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Urgent action needed on lake weed

Environment Waikato’s Biosecurity Committee wants urgent action taken to clear pockets of invasive egeria weed from parts of Lake Taupo.

This week’s Committee meeting heard the weed Egeria densa, a South American oxygen weed, has recently been found in the boat harbour marina at the lake and has the potential to displace native vegetation as well as other exotic oxygen weeds. Because it can form dense mats on the surface of the water, the weed could have serious recreation and economic effects on the lakes it invades.

Freshwater ecologist Grant Barnes said Lake Taupo was a lake where most shorelines were exposed to waves, which prevented nuisance growths from establishing. The lake had been invaded by a diverse group of introduced weeds, which presented the greatest recreational nuisance in the southern sheltered bays of Motuapa and Waahi.

Egeria grew in the areas favoured by another existing weed, lagarosiphon, and could out-compete it where wave action is low. It could also spread where swan populations are low and become a nuisance around boat ramps, reducing biodiversity of native plants.

Biosecurity officer Lisette Collins said that under the Council’s Regional Pest Management Strategy egeria was not currently a plant pest of regional significance, but was on the national surveillance list as a plant banned from sale, propagation and distribution. The strategy was now under review and egeria could be included.

Plant pests are generally required to be controlled by the landowners, and the lake bed is owned by Ngati Tuwharetoa. Environment Waikato can control pests that may become a serious threat if they become widespread and if the regional benefits of Environment Waikato carrying out control are greater than the benefits of control by the landowner.

Representatives of the Department of Conservation, Environment Waikato, Tuwharetoa Trust, Taupo District Council and NIWA have met to discuss the issue and wanted to see an investigation commissioned into the spread of the weed and other likely infestation points, develop management options and discuss control with the Tuwharetoa Trust.

Grant Barnes said it would be remiss not to deal with the issue immediately, and it may be too late when the weed is already affecting biodiversity. The weed was presently in small, isolated pockets and could be dealt with relatively inexpensively. A fuller survey would still need to be done to determine its full extent on the lake, which could take a week.

Committee chairperson Cr Helen Lane said there was a strong feeling the weed should be controlled quickly, and education was also needed to ensure the weed was not spread by boats and trailers. People may not realise the importance of controlling its spread.

The Committee supported the urgency of removing the weed as soon as possible and developing educational material for the public so they could also take action.

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