Relative competence of native and exotic fish hosts for two generalist native trematodes

Exotic fish species frequently acquire native parasites despite the absence of closely related native hosts. They thus have the potential to affect native counterparts by altering native host–parasite dynamics. In New Zealand, exotic brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss have acquired two native trematodes (Telogaster opisthorchis and Stegodexamene anguillae) from their native definitive host (the longfin eel Anguilla dieffenbachii). We used a combination of field surveys and experimental infections to determine the relative competence of native and exotic fish hosts for these native parasites. Field observations indicated that the longfin eel was the superior host for both parasites, although differences between native and exotic hosts were less apparent for S. anguillae. Experimental infections indicated that both parasites had poorer establishment and survival in salmonids, although some worms matured and attained similar sizes to those in eels before dying. Overall, the field surveys and experimental infections indicate that these exotic salmonids are poor hosts of both native trematodes and their presence may decrease native parasite flow to native hosts.