystem effects of fishing. Theoretical analysis of trophic web interactions permits the quantification of the ‘loss in production’ and provides an ecological basis for defining a new synthetic index (L index) that takes into account both ecosystem properties (primary production and transfer efficiency) and features of fishing activities (trophic level of catches and primary production required). Quantitative meta-analysis of ecological models, representing 51 exploited ecosystems previously classified as overexploited or sustainably exploited, allows the association of a probability of the ecosystem being sustainably fished with each index value. Moreover, by fixing the reference level for this probability, the index provides a basis for estimating the maximum allowable catches. The L index is applied here to several ecosystems worldwide using landings data and ecological models, providing quantification of the disruption of energy flows for ecosystems subjected to different types and levels of fishing pressure. Its application to outputs of calibrated dynamic models enables the evaluation of sustainability of fisheries in past and future scenarios of alternative fisheries management policies. Results highlight the usefulness of this index for quantifying the impacts of fishing and providing directional advice for fisheries management. Thus, the L index may be used to support ecosystem-based management of fisheries.