Scientists have made the first conclusive sighting of ruffe in Windermere - a fish species previously unrecorded in the lake but whose presence had been inferred by environmental DNA (eDNA) surveys.
Ruffe (Gymnocephalus cernuus), a relative of the more familiar perch, is not native to Windermere or the northern UK but has specific traits that mean it can thrive if introduced to new habitats. A single individual was found when CEH scientist Ben James and colleagues examined catches from the Centre for Ecology & Hydrology’s long-term fish community monitoring in September 2019.
The finding confirms the presence of ruffe in the lake, which had already been inferred from environmental DNA surveys carried out in 2015.
Ruffe have been recorded at Bassenthwaite Lake, Derwent Water and Thirlmere, also beyond their native species range, but are absent from many of the other larger Lake District water bodies. It is unclear whether the species was introduced into Windermere directly, possibly following their use as live bait by anglers, or whether it reached the lake from Rydal Water three kilometres upstream, where ruffe were previously introduced.
Dr Stephen Thackeray, Lake Ecosystems Group Leader at the Centre for Ecology & Hydrology (CEH), said, "We do not yet know the current size of the Windermere population. It could be present in low numbers and further work would need to be done to determine this. However ruffe are suited to successfully colonising new habitats: they can tolerate a wide range of water temperatures and chemical conditions; they feed on a variety of small fish and invertebrate prey and they can spawn at an early age and more than once a year.
"All of this translates into the potential for rapid population growth. Ruffe are also known to feed on the eggs and larvae of other fish, which may be a cause for concern."
This latest finding follows years of decline in the Windermere Arctic charr (Salvelinus alpinus) populations, as well as increases in non-native roach (Rutilus rutilus) in the late 1990s and, more recently, increases in non-native bream (Abramis brama).
Dr Thackeray added, “It is clear that the Windermere fish community is undergoing change, and that there is a need for continued vigilance if we are to understand the impacts of these changes on the wider ecosystem.”
CEH carries out long-term monitoring in the Cumbrian Lakes as part of the UK-SCAPE national capability programme.
Details of the eDNA surveys can be found in Hänfling et al (2016)