The effect of removing a natural migration barrier on a previously isolated population of Brown trout (Salmon trutta L.) – Kvåsfossen, Norway.

Abstract

In the years of 1970s and 1980s acidic rain threatened coastal freshwater fish populations in European rivers. Several fish populations in western and southern Norway were particularly affected including the Norwegian brown trout (Salmon trutta L.). Liming of lakes and rivers has had positive effects on re-establishment of fish populations in many Norwegian rivers. Migration barriers, natural or man-made is another challenge for anadromous coastal fish populations. By completely or partially isolating upstream populations from downstream migration, these barriers hinder gene flow and thereby reduce genetic diversity in the upstream populations. In 2014 a fish ladder was built in Kvåsfossen a waterfall in the river Lygna in Lyngdal municipality, Agder, Norway. Kvåsfossen completely isolated the upstream trout population from downstream migration before establishment of the fish ladder. This study has assessed the effects of removing a natural migration barrier on trout in the river Lygna by comparing genomic DNA from 2016 and 2022 using double digest RAD and Illumina sequencing. The results showed decreased genetic structure and genetic differentiation among sample locations in 2022, but no significant changes in genetic diversity and inbreeding. The decreased genetic structure and differentiation confirms migration from downstream to upstream locations after removal of the migration barrier.