Harvesting impacts on Antarctic marine ecosystems: changes in food web structure and loss of ecosystem value

International audienceEcosystems are composed of the physical environment and the species that live in them (the abiotic and biotic components of an ecosystem, respectively). Today there is great concern for the effects of climate change (change in the abiotic component) on Antarctic marine ecosystems, while relatively little attention is being given to the effects of harvesting of living resources (change in the biotic component). However, direct impacts on the biotic components of an ecosystem have the potential to affect the structure of the food web, including creating possible cascades that affect biogeochemistry, over time scales that are faster than those resulting from physical changes. Since the late 1800s, commercial removal of seals, whales and finfish changed food web structure in waters of the Antarctic Peninsula and insular shelves, e.g., that of the Scotia Sea, before baseline scientific data on these ecosystems were acquired and before the first signs of global warming became evident. Until 1997, the Ross Sea marine ecosystem remained relatively unaffected by human extraction, so that its food webs are arguably the closest worldwide to those that existed in a 'pristine' ocean prior to human harvesting. The consequences of harvesting are difficult to measure, but throughout the Southern Ocean important changes in ecosystem structure due to excessive exploitation have been observed. These changes include the collapse or diminishment of marine mammal and fish stocks and the reduction in average individual size of fish from targeted populations. Several conceptual models have been developed to explain these changes and/or to predict possible consequences (reduced population viability; trophic cascades; reduced carrying capacity). Robust predictions about the consequences of continuing the business as usual approach to harvesting Southern Ocean resources can be made based on comparisons with exploited marine ecosystems elsewhere. Current Ross Sea harvesting rates have the potential to reduce fish stocks below a level at which they may be self-sustaining, foment trophic cascades and alter food web structure. These changes will be accompanied by loss of ecosystem and commercial values, with those metrics related to conservation values still poorly defined and not taken into account in the management decision process