Stories: SAVING THE MARBLE TROUT

Fishermen and scientists in the River Soča basin in Slovenia have been working hard for more than 15 years to preserve and rehabilitate populations of the endangered genetically pure Marble trout in the upper parts of Soča basin, that have disappeared over the years of hybridization with Brown trout. Marble trout is famed for being illusive, mysterious, that it can grow to enormous sizes and weigh more than 30kg. In Slovenia, the largest one, found dead, weighed 24kg and was 117cm long. But the largest one caught by sports fishing weighed 22,5kg and was 120cm long. However, in 2008, most Marble trout in Soča river basin were hybrids. Since 1906, the Adriatic basin was stocked with non-autochthonous Brown trout that resulted in hybridization and subsequent disappearance of genetically pure native Marble trout. Before the start of the international project of rehabilitation and preservation of Marble trout, only one remaining population of pure Marble trout was known. In the next years, after the project started and provided with crucial information, fishermen and scientists have found seven additional remaining populations of pure Marble trout that are now used to breed their duplicates. The project was supposed to study the existing pure populations, create new ones by breeding them in captivity, and try to raise the level of pure DNA in hybrids until one day foreign DNA would be eliminated. The new pure populations are stocked in empty streams without connection to the habitat of hybrids. Many new streams have been populated with pure Marble trout since then. All streams are monitored twice a year. Fishermen and scientists go into the mountainous inaccessible gorges to count the fish, tag them, take fin and skin samples, weigh and measure them. These headwater streams in inaccessible gorges are isolated from hybridization zones (basically all other rivers) by waterfalls or other obstacles that Brown trout cannot defeat to migrate upstream and mix with Marble trout. Today, the captive breeding program involves seven original populations and their progeny. The fish are also stocked into Soča river to raise the pure DNA level in hybrids. In 2012, they toppled the scale. The level of pure Marble trout DNA reached over 50% and was advancing exponentially. The genetically pure Marble Trout is now again prevaling in the Soča river.