An invasive fish species has been detected in Kāpiti Coast District's local lakes.
Tench has recently been detected in Kotuku Lake and Waterstone Lake in Paraparaumu.
Council environment and ecological services team lead Andy McKay said the discovery of dead tench suggested fish had been illegally released.
"These fish are new in these waterways and have been released on purpose," he said.
Tench are an introduced fish which can grow to up to 2kg and 70cm in length in New Zealand.
McKay said they were usually released as a sports fish, however, they were an invasive species which could impact native freshwater communities.
"It's disappointing that people have released fish for their own entertainment and are disregarding the effects new fish species can have on freshwater ecosystems," he said.
"I'd like to remind everyone that the unauthorised introduction of fish into our waterways is prohibited so we can protect biodiversity in our aquatic environments.
"Transporting and releasing fish can attract enforcement penalties under the Conservation Act.
"Fishing in reserves without permission, regardless of the target species, is also prohibited under the Public Places Bylaw."
Kāpiti Coast District Council asks people to report if they see anyone actively fishing or sightings of pest fish through the Antenno app, by calling 0800 486 486, emailing kapiti.council@kapiticoast.govt.nz or making an online service request via the council website.