This metadata entry covers the data used to generate the Scottish Marine and Freshwater Science Report "Assessing spread of Didemnum vexillum in Scotland using environmental DNA", and the report itself.

Report executive summary:
Carpet sea squirt, Didemnum vexillum, is non-indigenous (NIS) (or invasive non-native) marine species which, when established outside its native range, can negatively impact marine habitats, outcompete natives species, and lead to economic losses in the aquaculture sector. In Scotland, D. vexillum is known to be present in parts of Firth of Clyde, Loch Creran and Loch Fyne.
Traditional rapid assessment surveys (RAS), applied to monitor for presence of NIS, rely on trained personnel to carry out a field surveys, collect samples and identify NIS, either directly in the field or in laboratory, using morphological features observed under a microscope.
The present pilot study explores use of environmental DNA (eDNA), DNA shed into water or sediment by organisms inhabiting marine environment, to assess the spread of D. vexillum in Scotland. This report shows the results of an eDNA-based survey for D. vexillum in Scotland, targeting NIS hotspots such as ports, harbours, ferry terminals, recreational marinas, aquaculture sites, private pontoons and jetties in the Firth of Clyde, Loch Creran, and the wider Lynn of Lorn areas.
Overall, presence of eDNA detected at studied sites suggested only a localized spread of D. vexillum from sites where established colonies had previously been identified. However, it is recommended that suspected presence of D. vexillum in Dunstaffnage Marina, where its presence has not yet been recorded, is further investigated using traditional rapid assessments to confirm eDNA-based survey results.
In common with any other novel monitoring tools used in the regulatory context, the need for standardized protocols is essential to ensure the robustness of analyses and to allow for benchmarking and comparisons between different data sets. The present study sets out protocols which were able to produce good agreement between outcomes of traditional RAS and the detection of D. vexillum eDNA. Moreover, the study demonstrates how an eDNA-based survey can be scaled up across large geographic regions, within a short time period, using simple equipment and potentially non-specialist personnel. It also illustrates that eDNA-based surveys can facilitate a risk-based monitoring approach by directing traditional RAS to areas with suspected presences of NIS, thereby allowing RAS to be utilised cost-effectively.

The data set includes:
1/ Spatial and temporal information of the surveyed sites
2/ Environmental DNA copy number of carpet sea squirt (Didemnum vexillum) target in individual water samples