| Summary: | Marine fisheries have been occurring for centuries but the last 50 years have seen a drastic increase in their reach and intensity. Fisheries now impact marine ecosystems worldwide and it is necessary to understand their impact, both historical and current, at this scale. A global perspective provides an efficient inter-disciplinary communication tool and allows to summarize and validate our current understanding of ecosystem functioning. The objectives of this thesis are to generate global estimates of biomass for marine ecosystems and evaluate the effects that fisheries have had on ocean biomass since the 1950s. A simple but versatile ecosystem model was used to represent ecosystems as a function of energy fluxes through trophic levels. Using primary production data, sea surface temperature, fisheries catch and trophic level of species, the model was applied on a half-degree grid covering all oceans. Estimates of biomass by trophic levels were derived for marine ecosystems in an unexploited state, as well as for all decades since the 1950s. Trends in the decline of marine biomass from the unexploited state were analyzed for all oceans, with a special emphasis on predator species since they are highly vulnerable to fishing. This thesis is the first application of a trophic modelling approach to a worldwide estimation of the effects of fishing. It provides an independent confirmation of previous reports by other researchers that were based on proxies of biomass or on meta-analyses of local datasets. The results presented highlight three main trends about the global effects of fishing: (1) predators are more affected than organisms at lower trophic levels; (2) declines in ecosystem biomass are stronger along coastlines than in the High Seas; (3) the extent of fishing and its impacts have expanded from north temperate to equatorial and southern waters in the last 50 years. More specifically, this work shows that many oceans historically exploited by humans have seen a drastic decline in their predator biomass, with about half of the coastal areas of the North Atlantic and North Pacific showing a decline in predator biomass of more than 90%. The spatial and temporal trends presented in this work provide a global synthesis of the effects of fishing on the biomass of marine ecosystems and point to a potential state of the world’s oceans if industrial fisheries maintain their current trajectory.
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