The brown trout (Salmo trutta), a popular angling species, was introduced to the waters of New Zealand in 1867 and has become very well established in the local freshwater system. The trout have made New Zealand their own all-you-can-eat buffet, feeding on many of New Zealand's native freshwater fishes. But other native fauna have also been getting intimate with the trout in a different way. It turns out that during its time in Aotearoa, the brown trout has also picking up a new parasite - Acanthocephalus galaxii, which normally infects a little native fish call the roundhead galaxias (Galaxias anomalus).
Furthermore, the parasitic worm has actually become more abundant in the introduced trout than in the native galaxids - presumably because when compared with the tiny native fish, the much larger trout gobbles up more amphipods (the crustacean which carries the larval stage of A. galaxii). But this isn't necessarily good news for the parasite. Once they get into the trout, because of physiological incompatibility with the introduced host, the parasites are unable to reach maturity. So the trout actually acts as a kind of dead-end sink for the worm, which in turn reduces parasite burden on the native fishes.
So even while the trout might be chomping up native galaxids by the mouthful, they also are inadvertently reducing their parasite burden - though I doubt that would give much comfort to the little galaxids fleeing from a hungry trout!
Paterson, R.A., Townsend, C.R., Poulin, R. and Tompkins, D.M. (2011) Introduced brown trout alter native acanthocephalan infections in native fish. Journal of Animal Ecology 88: 990-998.