The ectoparasite Ichthyophthirius multifiliis and the abundance of roach (Rutilus rutilus): larval fish epidemics in relation to host behaviour

Natural epizootics caused by the ectoparasite Ichthyophthirius multifiliis were followed in a 0+ roach (Rutilus rutilus) cohort in a biomanipulated lake, stocked with the predatory pike-perch (Stizostedion lucioperca). The parasite was not observed in three neighbouring lakes. Infection was first observed about 1 month after hatching of roach larvae, simultaneously with the spring increase in water temperature. Within 2 weeks the parasite had spread to almost all roach larvae observed. In the beginning of July, the abundance of roach larvae in the lake declined dramatically. I. multifiliis was not observed on 0+ of rudd (Scardinius erythrophthalmus), bream (Abramis brama) or perch (Perca fluviatilis), even though these species were numerous in the same habitats as roach. The importance of I. multifiliis as a factor in roach mortality was examined experimentally by comparing the mortality of uninfected and artificially infected roach during a laboratory epidemic, and by comparing the mortality of uninfected and naturally infected roach under laboratory conditions. Both the naturally infected roach larvae and the roach larvae artificially exposed to I. multifiliis died within a few days and before uninfected roach.