The occurrence of Henneguya psorospermica Thelohan, 1895 (Myxosporida) on perch, Perca fluviatilis L., from Llyn Tegid, Wales

Abstract. The occurrence of cysts of Henneguya psomspermica on a population of perch was studied over a periot of 19 months. Cysts were restricted to the gill filaments although no attempt was made to assess the occurrence of earlier developnental stages of the parasite. Cysts were not recorded from a small sample of perch fry (aged 4–6 weeks). There was a peak of prevalence, and intensity of the infection in juvenile perch (aged 8–9 months), while the prevalence and intensity in adult perch (aged 1–1 and over) was low. In adult perch there was a seasonal November-February.