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Weed treatment proposed for outstanding lake

Fuseworks Media
Fuseworks Media

Lake Kohangatera is to receive treatment to eliminate two new invasive weed species following surveys by NIWA.

Lake Kohangatera is one of two lakes returned to Taranaki Whanui in their Treaty of Waitangi settlement. The lakes are governed by the Roopu Tiaki a board established with iwi and Greater Wellington Regional Council.

Lake Kohangatera is located in the Parangarahu Lakes Area within the East Harbour Regional Park. Lake Kohangatera has been ranked in the top ten nationally outstanding lakes out of more than 240 lakes surveyed for aquatic plant values. The Roopu Tiaki through Greater Wellington Regional Council, commissioned NIWA to carry out assessments in March this year, after the aggressive exotic weeds Elodea canadensis and Egeria densa (Oxygen weed) was found in the uppermost reaches of Gollans Swamp.

"These latest survey findings confirm Lake Kohangatera is nationally outstanding for its aquatic plant values. However, the existing exotic weeds specifically Egeria densa in the upstream catchment has grown in area and this needs to be addressed to protect the lake’s special ecological values," says Greater Wellington Regional Council Environmental Scientist, Alton Perrie.

The affected area is relatively small at 1.8 hectares of the 72 hectare lake but the Roopu Tiaki, is keen to ensure the spread is controlled. And that total eradication is the final outcome. "The Parangarahu Lakes are extremely important to our region and to New Zealand. Our main concern is to protect Lake Kohangatera and to maintain its value as a place where our rare native plants can flourish unhindered by introduced species. After treatment we will work closely with the adjoining landowners to ensure the lake is not re-infested," says Roopu Tiaki Chair Liz Mellish,.

The course of action recommended by NIWA is aerial spraying of the aquatic weed with the herbicide diquat, which is non-toxic to fish and becomes inactive on contact with organic matter and sediment at the bottom of the lake. A helicopter with GPS guidance and specialist nozzles will apply the gel directly to the affected area in late spring and again in autumn.

"We are working closely with all our stakeholders in this special area. Once agreement has been reached the treatment plan will form part of our resource consent application" says Amanda Cox, the Regional Council’s Manager Parks. The survey reports can be downloaded from the document library on the Regional Council’s website - search for "Lake Kohangatera.

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