Karoro (southern black-backed gull) control
We run extensive control operations for karoro (southern black-backed gull) on braided rivers to safeguard endangered bird species and prevent E. coli contamination in popular swimming spots.
Why karoro are a threat
Karoro populations have been increasing in Aotearoa New Zealand for more than 100 years and are now abundant throughout the country. In Waitaha Canterbury, numbers are estimated to be greater than 100,000.
Like all scavengers, they are opportunistic predators that prey on the eggs and chicks of tarapirohe (black-fronted tern), tarāpuka (black-billed gull), ngutu pare (wrybill), and tūturiwhatu (banded dotterel), all of which are threatened birds that breed on braided rivers.
E.coli is a contaminant affecting rivers and popular swimming holes, making them unfit for use, and this has been traced to karoro colonies.
Southern black-backed gull/karoro
How to identify karoro
Karoro (Larus dominicans) also known as the kelp gull, Dominican gull, mollyhawk, and seagull, is the largest gull in New Zealand
- Adults have a white head and underparts, black back, yellow bill with red spot near the tip, and pale yellow-greenish legs.
- Juveniles are a dark mottled brown, with a black bill and legs; plumage lightens with age, moulting into adult plumage at three years.
Learn how to tell different bird species apart (JPG file, 101KB).
Karoro numbers are estimated to be greater than 100,000
Adults have a white head and underparts, black back and pale yellow-greenish legs
Karoro have a yellow bill with red spot near the tip
Karoro chick
Juveniles are a dark mottled brown, with a black bill and legs; plumage lightens with age, moulting into adult plumage at three years
Karoro stealing a gannet egg
Learn how to tell different bird species apart, Braided River Aid (BRaid)
About the karoro control programme
The karoro control is supported by local water zone committees, the Department of Conservation (DOC), the Canterbury Regional Council (Environment Canterbury) and BRaid.
It is part of a wider habitat enhancement programme. Other projects include clearing weedy vegetation from some river islands to create open, safe areas for threatened birds to breed, and bird surveying and monitoring.
This past summer (2023/24), there have been karoro control operations on several braided rivers throughout the region.
- Hurunui, Waiau Uwha
- Rakaia
- Waimakariri rivers
- Rakahuri Ashley River.
How we control karoro
Large-scale control operations use the toxin alpha-chloralose. For smaller operations and follow-up control, shooting and egg destruction are also used.
Alpha-chloralose use
Alpha-chloralose is an anaesthetic compound registered for the control of karoro. It is mixed with margarine, applied to a bread bait and hand-laid directly within gull colonies. Pre-feeding is carried out four to six times before the operation to make sure baits are readily taken up within minutes.
Toxicity to humans and pets
Alpha-chloralose in bait form (baited bread) is toxic to people and pets if directly eaten. Poisoning could also occur by eating poisoned gull carcasses.
Symptoms of alpha-chloralose poisoning include drowsiness, loss of coordination and, if large quantities are ingested, a reduction in body temperature.
Anyone suspecting ingestion of toxic baits by a person or by a pet should seek immediate medical help, or veterinary help, as there are effective treatments for accidental alpha-chloralose poisoning if sought immediately.
Toxicity to fish
Alpha-chloralose is moderately toxic to fish. Because of this, it is used in such a way that it does not come into direct contact with water because the baits are laid on river islands and riverbanks.
Further, pre-feeding is carried out with non-toxic bread baits (four to six times) to condition the black-backed gulls to the toxic baits and make sure they are eaten within minutes of being laid.
Clearing any uneaten baits and gull carcasses within 24 hours also prevents them from being washed into the river following rainfall.
Current control works
We engage with mana whenua, adjacent landowners and local veterinarians ahead of control operations.
All karoro control works are listed on our current works webpage and signage is erected on-site.