The prevalence and relative density of the monogenean Paradiplozoon homoion, were monitored over one year in populations of roach, Rutilus rutilus, captured in a thermally raised artificial lake located outside a nuclear power station and in a nearby reference locality. Recruitment of P. homoion started earlier in the warmer lake. A temporal shift of the life-cycle was thus observed but there were no differences between the total prevalence or the total relative density of the parasite in the two areas. Since there is no evidence of parasite-induced host mortality, or for the development of protective immunity, the main regulatory process in the heated area are most likely an increased mortality due to reduced thermal tolerance of the adult parasite.
