Distribution of Cottus rhenanus in the River Geul catchment area over the period 2000–2015. Data for the year 2000 were derived from Crombaghs et al. (2000). Data on the period 2005–2015 were collected in transects during standardized sampling for ecological status assessments according to the European Water Framework Directive and following the method of Bijkerk (2010). Sampled transects where the species was not found are depicted with open symbols in red.