Previous research has demonstrated the value of the spirlin (Alburnoides bipunctatus), a small freshwater fish, as an indicator species for monitoring the ecological health of European rivers. In this study, we examine the temporal occurrence of spirlin in a lowland section of the River Drzewiczka, Poland, at the centre of the European distribution of the species. Using a mixed modelling approach, we show a significant interaction of freshwater fish species richness and time period on the probability of occurrence of spirlin across multiple microhabitats, with a strong positive relationship between the probability of occurrence and species richness at the end of the 20^th Century, but an erosion in this relationship two decades later. This change in the predicted relationship of spirlin occurrence with freshwater fish species richness corresponds with a temporal decline in the water level of the River Drzewiczka. We discuss these findings in the context of the spirlin as an indicator species of the biotic integrity of European river environments.