Rats will be reintroduced to try to disperse pest birds roosting at Rotopiko and protect the water quality of a nationally significant peat lake.
Flocks of starlings, sparrows and pigeons – as many as 500,000 birds – have been roosting at Rotopiko, an important network of peat lakes near Ōhaupō in the Waipā district south of Hamilton.
The Rotopiko lake network is surrounded by a predator-proof fence, installed by the National Wetland Trust in 2013 as part of a wider restoration project at the site. Although the fence has successfully kept predators out of the wetland wildlife reserve, it has also created a haven for pest birds.
The Rotopiko lakes support healthy and diverse populations of aquatic plants. Most other peat lakes in the Waikato region have lost their underwater plants because of water quality. Further water quality decline risks loss or damage to the native aquatic plants found in the lake ecosystem, including Charophyte species such as Nitella aff. cristata and Chara australis, Potamogeton cheesemanii and Potamogeton ochreatus.
Native birds also live within the Rotopiko lakes. The spotless crake (pūweto), is a wetland bird that predominantly spends its time with its feet wet relatively close to the water. Copper skinks also move in and out of the enclosure.
One of the translocated skinks at its new home in Rotopiko. Photo/Supplied
National Wetland Trust (NWT) executive officer Karen Denyer says: “The volume of faeces from the sparrows, starlings and feral pigeons is a big threat to water quality. The pest birds are creating an unwanted nutrient loading in the lake estimated to be six times what we might expect to see from the wider catchment.”
The trust has tried to disperse the birds in recent years – including using non-toxic scent deterrents, noise and laser light disturbance, and even shooting and poison – but has not shifted the stubborn roost birds.
DOC Waikato operations manager Tinaka Mearns says the lake needs “immediate action” to retain the quality of the water. “Once we lose it, we cannot get it back - it is almost near impossible to return to a state of health again.
“It is a really challenging position for all of the agencies to be in - but we recognise the importance of having to put the quality of the water first, hence moving onto some of the tougher choices.”
Technical expert advice from several agencies has supported the difficult decision to temporarily reintroduce ship rats within the predator-proof fence – an experiment that will be closely managed and monitored.
“NWT has developed an operational plan, with support from partner agencies and mana whenua Ngāti Apakura, which will see a controlled reintroduction of a small number of locally captured wild ship rats inside the predator-proof fence,” says Karen.
“Retaining the fence is important to continue to exclude other pest animals – like hedgehogs and rabbits - that will not affect the pest birds but could impact other ecological values.”
The rats will be released over three months with regular and robust monitoring throughout so that the NWT will see any difference and make changes as they go.
Based on scientific advice, the rats will compel the roosting birds to disperse.
Rattus rattus - the Ship rat, aka. Black rat or Roof rat.
“The expert advises that the birds will not simply move en masse to another location. We believe they will disperse across the district to different sites presumably back to where they roosted prior to the eradication of rats from within the reserve,” Karen says.
Tinaka says the agency supports the approach being taken and views the health of the lake itself as of paramount importance.
Department of Conservations Tinaka Mearns (left) and The National Wetland Trusts Karen Denyer at Rotopiko. Photo / Supplied.
“DOC has responsibility for native bird and animal species found at Rotopiko, and we acknowledge there is a risk to those by potentially allowing rats back into the reserve.
“However, we’ve worked with NWT, Waipa District Council and Ngāti Apakura to make a very difficult choice between protecting water and native fauna: we cannot move the lakes, so we’ve placed importance on the protection of the water and its plants.
“[The birds] have other habitats elsewhere and are reasonably stable.”
Tinaka added that the spotless crake prefers to be in the water, the ship rats and prefer climbing so the trusts thinks there will be a natural distance between the two.”
Monitoring by Wintec and the NWT will determine if the reintroduction of rats is dispersing the birds. At the end of the experiment, the rats will be eradicated. If the operation is successful, rats may be periodically re-introduced as an ongoing pest bird management tool. The stakeholders will continue to explore other methods if the rat operation does not successfully disperse the pests.
“Predator-free is still the honourable goal that the DOC is still steering towards and know that it’s the right direction for us to strive towards, as a nation,” Tinaka says. “This experiment is unique as the size of the enclosure is 12 hectares, with 4 hectares of that being a fragile lake, which is why the lake becomes the priority.
“Through this experiment, all of the agencies will learn a lot more - and be able to consider that in the rest of the predator-free journey that we are heading towards.”
