Habitat Fragmentation and Degradation Driving Danube Salmon (Hucho hucho) to Extinction

Danube salmon, the largest salmonid species of the world, historically occupied most rivers of the grayling and barbel zone in the Danube catchment. The core area of distribution of this land-lock salmonid species was in the upper Danube catchment, mainly within the Austrian Alps. Nowadays, most of the populations are extinct. We investigated the largest remaining population in Austria located in the Mur river. The river is 290 km long and fragmented by hydropower plants into 71 reaches with 69 reaches historically occupied by Danube salmon. The Mur has an estimated total population size of about adult 1500 individuals which is already a critical minimum population size. The size of the subpopulation within reaches depends on the degree of fragmentation and habitat degradation, i.e. reach length of unfragmented river reach, number of barriers, length affected by impoundment and water abstraction.  The largest subpopulation with about 660 adult individuals and recorded recruitment is located in the longest unfragmented reach (58 km) with minor habitat degradation. Subpopulations < 500 adult individuals and recruitment are found in 6 reaches with increased habitat degradation and fragmentation.  Subpopulations < 50 adult individuals and lacking recruitment are located in the remaining 62 reaches that are heavily degraded. Any further habitat degradation, e.g. hydro power plant, will significantly increase the risk of extinction of Danube salmon. Long-term persistence of the species will depend on how much effort will be dedicated to habitat improvement and reconnection of subpopulations.