Water quality the focus as bird disturbance measures ramp up

Protecting the water quality of a nationally significant peat lake is the focus as efforts ramp up to disturb and disperse pest birds roosting at Rotopiko.

Flocks of starlings, sparrows and pigeons – believed to number as many as 500,000 individual birds – have been roosting at Rotopiko, an important network of peat lakes near Ohaupo in the Waipa 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 roosting at the location every night.

The Rotopiko lakes are nationally significant because they support a healthy and diverse populations of aquatic plants. Most other peat lakes in the Waikato region have lost their underwater plants due to declining water quality. Further water quality decline risks loss or damage to the native aquatic plants found in the lake eco-system, including Charophyte species such as Nitella aff. cristata and Chara australis, in association with Potamogeton cheesemanii and Potamogeton ochreatus.

National Wetland Trust 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 several methods to disperse the birds in recent years – including non-toxic scent deterrents, noise and laser light disturbance – but these have not shifted the stubborn roost birds.

Technical expert advice from several agencies has supported a difficult decision near the pointy end of the Trust’s list of options - to carry out a controlled, temporary reintroduction of ship rats within the predator-proof fence – an experimental method which 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 Denyer.

“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 expectation – based on scientific advice – is the temporary reintroduction of the rats will discourage the roosting birds and compel them to disperse.

“The expert advice we have is 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.

DOC Waikato Operations Manager Tinaka Mearns says the agency supports the approach being taken and views the health of the lake itself as of paramount importance.

“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 Ngati Apakura to make a very tough choice between protecting water and protecting native fauna: we cannot move the lakes, so we’ve placed importance on protection of the water and its plants.”

Waipā District Council’s community services manager, Brad Ward said council is responsible for the land management of Rotopiko.

“We have been working closely with National Wetland Trust, Ngāti Apakura and DOC, the work planned is crucial to ensure the current and future protection of the lake’s water quality.”

Monitoring will be undertaken by Wintec and the NWT to determine if the reintroduction of rats is working and dispersing the birds. At the end of the experiment the rats will be removed, however if the operation is successful, rats may need to 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 work.