Destruction of fisheries through dams

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.
Aggravated by 
(E) Emanations of other problems