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Invasive weed spreads in Rotorua's Lake Okareka

Monday, 15 April 2013 3:15 p.m.

An infestation of the invasive aquatic weed hornwort has spread so far in Lake Okareka it is unlikely to be eradicated, Bay of Plenty Regional Council says.

Hornwort is found in freshwater and grows very rapidly, forming dense weed beds or rafts that choke waterways and affect native plant and fish life. Regional Council Manager Land Management Greg Corbett said the weed was first found in May 2012 by a commercial operator.

“Surveillance was immediately undertaken which confirmed the presence of hornwort, but to determine the full extent of the weed problem we had to wait for the right time of year. March is the time of year when hornwort has new growth, and fragments are a light fluorescent green, which makes the plant more visible to surveillance divers,” Mr Corbett said.

“Unfortunately, it may be that the infestation has got past the stage where we can realistically consider eradicating it from the lake and we may need to learn to live with it always being there. We will be developing a control plan to keep it clear from key recreational areas.”

Two large infestations were found in the northern area of the lake, one with a combined size of around 50 square metres, and a couple of plants were found in the southern area of the lake, which may have originated from the larger beds.

Initial control operations are underway and the Regional Council is preparing an Incursion Response Plan to manage the weed.

“We have had success in Lake Okataina with reducing hornwort, but it does take years to manage once an infestation has occurred. The best way to control aquatic weed is to ensure weeds are not transported into the lake in the first place,” Mr Corbett said.

Boaties should always inspect trailers, engine wells, anchors, propellers, fishing gear and other wet equipment such as waders to ensure they are not carrying either weed fragments or pest fish species before they leave lakes or other waterways. All equipment should be thoroughly cleaned before it is used again, Mr Corbett said.

 

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Lake Okareka lowres (1)