Slovenia’s Soča River, known for its canyons and its turquoise color, is frequently called one of the most beautiful rivers in the Alps. It is also known for a rare species of fish that lives in its waters – the marble trout, known in Slovenia as the “Soča trout.” Endemic to the Adriatic watershed, the species is only found in a few rivers that flow into that sea, and the Soča is one of its key habitats.

For centuries, the marble trout, named for its distinctive marble-like pattern, was the only species of trout in the Soča. In 1906, however, the non-native brown trout was introduced to the river. Local fishermen hoped that the new species would increase their catch. Instead, the introduction proved to be disastrous for the marble trout population.

By the close of the 20th century, it had become apparent that the marble trout was becoming increasingly rare in the Soča. It wasn’t over overfishing that was to blame – it was interbreeding. Because the marble trout and the brown trout could breed, and produce hybrid offspring, the marble trout, which had once reigned in the river, had become endangered. The marble trout found itself on the IUCN Red List as an endangered species, and even the BBC produced a documentary on the uncertain future of Slovenia’s endemic species.

The threat spurred the local anglers’ societies to action. The Tolmin Anglers Association, in cooperation with its counterparts in other nearby towns, introduced a breeding program that would prevent the marble trout from going extinct. Fortunately for the species, genetically pure marble trout survived in the Soča’s tributaries, which the brown trout had been unable to reach. This stock made marble trout breeding possible, and the anglers’ society has farmed marble trout – exclusively this species -- at its fish ponds since the 1990s.

The marble trout remains endangered and only a lengthy process involving the gradual release of pure marble trout into the Soča will ensure the species’ long-term survival. Meanwhile, any additional introduction of the non-native brown trout has been banned.

Local anglers and environmentalists are optimistic that the marble trout will once again reign supreme in the turquoise waters of the Soča. Because public awareness plays such an important role in the process, the local authorities now organize a festival of the marble trout, which aims to introduce the endemic species – and its struggle for survival -- to more Slovenians, particularly the younger generations.