Council aims to eradicate water weed

Bay of Plenty Regional Council is planning to eradicate the invasive aquatic weed hornwort from Lake Ōkataina.

This week’s Regulation Monitoring and Operations Committee meeting heard that fragments of the weed were discovered by NIWA and Regional Council staff during surveillance work in April this year. Three sites were later identified in the lake’s south west bays.

Land Resources Manager Greg Corbett said Lake Ōkataina was an exceptional lake with very good water quality, and high biodiversity and recreational values. The lake had been free of two of the most damaging aquatic pest plants, hornwort and egeria. If hornwort were left to establish, it would significantly reduce the lake’s values.

Three hornwort incursions had been detected in the lake in recent years, with a relatively significant population of the pest detected earlier this year.

It’s not the first incursion. A single hornwort plant discovered in April 2007 growing near the main beach boat ramp was successfully controlled. The largest population found earlier this year covered about 50 square metres, and an initial control operation targeting 6 ha, including a substantial buffer, successfully destroyed the weed.

Mr Corbett said it was not known if there was any link between the hornwort discoveries in Lake Ōkataina over the last four years.

“It appears that the initial incursion may have occurred between three and five years ago. The initial introduction probably occurred via hornwort fragments (from another lake) attached to a vessel’s anchor warp, as this area is a well-known stop for recreational fishermen.”

Staff decided on eradication as the preferred course of action, which will involve on-going surveillance and control programme. It will also involve containment of the biggest infestation site using a weed cordon, and preventing human-assisted spread within the lake using regulatory powers under the Biosecurity Act to restrict public access to infested areas.

Known infestation areas would be controlled using Diquat herbicide and manual methods. The programme would be monitored, a weed cordon would be set up at the boat ramp to minimise further incursions and public awareness programmes would be increased for Lake Ōkataina users before October, he said.

ENDS

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