A large colony of black-billed gulls on the Ashley River. (Video first published November 2018)

The breeding season for the black-billed gull colony nesting on Canterbury's Waimakariri River has been ruined by the "senseless" shooting of two of its flock.

About 600 of the endangered gulls had been nesting at Groyne 44, on the north bank of the river, until the incident on Sunday. Empty cartridges, likely from a rifle, were found nearby.

Environment Canterbury (ECan) parks and forests team leader Simon Woods said fewer than 200 pairs remained at the site following the shooting.

IAIN MCGREGOR/STUFF The black-billed gull is considered nationally critically.

The fact most had left would have a huge impact on the remaining birds' ability to breed, he said.

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"These are endangered birds and to see them targeted like this is very concerning. We ask that people please be mindful of these birds and respect their habitat."

GRANT DAVEY Black-billed gulls stand among the remains of their flooded nests on the Ashley-Rakahuri River.

Shooting the birds was an offence under the Wildlife Act and the incident had left ECan and the Department of Conservation (DOC) extremely disappointed.

Target shooting with any type of firearm was prohibited within the Waimakariri River Regional Park, Woods said.

The black-billed gulls had been washed out of their previous nesting spots multiple times this season.

"They had chosen this spot as a last-ditch attempt to fledge chicks for the season. There's a strong possibility that will not happen now."

DOC senior ranger Anita Spencer said the deaths were senseless. Black-billed gull populations had declined steeply over the past few decades and they were now ranked as nationally critical.

"This incident has not only resulted in the death of two long-lived birds, but has effectively ended the breeding season for several hundred other gulls, which is a real setback.

"Across Canterbury many people are working to protect our endangered black-billed gulls by doing predator control, managing vehicle access, or monitoring our birds. The actions of the shooters have effectively cancelled out some of that effort this season."

Anyone with information about the shooting is asked to call the Mahaanui DOC office on 03 341 9100.