Invasive freshwater fish (Leuciscus leuciscus) acts as a sink for a parasite of native brown trout Salmo trutta

Invasive species tend to acquire native parasites from their invaded range over time. In these cases, native host-parasite dynamics may be altered as a result of differences in parasite population biology and host competency between invasive and native hosts. Competent invasive hosts are likely to increase transmission to native hosts while incompetent invasive hosts may dilute infection in natives. In this study, we used a freshwater fish system and a survey approach to compare the host competency and population biology of a dominant helminth parasite, Pomphorhynchus tereticollis, between native brown trout (Salmo trutta) hosts and invasive dace (Leuciscus leuciscus) hosts over 2 years at the core and edge of dace’s invasive range in Ireland. Our results show that, although dace acquired P. tereticollis, dace had prevalent and high intensity infections of immature extra-intestinal worms while prevalence of adult worms was consistently higher in brown trout. The majority of parasite individuals infecting dace were immature extra-intestinal forms and, in contrast to brown trout, parasites in dace did not attain sexual maturity. In addition, brown trout from the invasion core where dace have been established the longest had a lower abundance of P. tereticollis, indicating that by taking up but not transmitting infective stages of the parasite, invasive dace may dilute P. tereticollis infection in the native host.