Source: NZ Department of Conservation
Pest weeds, fish, microorganisms and now shellfish from overseas are estimated to cost New Zealand millions of dollars each year. Eradication is usually impossible. Containment and preventing further spread is a worthwhile investment – and a real team effort.
New Zealand’s worst freshwater weed
Brittany Earl, a DOC Freshwater Ranger, was in Christchurch catching up with a friend over dinner. She wandered over to say hi to her friend’s pet frogs, when some pondweed inside their terrarium caught her eye.
“It’s my job to keep an eye out for freshwater weeds and pests but I’d never seen hornwort in the South Island before, so it wasn’t on my mind.”
Hornwort thoroughly deserves its notorious reputation. It grows rapidly even from tiny fragments and can form huge, dense, free-floating mats that choke out native freshwater plants and block waterways.
“I’d seen it in the North Island the week before. But I thought, what are the chances it’s here?”
Brittany picked up the pondweed to get a closer look and feel it.
“It’s a unique-looking weed with bright green, feathery tendrils, and each spike has tiny, jagged horns that make it rough to the touch. I was pretty sure my initial ID was correct.”
She took some photos and explained the severity of the situation to her friend (it’s illegal to sell or propagate hornwort), who’d innocently bought it from someone else in Christchurch through Facebook.
Her discovery kickstarted an operation with Biosecurity New Zealand and Environment Canterbury.
“Environment Canterbury ran comms on social media asking people to report sightings. They tracked down the person who’d supplied it to check they hadn’t sourced it from a local waterbody.”
The frog’s weed was bundled up and burned.
“As far as we know, any hornwort in the South Island is contained in aquariums or garden ponds. It would be a massive issue if it got into the environment, not only for native freshwater species, but hydro lakes too. Their clear water would allow for rapid growth to significant depths, potentially clogging up turbines as well as boat propellors and fishing gear.”
New tool to detect invasive weeds early
Controlling a weed when it first invades is on average 40 times cheaper than control once it’s widespread. Using observations from iNaturalist and other sources, DOC’s smart weed alert tool will support faster, coordinated responses to protect native ecosystems and reduce future weed control costs.
Aggressive bottom feeders destroy habitat for native species
Koi carp are invasive fish with populations concentrated in Waikato and Auckland. They were illegally introduced to many New Zealand waterways by Stewart Smith on a personal campaign to make coarse fishing accessible for ordinary people. For 40 years, he bred and released more than 15,000 koi carp, rudd and perch across the country.
“Koi swim along and hoover up everything – weed, insects, worms, native fish – they’re not at all fussy,” says Phoenix Hale, Ngāti Porou ki Hauraki, DOC Freshwater Technical Advisor. “Like those underwater vacuum cleaners you can get for swimming pools.”
This feeding behaviour contributes to bank erosion. “Just like a vacuum cleaner, they spit silt back out of their gills, which releases sediment and nutrients into the water and contributes to algal blooms.”
Koi are very challenging to control. They reproduce rapidly, travel large distances and can be introduced to new areas during floods. “Waikato farmers have found them lying on their fields after floodwaters recede.”
DOC is responsible for protecting New Zealand’s native freshwater fish and habitats, and actively manages threats like koi, in collaboration with regional councils.
“Koi is one of the main pest species we focus on. Our goal is to eradicate them outside the Auckland-Waikato containment area.”
One of the most effective control tools is the toxin rotenone, a plant extract used as a traditional fishing method.
Phoenix helped plan and coordinate a rotenone operation earlier this year at a reservoir on private land near Dannevirke.
“The landowner alerted us after spotting big orange fish in the water. Sightings like this are very important for staying on top of the problem. Testing water with environmental DNA is also proving to be a useful surveillance tool for koi and other invasive species.”
Given rotenone affects all fish species, working with mana whenua to rescue native fish before application was a critical part of the project. “Things are looking good so far, but we will be monitoring for 3 years to be sure.”
Damaging intruder a recent arrival
Invasive freshwater clams had probably been in the Waikato River for a few years when they were spotted in Lake Karāpiro on 19 April 2023. It wasn’t welcome news.
Despite the biosecurity measures in place in Waikato, just this month mature clams were found in Lake Rotomanu, Taranaki. This lake is currently closed to motorised vessels.
Susan Emmitt leads DOC’s Ngā Riha Wai Māori freshwater pests programme. She says the clams are now present from Lake Maraetai to the sea, with high densities in some places.
Invasive clams found on the Waikato Riverbed at Lake Karapiro when water levels were reduced for maintenance work in August. Photos: Michelle Archer, Place Group.
“The clams have invaded all inhabited continents, and eradication has never been achieved.”
Take Ireland. First detected in 2010, the clams were present at 12 separate locations within a decade, with beds up to 15 cm deep across hundreds of metres. Record levels were found in the River Barrow – a staggering 17,872 individuals weighing 42 kg in one square metre.
Susan explains why they’re so easily spread. “The clams are self-fertile and able to produce up to 70,000 microscopic juveniles a year. The juveniles produce a thread that can help them attach to anything absorbent – like fishing gear, ropes, mats, waders or togs.”
Juvenile clams can be picked up in water intakes for town supply, irrigation or hydroelectric generation and grow there. Overseas, their economic impacts have been significant.
“It’s very worrying for New Zealand and our native species. Once you find invasive clams in a river, everything downstream will be part of the infestation. They can be very deep in the sediment, and unless it’s a contained site like a pond, very little can be done.”
Biosecurity New Zealand has imposed Controlled Area Notices on the Waikato River and Te Arawa Lakes to prevent the spread, supported by public awareness campaigns. In 2024 NIWA was awarded $10.2 million from MBIE’s Endeavour Fund to research the clam’s impacts and develop tools to manage it.
“There are many ways the clams could be spread, like building a new road, running a water-based event or fighting a fire – any activity involving transporting water or anything wet from the infested area of the Waikato River.”
Susan urges everyone to think through how they might inadvertently be transporting organisms to a new waterway, and to put more than one safeguard in place.
“Using different sets of gear for different sites can work well. For us, freezing everything that could be harbouring juvenile clams has proven to be the most convenient decontamination method.”
Raising awareness and containing the clam continues to be a big team effort, especially with summer just around the corner. “It’s essential to work together. We can contain their spread, but it takes everyone to be vigilant and do the right thing every time.”
How you can help stop the spread of freshwater pests
• Always Be Naturing by following Check, Clean, Dry for your watercraft and gear when moving between lakes and rivers.
• Ensure you have plans in place to manage biosecurity risks before undertaking water-based activities or transporting water from one place to another.
• Call the MPI hotline (0800 80 99 66) if you see invasive clams upriver of Lake Maraetai or outside the Waikato River and Lake Rotomanu in Taranaki, and koi carp or hornwort in the South Island.
• Report sightings of koi carp or hornwort outside containment areas to 0800 ASK DOC (0800 275 362).
Read more about freshwater pests and our work to contain them: Freshwater pests | Department of Conservation
This article was first published in the New Zealand Water Review.
