Description and distribution
Marble trout (Salmo marmoratus; Cuvier 1817), endemic species, characterized by distinctive marbled color pattern, inhabits some rivers and streams flowing into the Adriatic Sea. In Slovenia has cached the name Soca trout. The large head is one of the characteristics (taking around one fourth of the body length) which is predominant in the species, while huge variation in size and color between the habitat types exist.
Marble trout has cylindrical shaped long, lean body and relatively short dorsal fin. The strong mouth support many sharp teeth. Males can be distinguished from females by the conclusion of the lower jaw, which is directed downwards in males and is straight in females. In chalk streams the marble trout tend to grow stouter and may reach trophy dimensions, while in the upper reaches of Alpine rivers they grow smaller in size. For fly fisherman among us, the high growth capacity is one of the key reasons of species high esteem.
In general, anglers in Slovenia distinguish between Idrijca sub-type marble trout, being yellowish to olive in coloration, with possibility of having an off red mark on body. While Soca sub-type is less pigmented, almost grayish-blue earning nick name Soca ghost. Soca trout is one of the largest European salmonid species - in Slovenia second in size only to Huchen (Hucho hucho). Some specimen grows about one (1) meter in size, with largest Marble trout found at Soca lake - of 1.21 m and 25 kg. Marble trout spawn late autumn, early winter, in Slovenia the spawning peak is between late November and first half of December, ending before Christmas time.
In Slovenia the marble trout is found in Soča river and its tributaries, plus two smaller Rivers Rižana and Reka. Elsewhere, they populate few rivers of Southern Tyrol - Italy, Neretva system in Bosnia and Croatia, and the Skadar river system in Montenegro (see distribution map).
Natural co-existence with other trout
Most of the existing marble trout populations naturally coexist with native Adriatic brown trout. In Slovenia it is believed, that marble trout used to be the only naturally present trout species until the introduction of non-native brown trout. However, the latest research contradicts this notion, as Adriatic lineage of brown trout were found in the rivers Rižana and Učja (Balkan-trout.com).
Due to hybridization with brown trout, the marble trout was one of the most endangered freshwater species in Slovenia. From the first recorded brown trout stocking to Soča River in begin of the last century, to the late eighties, non-native brown trout and its hybrids dominated the entire Adriatic catchment area. To counteract the population-decrease of marble trout, French researchers started the genetic studies with help of Club Tolmin and Slovenian Fisheries Institute. Based on eight genetically pure marble trout micro-populations a repopulation was initiated. The genetic tests between 1997 and 2012 reveal the replacement of foreign genes with native ones, indicating a positive impact of ongoing conservation measures and successful marble trout restoration.
Juvenile marble trout feed primarily on aquatic invertebrates, or drowned land insects. Some favorite trout food includes the freshwater shrimp (Gammarus), members of the mayfly family (Ephemeroptera), caddis flies (Trichoptera) and stone flies (Plecoptera). Besides, trout are opportunists and will feed on anything edible that drops in the river.
Older trout tend to grow large to very large and feed almost exclusively on other fish, so called ‘piscivorous’ trout. Daily intake requirement is satisfied with bountiful protein meal.
As an outro. The native marble trout are amazing diverse species and worth preserving for future generations! They are a good indication of river or creek ecosystem health, and where they thrive man can be mostly sure that other wildlife forms will thrive too.
In next instalment we follow up with information to marble trout holding places, tactics, techniques...
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