On behalf of the County Administrative Board of Stockholm, a comprehensive eDNA survey of the fish fauna in Lake Mälaren was conducted, with a particular focus on detecting the presence of the invasive round goby (Neogobius melanostomus). A total of 20 eDNA samples were collected from different parts of the lake, including five locations that had also been surveyed in 2018. The study aimed to map the overall fish community and compare eDNA with traditional gillnet surveys in terms of species composition and its effectiveness as a long-term monitoring tool.
The survey identified 29 fish species, with European eel (Anguilla anguilla), asp (Leuciscus aspius), and vimba bream (Vimba vimba) being particularly important findings from a conservation perspective. Despite thorough screening, no round goby was detected, suggesting that the species had not yet established itself in Lake Mälaren at the time of the survey. Comparisons between eDNA and historical gillnet surveys revealed that eDNA was more effective in detecting a greater number of species, particularly elusive or rare ones, whereas gillnet fishing provided more detailed information on fish sex, size, and health.
The results also indicate that species composition in Lake Mälaren can vary seasonally, as evidenced by differences observed when comparing the five locations surveyed in both 2018 and 2022. The study confirms that eDNA is a cost-effective and non-invasive method for identifying and monitoring fish populations on a large geographical scale, making it a valuable tool for environmental monitoring and invasive species programs. However, it is recommended that eDNA be complemented with traditional methods to provide a more comprehensive assessment of fish populations and ecosystem health.
