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The Department and Fish and Game has been investigating the incursion of the pest fish rudd into Lake Ianthe/Matahi after an orange fish weighing about 1kg was caught by an angler last week in the lake and later confirmed as a rudd.
Netting surveys over the past two days, confirmed the presence of both juvenile and adult fish.
''Rudd are known as `the possums of the waterways','' Doc area manager Wayne Costello said.
''Their introduction had the potential to damage our lakes in the same way that possums and stoats have damaged our forests.''
The mature fish feed on endangered freshwater plants and invertebrates, destroy food sources and shelter for existing species, and damage water quality.
''This is a sad case of one or two people spoiling things for the rest of us. We pride ourselves on the quality of our waterways on the coast. They are an integral part of our pristine landscapes. The presence of rudd has the potential to damage both our ecosystems and our tourism industry.''
Mr Costello said there were positive steps the public could take to stop the spread.
''Check, clean and dry your gear. Make sure you aren't accidentally spreading weeds or fish eggs between waterways.
''If you see or catch an unusual or goldfish-like fish in West Coast waterways, please photograph or freeze it and hand it in to Doc or Fish and Game with an accurate description of where it was found.''
Possession and release of rudd without authority is illegal under the Freshwater Fisheries Regulations and Conservation Act with fines of up to $5000.