Native fish avoid parasite spillback from multiple exotic hosts: consequences of host density and parasite competency

Disease-mediated impacts of exotic species on their native counterparts are often ignored when parasite-free individuals are translocated. However, native parasites are frequently acquired by exotic species, thus providing a mechanism through which native host-parasite dynamics may be altered. In Argentina, multiple exotic salmonids are host to the native fish acanthocephalan parasite Acanthocephalus tumescens. Field evidence suggests that rainbow trout, Oncorhynchus mykiss, may be a major contributor to the native parasite's population. We used a combination of experimental infections (cystacanth-juvenile worm transmission from amphipod to fish; post-cyclic-adult worm transmission between definitive fish hosts) and dynamic population modelling to determine the extent to which exotic salmonid hosts may alter A. tumescens infections in native freshwater fish. Experimental cystacanth infections demonstrated that although A. tumescens establishes equally well in native and exotic hosts, parasite growth and maturity is superior in exotic O. mykiss. Experimental post-cyclic infections also showed greater establishment success of A. tumescens in O. mykiss, though post-cyclic transmission did not result in greater parasite size or maturity. Dynamic population modelling, however, suggested that exotic salmonids may have a very limited influence on the A. tumescens population overall, due to the majority of A. tumescens individuals being maintained by more abundant native hosts. This research highlights the importance of considering both a host's relative density and its competency for parasites when evaluating whether exotic species can modify native host-parasite dynamics.