The Soča trout or the marble trout is an endemic fish species which inhabits the rivers draining to the Adriatic Sea. In Slovenia, it can be found in the Soča and its tributaries, as well as in the Reka and the Rižana rivers.
The Soča trout is famous for its relatively large head and a characteristic marbled colour pattern on a grey-white skin. The pattern can range in colour from dark-gray, olive-green, copper-red to yellow-brown. In several varieties of the marble trout, this pattern can be interrupted by a varying number of red spots. The largest marble trout specimen in Slovenia weighed 25 kilos and was 121 cm long although the fish of 50 – 70 cm are more the norm.
Pollution, habitat destruction and hybridization of the native species with the brook trout, which was brought to this natural habitat at the beginning of the 20th century, have made the Soča trout one of the most endangered freshwater fish species of the Adriatic Basin. The Soča trout rehabilitation programme which is being implemented in the upper reaches of the Soča basin has already seen marked results: it has improved knowledge of the biology and ecology of the marble trout and led to increase in the number of genetically pure Soča trout in Slovenian rivers.