Ecosystems, on which fisheries depend, generally range from coastal areas to the open ocean, from tropical to polar oceans, including some semi-enclosed or enclosed seas. They include the lower end of river watersheds and their plumes, bays, estuaries and lagoons, coral and other reefs, continental shelves and slopes, and upwelling areas.
Large Marine Ecosystems (LMEs) are relatively large regions (200 000 km2 or more) of the oceans characterized by distinct bathymetry, hydrography, productivity and trophically dependent populations.
Neither pristine nor exploited ecosystems are static, but are subject to environmental variations. The frequency of these environmental changes and their amplitude vary widely. The greater and more frequent they are, the more significant the implications for productivity and for the management of fisheries. Natural variability adds to the uncertainty that should be considered in managing the harvesting of the resources of an ecosystem. A system's resilience to human impacts and their capacity to recover from serious disturbances varies with such natural cycles.
Of particular significance is the El Niño Southern Oscillation (ENSO) phenomenon, a naturally occurring global climate cycle involving complex interactions between the surface of the ocean and the atmosphere in the tropical Pacific. El Niño (warm) and La Niña (cold) events are extreme phases on the ENSO cycle. It has been increasingly accepted that the ENSO phenomenon has a global impact.
Fifty LMEs have now been identified extending across regions encompassing coastal areas from river basins and estuaries to the seaward boundary of continental shelves and the seaward margins of coastal current systems.