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Control work started this week to reduce the numbers of southern black-backed gulls livingĀ on the Hurunui River.

The project is aimed at boosting the populations of rare and threatened bird species, with gulls preying on threatened chicks and nests.

The Department of Conservation (DOC) is overseeing the control work, with the support of Environment Canterbury as part of the Hurunui Waiau Water Zone committee’s flagship project supporting threatened bird species.

Black-backed gulls are known to prey on the eggs and chicks of black-fronted terns, black-billed gulls, wrybills and banded dotterel – all native birds that breed on the river.

Zone Committee chairman John Faulkner says Canterbury’s braided rivers, including the Hurunui, are unique. “They support many important fish, animals and plants. The birds that depend on these rivers are declining.

“That is why we are supporting this important control operation.”

DOC science adviser Kerry Weston says control of the abundant black-backed gulls was being trialled on the Hurunui River after monitoring showed more than two-thirds of black-fronted tern nests there were preyed on last breeding season.

“We had only eight black-fronted tern chicks survive on the Hurunui last season, with 77 percent of nests destroyed after being preyed on by either mammalian or avian predators. A recent study on the Waitaki River found black-backed gulls were the main predator of black-fronted tern nests there, so we know they will be having an impact in the Hurunui where there are high densities of black-backed gulls.”

The black-backed gull control trial will involve comparing the breeding success of the threatened bird species this summer in locations where gulls are controlled and an area where they are not. If successful, it will be extended to the Waiau River next year.

Ecological consultancy Wildlife Management International will carry out the control work by targeting black-backed gull colonies over a 30-kilometre section of the river between the upper and lower Hurunui gorges.

Local landowners have been informed and will be contacted before the operation starts.

The toxin alpha-chloralose, an anaesthetic compound registered for black-backed gull control, will be mixed with margarine on bread bait and hand laid within gull colonies. All uneaten baits and gull carcasses will be removed within 24 hours. Warning signs will be in place.

Alpha-chloralose bread baits and gull carcasses are toxic to domestic animals, as well as people, if eaten. People are advised to avoid the control areas while signs remain in place and to keep pets away.