Simple SummaryMonogeneans are problematic parasites in fish production because of their epizootic effects and difficulty in administering control/treatment in the production environment. Understanding how abiotic and biotic factors influence the dynamics of these parasites is the first step towards developing efficient strategies for the management of monogeneans in intensive fish farming. This study showed that the highest parasitic indices of monogeneans in Nile tilapia (Oreochromis niloticus) were found in the coldest seasons with less precipitation. Furthermore, we showed that there was a clear positive correlation between the developmental culture stages of tilapia and the incidence of monogenean parasitism; that is, the larger the fish, the greater the degree of parasitism. In conclusion, we recommend the adoption of good management practices throughout breeding, especially in seasons that favor parasitism, as a promising strategy for reducing the mortality and economic losses associated with tilapia production.AbstractThe aim of this study was to observe how abiotic and biotic factors in a tropical region influence the rate of monogenean parasitism in Nile tilapia (Oreochromis niloticus) that are farmed in net cages. A total of 240 sexually reversed fish were analyzed, and 20 from each culture stage were collected during each sampling month. Overall, 60 fish were sampled in April (autumn), 60 in August (winter), 60 in November (spring), and 60 in February (summer). Fish were collected from a commercial fish farm located in Capivara Reservoir in the lower Paranapanema River region of Paraná, Brazil. In total, 3290 monogenean parasites were collected from fish gills of the following species:Cichlidogyrushalli,C. thurstonae,Scutogyruslongicornis,C. cirratus,C. sclerosus, andC. tilapiae. Higher parasitological indices were observed in colder seasons with lower precipitation. Autumn had the highest parasitic infection values compared to the other seasons. The occurrence of monogenean parasites showed a negative correlation with season, in contrast to the culture stage, in which there was a positive correlation. These results may provide a means for establishing adequate fish farm management to predict periods of high monogenean infestation.