Food and Agriculture Organization of the United Nation (FAO/UN), Viale delle Terme di Caracalla, I-00153 Rome, Italy, Gerd.Marmulla@fao.org
The threat to, or loss of, migratory fish species that reproduce in rivers (e.g. salmonids; sturgeons; shad) or grow in rivers up to a stage where, as adults, they leave the river for reproduction in the sea (e.g. eels) is increasing with the increasing number of dams and weirs. Once the dams and weirs are constructed, very often these fish can no longer freely migrate to reach the habitats that are essential for the completion of their lifecycles and fish passage facilities are only a limited tool of mitigation. In times of climate change, with more rain in absolute terms or more rain per unit of time in some parts of the world, and less rain – and hence a risk of drought – in other parts, there seems to be a perceived need for the construction of even more dams and weirs for water regulation (i.e. for flood control or irrigation) and it can be expected that therefore their number will still increase as a result of the climate change effects. However, not only the emblematic and economically important species mentioned above but all freshwater fish species, that need to make migrations to satisfy their lifecycle needs, are affected. This has negative consequences for the aquatic biodiversity and ultimately also for the fisheries, food supply, employment and income generation.