Population consumption via treated wastewater is a significant source of pharmaceuticals and personal care products (PPCPs) in the aquatic environment. This pollution results in a high impact on exposed organisms when recipient dilution is low . L aboratory studies are limited by artificial conditions, making it challenging to describe the complex effects of real-world pollution on exposed organisms in the environment. This study aimed to evaluate the impact of contamination from the sewage treatment plant (STP) effluent on early life stages development and sex determination of brown trout (Salmo trutta m. fario) in situ . This fish is a native cold-water species with long embryo-larval development. F loating egg incubators were used to support salmonid species reproduction . Brown trout eggs were placed upstream (the control group) and downstream (the exposure group) of the STP Prachatice in the Zivny Stream, Czech Republic , maintained in incubators for about three months, and then grown in the natural environment for nearly one year . Up to 72 PPCPs were detected in passive samplers deployed downstream of the STP effluent . In vitro bioassays of the sampler extracts also showed the strong endocrine-disrupting potential of the polluted water. Compared with the control group, the mortality of brown trout in the exposed group was significantly higher . Also, the body size, growth, and metabolic rates of exposed fish were significantly lower after statistical correction for temperature . After several months of natural development in the stream, the s ex ratio of randomly caught brown trout in the effluent-affected stretch was imbalanced, and sterile individuals were detected. These results suggest that STP effluents can negatively affect the early growth and development of fish in watersheds, and these advers e effects may further affect the population density of aquatic organisms and the balance of the whole freshwater ecosystem. The use of floating egg incubators proved to be a promising approach for studying the effect of pollution on the early developmental stages of fish in natural conditions.