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No quick fix for Otaki lakes weed invasion

Dec 5th, 2008 | By | Category: Latest News, News

PICTURES: Red duckweed covers the surface of Lake Waitawa – but the real problem lies beneath with infestations of other weeds (bottom picture).

BOATIES using Otaki’s Lake Waitawa are having to clear weed from the water before they can go fishing and boating.

The lake – which is used by fishing clubs, waka ama clubs, and water skiing clubs – is clogged with hornwort and egeria, both long, green weeds which have lots of branches and grow to huge lengths.

Nearby Crystals Lake on the Otaki River is inundated with the same pests. Egeria is a recent problem, spreading through Lake Waitawa in just the last 18 months.

The weed is so dense the boat motor gets stuck in it, says Hutt Valley coarse fishing club member Andrew Jones.

“It’s part of a bigger problem, weeds that have come into the country from aquariums and pet fish,” he says. They can grow prolifically from only a few centimetres, and are blocking the lake margins.

Mike Urlich, Kapiti biosecurity officer for Greater Wellington (GW), says they have been aware of the problem for some time.

Although Lake Waitawa is on private land, GW does assist landowners with advice on control of pest plants and animals, he says.

The hornwort was sprayed a few years ago, but that was a temporary measure and the weed has reappeared.

“Aquatic systems are very hard to deal with. There’s no quick fix with these weeds.”

GW’s role is co-ordinating between lake owners, users and iwi and it will be looking at a long term solution, he says.

As part of the process, GW may need to get a resource consent to spray, and other matters such as numbers of waterfowl and land use will also have to be investigated.

GW has been engaged by MAF Biosecurity New Zealand to put signs around the district telling people to clean their boats and equipment.

“Because we haven’t had didymo here, people have been pretty blasé,” says Mr Urlich. “At the moment people can just walk in there and throw in a vessel.”

A small, red duckweed on the surface of Lake Waitawa is not considered a pest plant, but Mr Urlich said it is an indication of high levels of nitrogen in the lake.

Lake Waitawa is part of Forest Lakes, which at 17 hectares is possibly the biggest natural body of water on the Western side of the Wellington region.

Forest Lakes Camping and Conference Centre manager Michael O’Hagan is applying to the Biodiversity Advice Fund of the Department of Conservation for the resources to draw up a management plan.

This would work out the best way to manage the site, he says. Possible improvements could be a riparian strip, more native planting, maybe grass carp in the lake to eat the weed.

He expects the process to take about three years, and the plan would be in consultation with other owners. As his business is the main user of the lake, he is prepared to shoulder the responsibility for the cleanup.

He said the egeria was not in the lake 18 months ago, and it would have been brought in by other users.

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