The surface of Lake Ianthe in Westland is ruffled by a terse early morning breeze. The forests at the water’s edge are still damp with last night’s rain. Two wetsuited divers slip into the water, clutching yellow underwater ‘scooters’.
The scooters drag the divers through the water at up to 5km/h, allowing them to patrol swiftly through the water in search of the enemy. The enemy is various species of alien water plants. They grow faster and stronger here than in their country of origin.
They spread across lake and river beds, pushing out native plants and animals. Their dense, tall weed beds reach higher than native water plants, and they can interfere with recreational use of lakes.
Ianthe is typical of the frontline of New Zealand’s efforts to keep these species out. On the surface all is quiet – picturesque. Underneath, invasions occur out of sight, allowing the plants to occupy and dominate lakes unchallenged.
Without human patrols of waterways, submerged water weeds are only discovered when they have grown into large, surface-reaching beds – when eradication is almost impossible.
The two scuba divers at Ianthe are from NIWA, contracted by the Ministry for Primary Industries to patrol for an aquatic weed called hornwort. Instead they find another weed, lagarosiphon. The official status of these weeds is ‘Unwanted Organism’, making them illegal to sell, propagate or distribute. Once these weeds reach a waterway, they grow rapidly, smothering native vegetation and reducing waterbound oxygen for fish (contrary to what the common name suggests).
Lagarosiphon is one of the three worst species commonly known as ‘oxygen weeds’ infiltrating waterways. Its discovery at Lake Ianthe by patrols illustrates the elusive nature of the enemy; it’s hard to know when and where the invasion will come, and by what.
Hide and seek
Detecting new alien freshwater pests within a lake or river is challenging. Underwater invasions occur out of sight, and water-borne dispersal easily and quickly distributes new arrivals from their point of introduction.
Managers of New Zealand’s waterways, like regional councils and the Department of Conservation, spend millions of dollars a year on weed patrols and eradication.
Many agencies help patrol waterways. The Ministry for Primary Industries looks for pest species new to New Zealand, and those on the list of ‘National Interest’ pests.
Other agencies sending searchers into the water are the nation’s territorial authorities (mostly regional councils), the Department of Conservation and Land Information New Zealand (on unoccupied Crown land). Weeds pose a particular threat to hydro-generation, so power companies are also part of the patrolling network.
John Hook, Acting Manager, Crown Property Management, at Land Information New Zealand (LINZ) says the partnership approach between various organisations is proving successful.
“The scientific monitoring and advice provided by NIWA is integral to ensuring that programmes are evidence-based and that resources are well directed.”
The search is a very hands-on effort; experienced people have to enter the lakes or waterways to look for the weeds.
Mary de Winton, a freshwater ecologist at NIWA, says there’s no substitute for direct observation.
“We are using new tools like drop cameras and sonar, but there still has to be a time when a person actually eyeballs the weeds.”
In fact, she says that the ‘intuition’ of people is critical to the patrols looking for first incursions.
“Great divers have an instinct – they’re able to think like the plant. They know where plant fragments are introduced, where they might float to and where they might wedge and grow.”
Once found, NIWA’s biologists make recommendations for eradication.
“We suggest control options that suit the weed and the conditions. How you eradicate it depends on the weed species, how much of it there is and characteristics of the site – like the profile of the shoreline.
“Eradication can take many forms,” explains John Hook.
“The approach we take varies slightly for each area, but generally involves a combination of herbicide control, suction dredging and hand weeding.”
After weeds are destroyed, NIWA personnel return to the waters to monitor if, and how quickly, the weed reestablishes itself.
To assist in their battle, the agencies are looking for improved methods to detect new pests at an early stage of establishment when it is most feasible to eradicate them.
Early identification is essential because even when a new infestation is found, most control options require that all colonies are located for treatment. The earlier they are found, the fewer weed colonies that need to be destroyed. Mary de Winton says the organisation is always looking to improve its techniques, as well as its tools.
“We’re always learning from our experiences, and constantly modifying techniques to do it better.”
Good news from the front
The good news from the frontline is that nine freshwater species designated as ‘Notifiable Organisms’ have not yet been detected. Five aquatic weeds threatening lakes, rivers and wetlands have been entirely eradicated from New Zealand.
Where weeds are taking hold in Hawke’s Bay, eradication efforts give reason to think some battles can be won. And, in the South Island, hornwort has been eradicated from all six known sites.
Mary de Winton says it has been rewarding to see the hornwort reduced. "We are down to just a few shoots being found, so the risks of spread to other lakes has been eliminated."
At Lake Tutira, grass carp have been introduced to eat hydrilla, a submerged aquatic weed that forms dense weed beds. The weed displaces native plant species and sucks oxygen out of the water at night, making it almost impossible for native fauna to live there. Hydrilla is not exactly a grass carp delicacy, but they happily eat it. Ian Gear, from InGearGlobal is overseeing the eradication of hydrilla from four Hawke’s Bay lakes for the Ministry for Primary Industries.
He told Water & Atmosphere that the grass carp had already chewed back great swathes of the weed within the first five years of introduction.
“Locals are already saying they havhave noticed a very significant decline in the amount of weed. That’s making the area habitable again for native fish and mussels along with trout. There are anecdotal reports that trout in the lake are getting bigger.”
Recording the decline of weed and return of other species is in the hands of NIWA. Early monitoring confirmed that the grass carp quickly removed hydrilla weed beds across the littoral (close to shore) zone of the lake. NIWA’s subsequent monitoring has found that some areas have then been colonised by native plants, and there was an increase in freshwater mussels.
Within three years of grass carp being released into the lake, live mussels were being recorded in water as deep as 8m. Previously, when the dense hydrilla weed beds were present, only dead or empty mussel shells were found at these depths.
Juvenile mussels grow in clean sand, so it is suggested that the increased abundance of mussels is due to an increase in available habitat since the removal of hydrilla. Gear hopes that when NIWA carries out the annual survey in April 2014, it will find that the invasion is well beaten back.
“At this rate we’ll need only another five years to eradicate the weed. But we’ll need another 10 years from that point to make sure it’s gone completely.”
Gear says the NIWA results are proof that grass carp are a good eradication option.
“This is the leading example of what grass carp can achieve. They are a good solution to restoring lakes invaded by exotic weeds they find palatable.”
A key advantage to grass carp is that their ideal breeding conditions are limited. “Grates prevent them leaving the lake, but if they did escape, they won’t find conditions outside the lake suitable.”
The success at Lake Tutira will help inform people about the concept of using grass carp.
“People are wary about the unknown and unfamiliar, but they’re willing to learn. It only took 18 months from commencing conversations with stakeholders to getting agreement from everyone to release the fish into the lakes.
“When we tell people about the success at Lake Tutira, it raises awareness of what they can do with grass carp,” Gear says.
De Winton says grass carp are clearly a useful tool that is appropriate for some weed problems. In the Bay of Plenty, close proximity of the lakes and their popularity for recreation, makes transfering invasive weeds easy.
Aquatic weeds such as hornwort, egeria, lagarosiphon and elodea are now well established in some lakes. Hornwort has spread through large areas of Lake Ōkāreka, for example, that the Bay of Plenty Regional Council (BOPRC) says it may never be eradicated. NIWA is currently investigating whether eradication is feasible. Regional Council Manager Land Management Greg Corbett says the infestation has got to the point where they may not be able to eradicate it.
“While we have asked NIWA to test and trial new approaches for surveying and controlling hornwort, we may need to learn to live with it always being there. We will decide whether to attempt to eradicate it once NIWA has reported back to us later this year. In the meantime we are working with our partners to intensively control all hornwort we find.
“We have had success in Lake Okataina with reducing hornwort, but it does take years to manage once an infestation has occurred.”
Spread mainly by boats and trailers, three Rotorua Te Arawa lakes contained hornwort by 1990. It has infested another four lakes since 2005, and there are only four lakes where this invader has not yet been sighted. BOPRC carries out annual surveillance for water weeds, but experienced difficulties in pinpointing the exact location of the recent infestations. In some cases it was evident the weed had been present for some time before it was detected.
LINZ funds an annual spray programme within the Rotorua Te Arawa lakes. BOPRC advises LINZ on areas of lakes that would benefit from spray work.
Steph Bathgate, Land Management Officer at the BOPRC, told Water & Atmosphere that there is regular monitoring of 10 lakes, and an immediate response team dives any of the lakes when weeds are sighted.
“A considerable amount of monitoring and control goes into containing, managing and preventing further spread of aquatic pests.”
When hornwort fragments were found at Lake Rotomā (which is free of the plant) in 2013 and 2014, divers checked weed cordons and boat ramps to determine the extent of the spread. They found no further plant fragments.
“We aim to contain and minimise the further spread of new and existing infestations of aquatic pest plants to lakes. People from a variety of agencies within the Bay of Plenty work hard to keep it that way,” Steph Bathgate says.
Testing times
In response to the influx of weeds across Rotorua Te Arawa lakes, BOPRC asked NIWA to test the effectiveness and efficiency of surveillance methods, including direct observation and remote detection of submerged weeds. The lessons learnt will also be applicable to surveillance for other macroscopic pests, such as alien snails, or possible future invaders.
Long lengths of shoreline are scanned for weed by snorkel divers towed behind a boat. But their detection abilities are limited when viewing through deep or dirty water. At these times, scuba divers are towed underwater on ‘manta boards’. Alternatively, they use underwater scooters to cover the ground quickly. Grid-based or focused searches like those used for airborne sea rescue searches are particularly thorough when used by free-swimming divers looking for weeds. But the cost of thoroughness is time and resources.
A method being trialled is dropping cameras into the water from boats to conduct spot checks. It is less effective than direct eyesight because of the limited focus of cameras and their tendency to shake when moving through water. Sonar is an alternative method of remote observation(see below), but has limited detection ability when new invaders are in low numbers.
NIWA is testing the efficiency of these different methods by looking at detection rates for items that mimic weeds (such as tree branches). The testing team places branches within a trial area at a Rotorua Te Arawa lake, then assesses how many are found using the available techniques.
The team tries out the methods in various conditions, such as the clarity of water and the amount of existing vegetation.
Sounding Horowhenua
Lake Horowhenua used to be surrounded by podocarp forest at the centre of a wetland. It is now surrounded by farms and Levin township. Until the late 1980s it was used as a discharge for Levin’s treated sewage. Fixing the resultant eutrophic state has become a local goal and community effort, aided by a $1.27 million clean-up fund for the lake.
Weed harvesting was one of eight projects cleared to use the clean-up fund. The first stage of the harvest was finding the weed.
NIWA has just completed a survey using sonar to determine areas of weed in Lake Horowhenua that could be harvested for amenity and nutrient remediation purposes. Sonar technology was once the domain of the military, but is becoming cheap and accessible enough to be used for the battles against weeds.
Sonar uses acoustic messages, ‘pings’, which bounce off objects in the water such as weeds, the lake bed or river bed. The returned signals can be measured and visualised to show the location of weed beds within the water body, their density and their height in the water column.
Mary de Winton says one of the more useful features is that sonar can be used to show weed to the side of the research boat as well as straight down.
As well as revealing where dense vegetation is, sonar can be used to document changes after weed eradication efforts. It can also pick out patches of alien weeds where these grow much taller than native freshwater vegetation.
Sonar surveys are generally ‘ground-truthed’ – which means NIWA divers manually check the presence, composition and extent of weed development at a selection of sites before accepting or interpreting the sonar data. Over 79 per cent of Lake Horowhenua was covered by the boat-based survey using sonar.
The survey mapped vegetation bio-volume (space occupied by weed in the water column). This allowed the extent of dense weed to be defined and measured.
NIWA’s sonar survey will provide information to identify harvester requirements and help optimise a weed harvesting strategy.
Home by Christmas?
In most battles there’s a sense that there may be a day that those on patrol can pack up their kit and go home.
But Steph Bathgate says research shows that even if you think you’ve won, re-infestation can still occur.
“You can never be 100 per cent sure of success with controlling aquatic pests. The environment they inhabit can be cryptic and ever-changing, so having a complete understanding of extent of infestations can be difficult.”
Bathgate warns that people using waterways have a responsibility not to make the job of the weed patrols harder.
“People must ensure they are not part of the infestation process by spreading plant fragments and fish eggs via equipment they use in the lakes.”
NIWA enlists
Science is at the forefront of the battle against freshwater invaders, developing strategic and tactical defences. NIWA designs and tests surveillance strategies and approaches, and prioritises the bodies of water to survey, where to look and how frequently. It also identifies the best tools to control, contain or eradicate pests.
Scientists on the frontline
One of the most notable features of this war is that the scientists are themselves the frontline soldiers. Mary de Winton is part of the team of about 10 biologists at NIWA who don wetsuits to conduct the searches themselves.
“This is frontline, hands-on science. To spot the weeds, you’ve got to know a lot about them. And the more we do this, the more we learn.”
Mary wasn’t really a diver before she started the job.
“I’d had some experience, but I’ve had a lot of training through NIWA, and acquired a lot of diving hours.
"It’s not very glamorous but the dark holes and murky water are compensated by some beautiful locations.”
By the numbers
- 0.5km/h to 1.5km/h – average speed of free-swimming diver
- 3km/h to 5km/h – top speed of underwater scooter
- 5km/h to 6km/h – the maximum tow speed for divers behind a boat (before your mask is ripped off!)
- 2m to 4m – the most common invasion depth by lagarosiphon
- 28 per cent of 344 lakes – found by NIWA to contain one or more of six alien submerged weeds
- 9 freshwater species – plants, fish, molluscs and crustaceans designated as ‘Notifiable Organisms’ that have not yet been detected in New Zealand waters
- 30 – the number of aquatic plant species banned from sale and distribution in New Zealand
- 6+ – number of lakes where NIWA staff have found new weed incursions since 2005
Sonar power
- 83/200 kilohertz – the sound frequencies used to detect vegetation
- 15 to 20 per second – rate of ‘pings’ sent from the sonar unit
- 25m – the maximum horizontal distance sonar can scan from the boat under favourable conditions
- 10m – the height of the tallest alien weed bed measured in a sonar survey by NIWA
