In many reservoirs, fisheries have been overexploited
Courtesy of Global Hydrologic Archive and Analysis System, University of New Hampshire
Worldwide there are about 60 000 large reservoirs (>15-m dam height), totalling
In many reservoirs, fisheries have been pushed to the level reaching overexploitation due to excessive fishing. A study on the status of fish stocks and fisheries of thirteen medium-sized African reservoirs over 300 km3 concluded that adequate data exist in only a few cases to produce annual yield figures or estimates. Deterioration in data collection has also been noticed in the countries of the Commonwealth of Independent States (ex-USSR), where previously long-term data series greatly assisted in the management of fish stocks and fisheries in reservoirs.
Fish resources in reservoirs
Deep reservoirs tend to stratify, especially in the tropics, creating an environment with little or no oxygen along the bottom where fish cannot survive; effectively limiting fish habitats to the near shore region and the open water. Where pelagic species are present these will quickly proliferate and form large populations that can be exploited by fisheries. However, in river systems without permanent lakes, there may be no or only few pelagic species -- and a large part of the fish fauna may not be able to adapt to life in standing water in a reservoir. To compensate for any loss in fish production it is common practice to stock reservoirs with fish species that are better suited for this type of aquatic environment.
Reservoirs in developing countries
For many years FAO has focused on the development and management of reservoir fisheries in developing countries, especially in Africa and Asia. Since their establishment, the four largest reservoirs in Africa (Volta, Kariba, Kainji, Nasser/Nubia) have benefited from attention to fish stocks and fisheries. However, statistical data on catches and landings for these and many other reservoirs in the world are usually unreliable, or unavailable, and must be extrapolated from limited information, such as localized short-term surveys. In the 30 years since completion of the Volta River dam, estimates of actual catch are still inaccurate, showing the need for continual management assistance to achieve sustainability.
The Volta Lake Dam (Ghana)
Courtesy of International Lake Environment Committee Foundation, World Lakes Database
In Asia, reservoirs represent an important inland fishery base in a number of countries. India has close to 20 000 reservoirs, including numerous irrigation tanks >10 ha. The total reservoir area is estimated to be close to 3.2 million hectars. In China, large-scale construction of reservoirs took place in the second half of the twentieth century, resulting in a steep rise in stored water surface area. While this caused an increase in capture fisheries, the growth in culture-based fisheries has been much more spectacular. Almost all of Sri Lanka's inland fisheries are based on fish harvested from reservoirs. Enhancement methods, such as cage culture, contribute a growing share to total fishery production in reservoirs.
In Latin America, Cuba, Mexico and Brazil have large numbers of reservoirs, with those in Brazil exceeding India's in surface area. In Cuba, reservoirs provide the bulk of the country's inland fish production. In some countries, the largest contributors to fish catches from reservoirs are introduced exotic fish species, such as tilapias (Cuba, Sri Lanka) or Chinese carps (some Indian reservoirs). Regular enhancement of fish stocks has improved fish production in some reservoirs but reservoirs generally have a lower productivity than lakes.