Salmo trutta

Alexander, Timothy & Seehausen, Ole, 2021, Diversity, distribution and community composition of fish in perialpine lakes – “ Projet Lac ” synthesis report, Eawag: Swiss Federal Institute of Aquatic Science and Technology : 135

Atlantic trout ( Salmo trutta )

are native in the northern perialpine lakes and probably also in Lake Sils in the alpine section of the Inn-Danube catchment. Atlantic trout were recorded in all lakes in the Rhine catchment except Upper Zurich, Biel and Rousses. In the Rhone catchment, Atlantic S. trutta have been widely introduced, any may be native only in Lake Geneva. In the Rhone lakes this species was missing from Projet Lac catches of only the smallest lakes: Remoray, Bonlieu and Bret. It is not known which of these lakes may have hosted the native Rhone or zebra tout Salmo rhodanensis in the past, and what happened to these populations.

Atlantic trout S. trutta has also been introduced into all surveyed southern perialpine lakes, as well as Lake Poschiavo. Indeed, S. trutta were recorded in every lake of the Po catchment, except Varese (which did not have any trout). Salmo trutta is known to hybridize with southern Salmo species in some water bodies where they have been introduced, and remain distinct in others. The massive stocking of S. trutta into southern drainages is thought to have led to nearly complete displacement of the native species in some drainages [164]. This makes the proper assessment of trout diversity in southern drainage systems very important, but also complicated.

Projet Lac recorded Atlantic trout Salmo trutta and three Adriatic-endemic Salmo species in the southern lakes: Salmo marmoratus (marbled trout) in lakes Maggiore, Lugano and Poschiavo, Salmo cenerinus (northern Italian brook trout) only in Lake Poschiavo, and three individuals resembling Salmo carpio (carpione) in Lake Garda. “Engadiner trout” were also identified in Lake Sils, characterized by very few large black spots and genetically belonging to the Danubian Salmo labrax . This taxon coexists with Salmo trutta in Lake Sils as two genomically distinct species ( Figure 46 ). Finally, a local lake trout type was documented in Lake Poschiavo, with an unusual pattern of very dense large black spots. This taxa genetically resembled S. marmorata in the mitochondrial barcode, but Salmo cenerinus at microsatellite markers, with considerable introgression from Salmo trutta ( Figure 47 ). This likely native form of lake trout in Poschiavo and is referred to in this report as Salmo sp. “Blackspot”. Based on information from local fisheries authorities, this trout form spawns in the lake, where an annual spawn fishery takes place.This makes it ecologically unique in Switzerland.The only other known lake-spawning trout in the perialpine region is S. carpio of Lake Garda, which is interestingly of similar hybrid origins (mitochondrially S. marmoratus , otherwise mostly S. cenerinus ).

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It is noteworthy that Projet Lac recorded lake trout forms of S. marmorata in Lake Maggiore.The highest abundance and diversity of trout was recorded in the alpine lakes Sils (Danube catchment) with two native species, and Poschiavo (Po catchment) with three native and two non-native species. The Salmo labrax (Danube trout) phenotype was recorded in both lakes.This lineage is native to the Danube catchment, and has been introduced into Lake Poschiavo. Salmo sp. “Blackspot” was recorded also in Lake Sils where it was possibly introduced from Lake Poschiavo. In Poschiavo, the two species native to the Po river catchment, Salmo cenerinus and Salmo marmoratus were confirmed as genetically distinct from one another and from S. trutta , but with strong signs of hybridization ( Figure 47 ), possibly associated with the arrival of S. trutta and S. labrax in the lake. Some genetic differences remain that correspond to the different phenotypes ( Figure 47 ).

Figures

Figure 46: RAxML phylogeny tree depicting the genetic relationship among individual trouts from Lakes Sils and Poschiavo and reference populations of the five species S. trutta, S. marmorata, S. labrax, S. cenerinus and S. carpio based on several thousand Single Nucleodide Polymorphisms. Note that phenotypic S. trutta from Poschiavo are genetically close to S. trutta from reference populations in the Rhine, whereas phenotypic marmorata, cenerinus and “blackspot” from Poschiavo are genetically intermediate between nonintrogressed S. trutta, S. marmorata and S. cenerinus references.

Figure 47: Weak genetic differences among phenotypically assigned trout populations in Lake Poschiavo. Discriminant Analysis of Principal Components based on several thousand Single Nucleodide Polymorphisms.