Spawning migration is an integral part of the natural life cycle of all Danube sturgeons. This makes them especially sensitive to the impacts of physical barriers such as dams.
After damming, fish are confined, unable to complete their migration to spawning sites. Enclosed sturgeon populations can experience the negative effects of inbreeding and loss of genetic variability.
Located just below the Iron Gates gorge (Djerdap) between Romania and Serbia, Iron Gates is the largest hydropower dam and reservoir system along the entire Danube. The system consists of two main dams, Iron Gates I and II, built in 1972 and 1985 respectively. The dams are constructed at river km 942 and river km 863 upstream of the Danube delta, in effect confining migratory sturgeons to 863 km of the river and cutting off important spawning sites in the Middle Danube. Iron Gates is jointly operated by Romania and Serbia.
It has been reported that catches of Beluga Sturgeon (Huso huso) and Russian Sturgeon (Acipenser gueldenstaedti)reached a peak following completion of Iron Gates I due to many migrating sturgeons being trapped below the dam. In the period 1972-1976, 115.7 t of Beluga and Russian Sturgeon were caught below the dam, representing an almost 25% increase over the five-year period prior to dam construction. However, a sharp decline followed directly after 1976, dropping to only 37.3 t for the period 1980-1984, when Iron Gates II was built, and stocks have continued to fall since.
The Iron Gates dams do not have technical equipment such as fish passes or bypasses, designed to assist fish migration. However, the sporadic capture of migratory sturgeons upstream of the Iron Gatesshows that a very few individuals manage to negotiate the locks used by ships. WWF is supporting the International Commission for the Protection of the Danube River (ICPDR), the EU Strategy for the Danube Region (EUSDR) and other partners to make the Iron Gates dams passable to sturgeons. This would be a big achievement for the conservation of Danube sturgeons and more than double their range by making nearly 1000 kilometres of free flowing Danube accessible (up to Gabcikovo dam in Western Slovakia).