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aahydrilla verticillata

As it grows, Hydrilla verticillata becomes very dense and crowds out native aquatic plants, restricts light, and depletes oxygen. Supplied.

A survey has revealed "great progress" has been made in removing Hydrilla verticillata, a highly invasive aquatic weed, from the Tutira, Waikōpiro and Opouahi lakes in Hawke’s Bay.

The recent annual flora and fauna survey conducted by the National Institute for Water and Atmospheric Research (NIWA) shows for the first time in more than five decades, the Hydrilla weed has not been found in the three Hawke’s Bay lakes where it was first found.

"After over seven years of dedicated work, we are well on our way to reaching our goal of eradicating this invasive weed," said Dr Mike Taylor, the Ministry of Primary Industries' (MPI)  Biosecurity Response manager.

Hydrilla is a submerged, rooted freshwater aquatic plant which grows up to nine metres. As it grows, it becomes very dense and crowds out native aquatic plants, restricts light, and depletes oxygen. It is considered one of the world’s most invasive water weeds.

Hydrillais one of nine species currently managed as part of the National Interest Pest Responses, which is an MPI programme that focuses on responding to organisms that present significant risks to New Zealand’s biodiversity.

In collaboration with the Hawkes bay Regional Council, Department of Conservation, Fish and Game, and local iwi, MPI has been actively working on removing Hydrilla from Hawke’s Bay’s lakes since 2008.

The programme started off with the use of an aquatic herbicide, which was then followed up with the introduction of Ctenopharyngodon idella, the herbivorous grass carp (a type of fish), into the affected lakes in December 2008 and again in 2014.

"This has been successful as Hydrilla is a preferred food plant by grass carp. The carp used will not breed in New Zealand waters, so we aren’t having to deal with an increase of carp numbers in the local waters," said Dr Taylor.

"Being able to remove Hydrilla from these lakes will remove the likelihood that Hydrilla can be transported to other water bodies."

Native fauna, such as freshwater mussels, are re-colonising their preferred habitat which was previously smothered by the dense Hydrilla weed beds.

MPI says it will be contracting NIWA to conduct a further flora and fauna survey in autumn 2017 to monitor the progress of the programme.

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