Two conflicting coalitions would advocate opposite conceptions of river pollution in the French public space at the end of the 1950s. River fishermen would consider pollution as a crime and they would draw on biology to make these offenses visible. Engineers, urbanists and hygienists would, by contrast, perceive pollution as resulting from dearth of sewerage and sewage treatment, the cost of which they would try to reckon. Both mobilizations shaped the regulation of waste water discharge and the indicators used to evaluate the scope of river pollution– a problem never monitored at national level before. Three biologists crafted the biotic index so that the first inventory of river pollution included biological data. This indicator rests on categories which resulted from mutual enrolments between angling organizations and scientists.