Demand for exotic aquarium pets is enabling alien fish, plants and snails to become established in New Zealand waterways.
National Institute of Water and Atmospheric Research (NIWA) scientist John Clayton is monitoring a range of exotic species.
They include the ear pond snail, which has quickly become widespread through many of Rotorua's lakes and has now also been found in the remote Ashburton Lakes in the Southern Alps.
Dr Clayton said the only way these snails, and other exotic fish and plant species, could end up in lakes in the first place is that they are released.
He said sometimes people get tired of looking after their aquariums and release the contents into the nearest water body.