Aquatic ecosystems provide resources, food and livelihoods every day; they perform many important environmental functions, contribute to general human well-being and are exploited by both capture fisheries and aquaculture sectors. Achieving sustainable use of aquatic ecosystems has been the main and largely failed objective of fisheries management for decades. Nontheless, the formally adopted Ecosystem Approach to Fisheries (EAF) imposes further efforts in that direction, implying better understanding and better governance.

Marine fisheries resources exist as part of marine and coastal ecosystems, that is, as part of functioning, interacting aquatic systems composed of living organisms around a grain of sand and their environments. Conceptually the marine ecosystems can vary greatly in size, from encompassing the whole world ocean as a loosely integrated system, to a microscopic colony of organisms and its immediate surroundings. However, the more usual and useful use of the concept for fisheries relates to some intermediate ecosystem size range, corresponding to the size of the resources sought and of social structures in charge of their stewardship.

Ecosystem-based fisheries management could be considered at the level of a fishery (with its habitats, target, dependent and associated species), an estuary, a large bay, a coastal zone, an exclusive economic zone or a Large Marine Ecosystem (LME). In each case, adjustments will be needed to account for the possible mismatch between the limits of the human jurisdictions and those of the species distributions and ecological processes.

The exploited ecosystem is unavoidably affected by fishery activities. Wild fishery stocks and other organisms found within the ecosystem affect each other e.g. through predator-prey relationships or transfer of diseases. The same may happen, to a lesser extent perhaps, for species cultured in an extensive mode (e.g. through ranching). The impact of fishery activities, either in raising or catching, on the resource itself as well as on the associated and dependent species and the habitat. Impact by mobile gear on the bottom and the benthic flora and fauna can be particularly severe and longlasting.

Technological progress may improve productivity and reduce impact. Engineering imported from offshore oil rig construction, increase the possibilities for offshore aquaculture using robust cages. Sea ranching, the release of young fish into the wild to improve the harvest in capture fisheries, has also made a start but its long term viability is still to be assessed. Major advances are also being made in the technology of the production of aquafeeds, which generally require the combining of a large number of ingredients into very small feed pellets. All of these need to be considered together with their ecosystem impacts.

The ecosystems in which fisheries and aquaculture operate are also vulnerable to the effects of other industries, ocean- or land-based which pollute the marine ecosystem and induce environmental degradations which may be very longlasting or irreversible, with significant effects on the production of fisheries, the quality of seafood and fishers' livelihoods.

Natural variability is a feature of marine ecosystems and has very significant implications for the productivity of fish stocks and management of fisheries and aquaculture (e.g. El Nino). Environmental events (such as typhoons, earthquakes and tsunamis) may have significant impacts on exploited organisms and industrial infrastructures. They may result in environmental emergencies. Understanding and predicting them - and taking them into account in management - is a significant challenge of the next decade.