Mean density of Cottus rhenanus (number of individuals 300 m⁻²) in the River Geul catchment area per sampling year for each stream. Dots represent the density of each sampled transect. Error bars depict the standard error of the mean. Significant differences between sampling years tested with negative distributed regression models are indicated with letters. Sampling years of the Mechelderbeek could not be statistically compared due to too few sampled transects. *p < 0.05, **p < 0.01, ***p < 0.001.