Parasite sedimentary DNA reveals fish introduction into a European high-mountain lake by the seventh century.

High-mountain lakes were historically fishless due to natural barriers, but human introductions have led to widespread fish presence. Although particularly intensive during the last decades, historical documents indicate introductions in European high mountains already during the 14th and 15th centuries, but they could have occurred before, provided the intensive land use of the high mountain had started earlier. We used ancient environmental DNA from lake sediments (sedDNA) to investigate this hypothesis. Fish ectoparasites from various clades were identified using the 18S rRNA gene in the sediment record of a deep, high-mountain Pyrenean lake, with Ichthyobodo (Kinetoplastea) being of particular interest due to its consistent occurrence. The study shows a continued presence of fish parasites in the lake since the 7th century, which coincides with the Late-Roman and Visigothic extensive mountain use for sheep pasturing as supported by nearby archeological remains and increased lake primary production evidenced by photosynthetic pigments.