Black-backed gulls seem an unusual culprit when it comes to affecting water quality – but they are one of the targets of a project to clean up rivers and restore much-loved swimming holes in North Canterbury.

Our largest and most common seagull, the gulls are recognisable from their mostly white body, black wings and yellow bill with a red spot at the tip – the one that'll steal your fish'n'chips if you turn your head away for a moment.

According to ESR, the Crown research institute, black-backed gulls are the primary source of E coli in Hurunui, which runs from the east coast to the Southern Alps, north of Christchurch.

In 2018, Environment Canterbury (ECan) launched the Hurunui Splash project to reinstate two swimming holes in the Hurunui and Waiau rivers that had been used by generations of locals but had become unsuitable for swimming. The project also created two more swimming holes.

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The project is led by the Hurunui Waiau Water Zone Committee and intends to provide improved recreational opportunities for residents and tourists over the summer months.

The large population of gulls is being targeted not just because of Ecoli but also because of the effect they have on native birds.

ECan sought the Department of Conservation's (DOC) assistance in culling the population of black-backed gulls – which prey on the chicks and eggs of rare and threatened bird species. The black-backed gulls form large colonies and impact the endangered black-fronted tern/tarapirohe, black-billed gull/tarāpuka, wrybill/ngutu pare, and banded dotterel/tūturiwhatu, which breed on Canterbury's braided rivers.

DOC is hoping to reduce the population of black-backed gulls in the rivers by more than 80 per cent over five years.

To measure the success of its control operations, DOC compares the breeding success of the threatened bird species in areas where gulls are controlled with areas where they are not.

DOC Science Advisor Kerry Weston, quoted on DOC's website, says the work is part of a five-year control programme and follows a successful trial in the Hurunui River last year: "This will be the first time we've controlled black-backed gulls in the Waiau River where we know they have established several large breeding colonies."

"Last year the control operation was really successful, we managed to reduce 90 percent of gulls in some locations."

Zone Committee Deputy Chair Ken Hughey says Canterbury's braided rivers, including the Hurunui and Waiau, are unique.

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"They support many important fish, animals and plants. Many of the special bird species that depend on these rivers are declining," he says on the DOC website. "That is why we are supporting these important control operations".

Photo / Supplied
Photo / Supplied

In January, ECan was able to tell Hurunui visitors and residents they could use the four newly dug swimming holes but advised they should check the current water quality at the Land Air Water Aotearoa (LAWA) website before jumping in.

Water quality at the two older sites has been monitored for some time; the two new sites are not monitored though 100 popular Canterbury swimming sites are updated with weekly results on the LAWA website. That includes the Waiau Uwha and Balmoral Reserve sites.

One of the four sites, at Balmoral Reserve, has been used by generations of locals – including the Hurunui District Council's first woman mayor, Marie Black. Elected in October, she grew up a stone's throw away.

Many Kiwi and overseas visitors are familiar with this swimming hole because it's part of a large, popular riverside camping ground in Hanmer Springs.

That and another swimming hole on the Waiau Uwha River were re-dug to deepen them and remove the build-up of silt. Two new holes, on the Waitohi River and at Dog Stream in Hanmer, were completed in December.

ECan's North Canterbury zone delivery lead Marco Cataloni said the $19,000 project had been in the works for a long time and the agency was glad to announce its conclusion as summer kicked into gear.

The Hurunui Waiau water zone includes Lake Sumner, the alpine Hurunui and Waiau rivers, the hill-fed Waipara and Conway rivers, as well as the north Pegasus Bay coastal wetlands and coastal hills.

ECan and the Hurunui District Council have jointly formed the Hurunui Waiau Water Zone Committee to work with the community to develop actions and tactics to deliver on the Canterbury Water Management Strategy, to foster a thriving natural environment and healthy waterways in the region.