COLONIZATION AND ESTABLISHMENT OF POMPHORHYNCHUS-LAEVIS (ACANTHOCEPHALA) IN AN ISOLATED ENGLISH RIVER

The successful colonization, establishment and spread of Pomphorhynchu s 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 para site was first detected in Anguilla anguilla and Platichthys flesus in 1988: by 1995 it had attained prevalence levels of 22.6% in A. anguil la and 43.6% in P. flesus and also occurred in 100% Salmo trutta, 50% Cottus gobio and Noemacheilus barbatulus. As judged by prevalence, abu ndance, proportion of females gravid and weight of gravid females, S. trutta was the preferred definitive host although C. gobio was a suita ble host and may play a role in cycling the parasite: the other three species were unsuitable hosts. The intermediate host was the freshwate r Gammarus pulex: the euryhaline G. zaddachi was not infected. On biol ogical grounds, the P. laevis could be assigned to the English freshwa ter strain and was almost certainly introduced to the river by anthrop ochore stocking of S. trutta from a Dorset hatchery. The findings demo nstrate conclusively that the English strain of P. laevis can colonize and establish in a new locality from which Leuciscus cephalus and Bar bus barbus, its normal preferred hosts, are absent and use S. trutta i nstead. The results also confirm that P. laevis is a poor natural colo nizer and appears always to be introduced to new localities by anthrop ochore transfers of fish. The implications of these conclusions for un derstanding the present distribution of P. laevis are discussed and it is considered that they provide direct evidence in support of the hyp othesis that P. laevis was introduced to Ireland from England and subs equently adapted to use of S. trutta and G. duebeni there.