General Impact
Brown trout have been implicated in reducing native fish populations (especially other salmonids) through predation, displacement, and food competition (Taylor et al. 1984, in Fuller, 1999). Although it rarely occurs, in America the brown trout is one of the few foreign species able to hybridize with natives, (Fuller, 1999).
Location Specific Impacts:Australia 
Predation: The impact of predation by brown trout on many native species, especially representatives of the family galaxiidae, has been severe ( M. Allen 2002, Field guide to the freshwater fishes of Australia). New South Wales (Australia)
Predation: The impact of predation by brown trout on many native species, especially representatives of the family galaxiidae, has been severe ( M. Allen 2002, Field guide to the freshwater fishes of Australia). South Australia (Australia)
Predation: The impact of predation by brown trout on many native species, especially representatives of the family galaxiidae, has been severe ( M. Allen 2002, Field guide to the freshwater fishes of Australia). Tasmania (Australia)
Predation: May have caused the decline of the Tasmanian mountain shrimp Anaspides tasmaniae. The impact of predation by brown trout on many native species, especially representatives of the family galaxiidae, has been severe ( M. Allen 2002, Field guide to the freshwater fishes of Australia). Victoria (Australia)
Predation: The brown trout may have caused the decline of the Tasmanian mountain shrimp Anaspides tasmaniae, and has eliminated or reduced several Plecoptera and Trichoptera in Victorian streams (Fishbase, 2003). The impact of predation by brown trout on many native species, especially representatives of the family galaxiidae, has been severe ( M. Allen 2002, Field guide to the freshwater fishes of Australia). Falkland Islands (Malvinas) (sub-Antarctic)
Reduction in native biodiversity: Native zebra trout (Aplochiton zebra) have suffered massive declines since the introduction of brown trout and remain largely in restricted areas where brown trout have not yet invaded (including some land-locked freshwater populations). The introduced trout are migratory and it is considered likely that they will eventually invade all the streams currently inhabited only by zebra trout. (McDowall et al., 2001 in Varnham, 2006) Italy
Hybridisation: North Italy (Croatia also) has an endemic trout, Salmo marmoratus which creates fertile hybrids with the brown trout. Nowadays it is very
hard to know if a trout is a hybrid or not. Japan
Competition: Introductions of non-native fishes can lead to the decline or extinction of native species through interspecific interactions (Moyle and Light 1996, in Hasegawa and Maekawa 2006). In particular, many species of salmonids have been introduced into streams throughout the world, primarily for recreational fishing (Fausch 1988, in Hasegawa and Maekawa 2006). Closely related native and introduced salmonids have not coevolved to partition niches, thus interspecific competition can often occur between such species (Krueger and May 1991, in Hasegawa and Maekawa 2006).
Using an artificial stream, habitat use by two sympatric native salmonids in the presence and absence of introduced salmonid species was investigated experimentally. When only native white-spotted charr Salvelinus leucomaenis and masu salmon Oncorhynchus masou were sympatric, they occupied different microhabitats. In the presence of introduced brown trout S. trutta or rainbow trout Oncorhynchus mykiss, however, white-spotted charr and masu salmon were observed to use a similar habitat and interspecific competition between white-spotted charr and masu salmon was initiated. The study suggested that the coexistence of native salmonids was negatively affected through interspecific competition between native and introduced salmonids. New Zealand
Predation: The impact of the introduction on native fish has been apparent on in reduction of riverine and lake populations of galaxiids. Many authors conclude that the native galaxiid fauna has retreated to headwater areas where trout are absent. The native eleotrid fauna remain an important prey item of trout (Ref: McDowall, R.M., 1990, in Fishbase, 2003) Michigan (United States (USA))
Competition: Fausch and White (1981) stated adult brown trout displaced adult native brook trout Salvelinus fontinalis from the best habitats in a Michigan stream.
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