Danube salmon  (Hucho hucho)

One of the world's biggest salmonids, the Danube salmon is now amongst the most endangered fish species in Europe (3) (4). This sizeable freshwater fish has an elongate, almost cylindrical body (5), a large head and mouth, and strong conical teeth (6). The back is a grey-brown to red-brown colour and patterned with numerous dark spots, the sides are reddish-grey with a copper-coloured gloss, and the belly is silvery-white (5) (7).

As implied by its common name, this fish is native to the rivers of the Danube basin in Europe (2). However, in recent years it has also been introduced into other European river basins when their numbers declined due to ecological changes in the Danube (2).

A freshwater species found in deeper regions of strong to moderate flowing waters, which are cold, clear and oxygen-rich, with deep scours as hiding places (2) (7).

The Danube salmon is a territorial species, but not solitary, with large individuals occupying and defending territories such as a large pool, which may be inhabited by several other individuals (2) (6). These fish undertake short migrations upstream for spawning between April and May (2) (5). Here, females make a pit in the gravel, in which the male fertilises the eggs (5). One female mates with one male, which warns off other males that approach. The length of incubation depends on water temperature, but larvae generally hatch 16 to 24 days after the eggs are ‘activated' by appropriate water temperatures (6). Individuals then proceed to grow very rapidly, reaching sexual maturity by between four and six years of age (5). Juveniles feed primarily on invertebrates such as insect larvae, whilst adults prey mostly on fish, but also on amphibians, reptiles, small mammals and waterfowl (6).

Once widespread, the Danube salmon is now amongst the most endangered fish species in Europe (3) (8). Numbers have been massively depleted due to overexploitation, industrial and agricultural pollution, deforestation (causing increased water temperatures), water redirection and badly designed or non-existent fish ladders in dams and reservoirs (2) (6). Barriers and dams are major obstacles for spawning fish and prevent genetic exchange between sub-populations (8). In addition, the fish's popularity with anglers as a sports fish has probably also contributed to the species' decline (2). As a result of these combined threats, the Danube salmon is now common in only around 33% of its former range, rare in 28 % and has disappeared completely from 39 % (4).

Conservation efforts to date have involved the establishment of reserves, restocking of populations and introductions into rivers not previously inhabited by the species, legal restrictions on fishing times and quotas, and even a total ban on fishing the Danube salmon in most European countries (4) (6). Unfortunately, none of the measures have had long-term success, partly due to the continuing pollution of rivers (4) (6). It has therefore been advocated that total protection should be given to the Danube salmon's habitat, with particular focus on the halting of pollution, and that more ecologically sound fishery management protocols be implemented (6).

For more information on the Danube salmon see:

FishBase:
http://filaman.ifm-geomar.de/Summary/SpeciesSummary.php?id=4664

Holcik, J. (1995) Threatened fishes of the world: Hucho hucho (Linnaeus, 1757) (Salmonidae). Environmental Biology of Fishes, 43: 105-106.

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