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

Abstract

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 novel 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 parasites population. We used a combination of experimental infections (cystacanth and post-cyclic) and dynamics population modelling to determine the extent to which additional exotic salmonid hosts may alter A. tumescens infections in native freshwater fish. Experimental cystacanth infections demonstrated that although A. tumescens established equally well in native and exotic hosts, growth and maturity was 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 fitness in additional hosts (measured as length 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 hosts relative density and its competency for parasites when evaluating whether exotic species can modify native host-parasite dynamics.