The 'reappearance' of vendace (Coregonus albula) in the face of multiple stressors in Bassenthwaite Lake, U. K.

Abstract

The vendace (Coregonus albula) is the U.K.’s rarest freshwater fish with only four native populations ever having been recorded. These occur or occurred in two lochs (Castle Loch and Mill Loch) in south-west Scotland and in two lakes (Bassenthwaite Lake and Derwent Water) in north-west England. The two vendace populations in Scotland were extirpated many decades ago, but since 1995 and 1998 those in Bassenthwaite Lake and Derwent Water, respectively, have been monitored by gill netting and hydroacoustics within annual fish community surveys. The population in Bassenthwaite Lake was recorded up to 2000 but not in the following 12 years, while that in Derwent Water has been consistently recorded each year. Likely reasons for the apparent extirpation in Bassenthwaite Lake include the multiple stressors of eutrophication, sedimentation and species introductions including roach (Rutilus rutilus) and ruffe (Gymnocephalus cernuus). Measures are in progress to improve environmental conditions, although lake recovery is anticipated to be slow. In 2013, the fish community survey of Bassenthwaite Lake unexpectedly recorded a single underyearling vendace representing a ‘reappearance’ of this species after an apparent absence of 12 years. In 2014, this was followed by the recording of two adult vendace. There are three possible origins for these individuals. First, vendace may have actually survived locally at very low abundance below the limit of detection and may now be increasing in abundance. Second, the fish may have arrived in Bassenthwaite Lake by moving down the River Derwent from Derwent Water. Third, such downstream movement may have happened some time ago and the individuals are the locally-spawned offspring of such recolonisers. Continued monitoring of the fish community of Bassenthwaite Lake promises to be highly informative for both local conservation management and fundamental coregonid population biology.