Monitoring of the invasive round goby in an estuarine seascape based on eDNA

Non-indigenous species (NIS) is one of the five most global concerns when it comes to ecosystem services and threats to native biodiversity. This is especially true in aquatic environments which are harder to monitor than terrestrial environments, and NIS are often found first when they are fully established and basically impossible to eradicate. In marine environments, this is further complicated due to the connectivity and difficulty of eradication. The development and implementation of effective monitoring methods for marine NIS are therefore crucial to enable early detection useful for management strategies. In this study, we develop and evaluate environmental DNA monitoring using quantitative (q)PCR as a means to assess presence of the euryhaline round goby fish ( Neogobius melanostomus ) in a seascape environment close to Scandinavia’s largest shipping port. We developed a dPCR assay for the species, targeting a region of 12S gene, and verified its specificity compared to other common species from the gobiid-family in the region. We also experimentally determined the decaying rate of round goby DNA in water to a half-life of 9.8 hours in 15 PSU and 15°C with live fish in captivity. Finally, we sampled 10 sites within a 400 km2 area for eDNA and presence of the species using fyke-nets and baited remote video to validate the accuracy of the water samples to predict presence, and abundance. We found that the number of DNA copies extracted from the water samples varied strongly at sites where round gobies were caught in nets or on video, but that the average value from four water samples significantly correlated with an average value from four video samples, and also with the total catch at each site. The eDNA assay also detected signals from the species at sites where no fish were caught by fishing or on video. These results show that this method is highly sensitive for the species even in low abundance, and with sufficient amounts of replicates, it can be possible to determine the relative abundance between sites. ### Competing Interest Statement The authors have declared no competing interest.