Biology of Cucullanus truttae (nematoda) in a trout stream

The life-history of Cucullanus truttae (fabricius, 1794) from a trout stream (Cernovický Brook) in Czechoslovakia is described on the basis of observations in the years 1977--78. Larval lampreys, Lampetra planeri, were found to serve as natural obligate intermediate hosts, whereas the definitive and probably also postcyclic hosts are brown trout, Salmo trutta m. fario. No distinct seasonal changes were observed in the occurrence of infective nematode larvae in the intermediate hosts; the rate of their infestation increases with increasing size of their bodies. Only trout older than two years become infected with the nematodes; the frequency distribution of nematodes is typically overdispersed. There are no seasonal cycles in the maturation of nematodes and gravid females are present in fish throughout the year. A case of conspecific hyperparasitism in this species is recorded for the first time.