I remember about 10yrs ago a piranha was found alive in a uk river....
The Poster is right that the people that release aquarium fish in to lakes and rivers should be herded (and shot for being niaive)
Near me, in surrey U.K. A wealthy individual released 2 puma from his private collection gin the 70-80's, they killed some sheep) there are thought to be around 8 puma on bodmin, giving rise to "the bodmin beast" stories....