Dams reduce the catch of migratory fish by preventing them from reaching their spawning grounds, vast reservoirs reduce the flow and level of rivers, increase the salinity of many rivers, trap the silt which contains nutrients vital to the survival of fisheries in the lower reaches of the river and in the sea, help the invasion of aquatic weeds which reduce the biological productivity of the reservoir and the number of micro-organisms on which fish can feed. When a reservoir is filled, there is a rise in the population of fish species favoured by the new lacustrinine conditions, but it is followed by failure. Besides, the river population decreases substantially, and some species disappear completely.
The Encyclopedia of World Problems and Human Potential is a collaboration between UIA and Mankind 2000, started in 1972. It is the result of an ambitious effort to collect and present information on the problems with which humanity is confronted, as well as the challenges such problems pose to concept formation, values and development strategies. Problems included are those identified in international periodicals but especially in the documents of some 60,000 international non-profit organizations, profiled in the Yearbook of International Organizations.
The Encyclopedia includes problems which such groups choose to perceive and act upon, whether or not their existence is denied by others claiming greater expertise. Indeed such claims and counter-claims figure in many of the problem descriptions in order to reflect the often paralyzing dynamics of international debate. In the light of the interdependence demonstrated among world problems in every sector, emphasis is placed on the need for approaches which are sufficiently complex to encompass the factions, conflicts and rival worldviews that undermine collective initiative towards a promising future.
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