Fabricio R. Gómez, Agustina Valenzuela, Hector J. Villavicencio, Martín Miguel Montes
ABSTRACT This study reports the first record of the trematode Ascocotyle sp. parasitizing the Andean torrent catfish Hatcheria macraei and evaluates host and seasonal predictors of parasite abundance in an arid river system. A total of 365 fish were collected seasonally from the San Juan River (Argentina) between 2022 and 2025. Infection was highly prevalent (89.0%), and parasite abundance showed a strongly aggregated distribution that was analysed using generalized linear models with a negative binomial distribution. Host total length was the main predictor of parasite abundance, supporting a pattern of size‐dependent accumulation. A significant interaction between length and sex indicated that parasite abundance increased with body size and was higher in females than in males, particularly among larger individuals, whereas smaller fish exhibited low parasite abundance. Seasonality had a limited effect, with parasite abundance being approximately 24% lower in autumn than in winter ( p = 0.041), while no significant differences were detected among the remaining seasons. These findings suggest that cumulative infection processes may obscure short‐term seasonal variation in transmission. In this arid river system, parasite abundance appeared to be more strongly associated with host characteristics, particularly body size and sex, than with seasonal variation. This study provides new insights into host–parasite interactions in arid Andean freshwater ecosystems.