Colonization and establishment of Pomphorhynchus laevis (Acanthocephala) in an isolated English river.

The successful colonization, establishment and spread of Pomphorhynchus laevis in a small, isolated, Devon river 128 km from the parasite's nearest focus in Dorset was followed over 11 years from 1985. The parasite was first detected in Anguilla anguilla and Platichthys flesus in 1988: by 1995 it had attained prevalence levels of 22.6% in A. anguilla and 43.6% in P. flesus and also occurred in 100% Salmo trutta, 50% Cottus gobio and Noemacheilus barbatulus. As judged by prevalence, abundance, proportion of females gravid and weight of gravid females, S. trutta was the preferred definitive host although C. gobio was a suitable host and may play a role in cycling the parasite: the other three species were unsuitable hosts. The intermediate host was the freshwater Gammarus pulex: the euryhaline G. zaddachi was not infected. On biological grounds, the P. laevis could be assigned to the English freshwater strain and was almost certainly introduced to the river by anthropochore stocking of S. trutta from a Dorset hatchery. The findings demonstrate conclusively that the English strain of P. laevis can colonize and establish in a new locality from which Leuciscus cephalus and Barbus barbus, its normal preferred hosts, are absent and use S. trutta instead. The results also confirm that P. laevis is a poor natural colonizer and appears always to be introduced to new localities by anthropochore transfers of fish. The implications of these conclusions for understanding the present distribution of P. laevis are discussed and it is considered that they provide direct evidence in support of the hypothesis that P. laevis was introduced to Ireland from England and subsequently adapted to use of S. trutta and G. duebeni there.