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Lake Mjörn in southwestern Sweden harbors a large lake-migrating trout which is classified as genuine and worthy of protection. Trout reproduction occurs in several of the lake's tributaries, but the abundance of the larger lake-migrating form has declined in recent decades. Releases of reared trout also occur in Lake Mjörn. The hatchery strain used to produce fish for release was founded in 1969 with spawners caught in one of the most important tributaries, and has since been genetically isolated, i.e. no new breeding material has been added.
The aim of this study was to carry out a genetic survey of the trout in Lake Mjörn. This survey includes identification of the population genetic structure, an estimation of genetically effective population sizes, as well as an attempt to evaluate the origin of the Lake Mjörn trout through comparisons with trout from other areas in Sweden.

The results show that the trout populations in the lake are small with relatively low levels of genetic variation. The wild trout in the lake's tributaries form a separate and distinct group in analyses including also trout from other areas in Sweden. However, some of the analyses indicate subtle genetic similarities with sea trout from the Swedish west coast. Although the wild trout in Lake Mjörn forms a distinct group when compared to trout from other areas, there are clear genetic differences also within Mjörn indicating that the gene flow between tributaries is limited. Therefore, releases of trout of foreign origin, as well as movements of trout between streams in Mjörn should be avoided.

The hatchery strain clearly deviates genetically from the wild trout found in the lake. The hatchery strain is genetically most similar to trout from River Gullspångsälven in Lake Vänern. Historic information on releases in Mjörn in combination with the results of this genetic study suggest that the spawners that were used to found the hatchery strain more than 40 years ago most likely included trout of Gullspång origin, previously released into the lake. The conservation value of the hatchery strain is therefore questionable. However, there are alternative, albeit less likely, explanations for the observed results that cannot be entirely ruled out and should be investigated further.