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30th July 2021

Slender Naiad (Najas Flexilis) Habitat - Site Prioritisation

The Slender Naiad (Najas flexilis) is a rare submerged, rooted aquatic plant, typically found in clear-water, lowland lakes. Within the UK sites it is now currently found only in Scotland, with its strongholds historically in Argyll, the Hebrides and Perthshire. Its mainland sites are under increasing threat, primarily from nutrient pollution and competition from invasive non-native species. To help stem the decline of the Slender Naiad and hopefully restore its populations, a research programme was set up and funded by CREW, and carried out by the UK Centre for Ecology & Hydrology in collaboration with the University of Stirling. In the first phase of this project, Gunn and Carvalho (2020), with the support of a number of aquatic plant experts, reviewed the available information on the habitat requirements of the Slender Naiad (

In the second phase of this project, the phase one findings were used to identify lochs in Scotland with suitable habitat for Slender Naiad. It could either be present at these sites yet unrecorded, or they would be lochs to be prioritised for its re-introduction. Suitable sites were first identified using water quality (alkalinity and nutrient) data. Applying a combination of thresholds for alkalinity and nutrient concentrations produced an initial list of 4092 potentially suitable lochs across Scotland. To refine this list, a novel method was developed to use existing data on other aquatic plants to identify habitat suitability. This process was carried out by evaluating a set of 80 “associated” aquatic plant species that have previously been recorded at known Slender Naiad sites. For each of these 80 species an “Indicator Value” score was calculated based on their occupancy and fidelity at sites with, and without, the Slender Naiad. Application of this methodology generated a shorter list of 867 potentially suitable lochs. Further prioritisation of sites on the mainland and sites within 60 km of an existing Slender Naiad sites produced a final short-list of 156 lochs as priority sites for further conservation action.

Of the top 20 ranked sites from the short-list of 156 lochs, the majority occur in Argyll, Dumfries, Galloway, and Stirlingshire, all close to known existing Slender Naiad sites. Additional criteria that could also be used to rank the priority list of sites for further investigations, such as connectivity to other lochs or absence of invasive non-native species in the area, were also highlighted.

It is quite possible that the Slender Naiad may already be present at some of these selected priority sites but may have been missed previously if the site was not surveyed sufficiently, or was previously surveyed too early in the summer season. It is recommended that field surveys be undertaken to first check whether the listed priority lochs hold undiscovered Slender Naiad populations and check the environmental quality (water quality and absence of invasive species). Because of the difficulties in surveying rare submerged aquatic plants, we also recommend sampling loch sediments to check if the seeds of Slender Naiad are present. If it is confirmed through site survey that the slender Naiad is not present, and that these lochs still meet the species’ environmental requirements, then these would be strong candidates for either the re-introduction of the Slender Naiad.