A fish species is considered invasive if its position in an ecosystem negatively impacts other species.  Generally, these are non-native species which have been introduced into a region by humans (either intentionally or unintentionally) with detrimental consequence to resident fish and other aquatic organisms.  However, some argue that a native species can also be considered invasive if humans have altered its natural ecosystem to the point where the species can throw the whole system into disequilibrium.(fishionary.fisheries.org/invasive)

For further information, please check out a related post on The Fisheries Blog – Blurred lines: Can climate change-induced range expansion qualify a species as invasive?