A threat to nationally endangered birds in the Waitaki Valley forced the Department of Conservation (Doc) to step in last week.
Doc undertook control work to remove southern black-backed gull colonies from the Waitaki River, east of Kurow, as the gulls were preying on black-fronted tern and wrybill colonies.
There were “unnaturally high numbers” of the gulls in the lower Waitaki and the colonies targeted had about 600 birds in them, Geraldine operations manager Duncan Toogood said.
It was the first season Doc had undertaken black-backed gull control work in the area, and it was not a situation the group enjoyed, Mr Toogood said.
“Sometimes conservation involves making tough choices to give our threatened species the best possible chance of survival. [This] is the best way to give black-fronted terns the chance to build up their numbers in the lower Waitaki,” he said.
Doc contractors laid bread and margarine baits at the colonies between February 4 and 12.
After three “pre-feeds”, baits using margarine mixed with poison were laid to kill the gulls.
The control was not expected to affect other birds as they did not feed near the gull’s colonies, and Doc observed the areas to ensure other species did not eat the bait, he said.
Earlier in the season Doc attempted to shoot the lower Waitaki gulls, but it did not reduce the population significantly to stop the predation of the native birds.
Egg pricking was also used as a long-term strategy, but it did not reduce the numbers either.
“Poison is now being used because it has proven to be an effective and safe way to control black-backed gulls at colonies in other areas such as the Mackenzie Basin,” Mr Toogood said.
All dead gulls and uneaten toxic bait would be collected by staff to reduce the chance of other animals eating it, he said.