Three years of fish monitoring in Leiden Over a three-year period (2013-2015), 50 volunteers surveyed freshwater fish communities in the urban canals located in the Havenwijk (Leiden, the Netherlands). A total of eight monitoring methods were employed to locate and count fish. The most effective methods in this inner-city environment are “Torch light surveys”, snorkelling and fishing for debris in which fish turned out to be sometimes present. During 255 field trips 17 species of fish were recorded. Common perch (Perca fluviatilis) proved the most common species but also visitors like the flounder (Platichthys flesus) were occasionally observed. The western tube-nosed goby (Proterorhinus semilunaris) and the flounder were both found whilst using the snorkelling method. Many species of aquatic wildlife are caught in or near rubbish in the canals. Too rigorous a clean-up of sunken bicycles etc. however, will result in a smaller abundance of fish, since these artificial reefs provide food, shelter and breeding opportunities. To compensate for the loss of habitat, the bullhead (Cottus perifretum) has been provided with ‘aquatic hotels’. Locals placed three or more stacked roof tiles on the bottom of the canal and bullheads were indeed found there. Since the output of the field trips is usually amazing (at least six species are spotted per trip), hundreds of people are now aware of the unique fish communities in their own city. Future goals include promotion of clean and flourishing canals with the help of enthusiastic new ambassadors and citizen scientists which are recruited by the many excursions and surveys. Veel Havenwijkbewoners zijn bijna dagelijks op of aan de grachten te vinden. Ondersteund door de KNNV-afdeling Leiden en omstreken, en getraind door RAVON-medewerkers telden ze drie jaar lang de vissen in hun wijk. Dit artikel is een samenvatting uit het rapport dat hierover in 2016 verscheen. Zo’n vijftig vrijwilligers voerden overdag en ’s avonds 255 inventarisaties uit. Er zijn acht telmethodes gebruikt, waarvan het snorkelen, uitpluizen van opgevist grofvuil, zaklampvissen en watervogelmethode wellicht de meest inventieve en praktische bleken. Vrijwilligers ontdekten onder meer dat kunstriffen, zoals winkelwagens, een groter aantal viswaarnemingen opleveren.