A fish species is considered non-native to a location if it does not occur naturally there and only is present as a result of direct or unintentional human introduction. This does not necessarily mean that these fish cannot thrive in their non-native habitat. In fact, some fish, such as Sea Lamprey (Petromyzon marinus), can be in decline in their native range but can be considered invasive and even imperil native species in their non-native locations.
A fish species is considered native to a location if it occurs naturally there. It may have evolved in that region or dispersed and become established there without human assistance. For example, Lake Trout (Salvelinus namaycush) is native to some locations in North America but is considered non-native in other locations where it has been introduced by people.