Modelling the Effects of Fishing on the Biomass of the World’s Oceans from 1950 to 2006

L. Tremblay-Boyer, D. Gascuel, R. Watson, V. Christensen, D. Pauly, (2011). Marine Ecology Progress Series 442, 169-185.

Marine fisheries have endured for centuries but the last 50 years have seen a drastic increase in their reach and intensity. We 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, transfer efficiency, fisheries catch and trophic level of species, the model was applied on a half-degree spatial 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 with a special emphasis on predator species since they are highly vulnerable to overexploitation. This study highlights 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 specfically, this modelling 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%.