Ecosystems are subject to increasing anthropogenic pressures worldwide. Assessing cumulative effects of multiple pressures and their impacts on recovery processes is a daunting scientific and technical challenge due to systems' complexity. However, this is of paramount importance in the context of ecosystem-based management of natural systems.

Our study provides major insights into the assessment of cumulative effects on Northeast Atlantic ecosystems. Using an Ecopath with Ecosim (EwE) tropho-dynamic model for the Celtic Sea ecosystem including 53 functional groups, we (1) assess individual and cumulative effects of fishing and climate change and (2) explore the impact of fishing intensity and climate change on ecosystem resilience. Various levels of increasing fishing intensities are simulated over the whole 21st century, by forcing the EwE model with time series of sea temperature, primary production and secondary producer's biomass from the regional POLCOMS-ERSEM climate model, under both RCP4.5 and RCP8.5 scenarios. Cumulative impacts on the ecosystem's health and its capacity to recover after the cessation of fishing activities were assessed through a set of 45 indicators (biomass-based, diversity, trait-based and habitat-based indicators), using a theoretical non-fishing and climate-constant scenario as a reference.

Our results reveal climate change impacts on Boreal, pelagic species and on ecosystem stability. Fishing preferentially removes apex predators and is predicted to increase the likelihood of a regime shift by decreasing ecosystems' capacity to recover. Predicted cumulative effects are mainly additive and antagonistic but synergies are observed for high fishing effort levels, and finally climate change had minor impacts on ecosystem recovery to fishing. Fishing is shown to be the main driver of cumulative impacts and of ecosystem resilience over the next decades. Our results suggest that slight reduction in fishing effort is enough to compensate the impact of climate change. Future research should then be directed towards exploring and evaluating ecosystem-based climate-adaptive fisheries management strategies.