Scientists and volunteers scouring Lake Rotokare believe perch may be responsible for the toxic blue-green algae plaguing the reserve.
Earlier this month, a team of experts from the University of Waikato joined members of the Rotokare Scenic Reserve Trust in a "bioblitz" - a two day search for as many species as possible living in the waters and surrounding bush.
It was hoped the search could offer a clue to combating the poisonous algae.
The lake has been out of bounds to swimmers and boaties since November under the advice of the Taranaki District Health Board.
The algae, also known as cyanobacteria, can cause severe irritation of the skin, eyes and throat and can bring on asthma.
Sanctuary manager Maj De Poorter said the scientists hypothesised growing numbers of perch could be blamed.
Perch were introduced as a sports fish species and have been implicated in algal bloom formation, she said.
The fish eats microscopic animals (zooplankton), which affects the foodchain and water quality.
If most of the zooplankton have been eaten, then the microscopic plants usually eaten by zooplankton can explode in numbers, causing algal blooms.
Reserve Trust chairman Mike Weren said they had now formed some good theories and would continue to work with the university.
"Knowing more about the wetland ecology should eventually provide insight into what causes those blooms here, and whether there are management options that could result in fewer or shorter closure periods."
The potentially toxic algae tends to form in fine weather, on the rocks of river beds when the water is relatively calm.
During the blitz, the 30-strong team also found the endangered longfin eel was living in the lake.
- © Fairfax NZ News
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