Endoparasites of European perch Perca fluviatilis fry: role of spatial segregation.

A total of 246 perch Perca fluviatilis L. fry, 20 to 106 d old from 3 different reservoir subpopulations (epipelagic perch fry, EPF; bathypelagic perch fry, BPF; littoral perch fry, LPF), were examined for parasites. Six species of endoparasites were found: the nematode Camallanus lacustris was the most common, followed by the cestodes Proteocephalus percae, Bothriocephalus claviceps, Glanitaenia osculata and the acanthocephalan Acanthocephalus lucii. All worms were juvenile or immature and were recovered from the intestinal lumen, with the exception of plerocercoids of Triaenophorus nodulosus, which were found in the body cavity or already encysted in the liver (the final site of infection of metacestodes). A marked difference was found in infection rates in the 3 spatially segregated subpopulations of perch fry. Parasites were found almost exclusively in LPF, which were heavily infected (overall prevalence = 30%) compared with the other studied subpopulations. Two species (C. lacustris and T. nodulosus) were found in 1 fish each (prevalence = 3%) in BPF, whereas EPF were uninfected. The species richness and prevalence of parasites in LPF increased from 20-24 d old fry (2 species of parasites; prevalence = 13%) to 106 d old fry (5 species of parasites; prevalence = 80%).