The World Summit on Sustainable Development (Johannesburg, 2002) encouraged
the application of the ecosystem approach by 2010. However, at the same Summit,
the signatory States undertook to restore and exploit their stocks at maximum
sustainable yield (MSY), a concept and practice without ecosystemic dimension,
since MSY is computed species by species, on the basis of a monospecific model.
Acknowledging this gap, we propose a definition of "ecosystem viable yields"
(EVY) as yields compatible i) with guaranteed biological safety levels for all
time and ii) with an ecosystem dynamics. To the difference of MSY, this notion
is not based on equilibrium, but on viability theory, which offers advantages
for robustness. For a generic class of multispecies models with harvesting, we
provide explicit expressions for the EVY. We apply our approach to the
anchovy--hake couple in the Peruvian upwelling ecosystem
