Black-backed gull control - What's the story?

There are several reasons for controlling colonies of this abundant native gull. One is to enhance populations of rare and threatened endemic bird species that nest on the river bed.

Black-backed gulls prey on the eggs and chicks of black-fronted tern/tarapirohe, black-billed gull/tarāpuka, wrybill/ngutu pare, and banded dotterel/tūturiwhatu — all threatened birds that breed on braided rivers.

Another reason is to protect rivers and popular swimming sites from E.coli, when this contaminant has been traced to black-backed gull colonies.

Southern black-backed gull populations have been increasing in New Zealand for more than 100 years and are abundant throughout the country.  They have exceeded previous record numbers in some areas because they are able to exploit favourable conditions. Greater food availability in urban areas and development of rural land have increased favourable habitat.

In Canterbury, numbers are estimated at greater than 100,000. Recent research has brought to light the impacts of these gulls on endemic braided river bird populations.

A study on the Hurunui River over the 2017/18 breeding season showed that 77% of black-fronted tern nests were destroyed by mammalian and avian predators.  While the specific impact of black-backed gulls wasn’t determined in that study, other research on the Waitaki River (published in 2018) using remote cameras at nests showed gulls were the primary predator of endangered terns there. 

The large black-backed gull colonies on the Hurunui and Rakaia rivers are having a substantial impact on the small populations of black-fronted tern, banded dotterel, wrybill and black-billed gull.

Braided River Aid (BRaid) has provided this identifier:

Black-backed gulls once co-existed with these other endemic braided river birds, but in much lower numbers than today.  However, black-backed gull numbers have increased markedly, and they now occupy large colonies on braided river beds. In contrast, populations of black-fronted tern, banded dotterel, wrybill and black-billed gull have all declined significantly due to factors such as habitat loss and degradation, invasion of river beds by woody weeds, and predation by introduced mammalian predators as well as black-backed gulls.

Last summer the Department of Conservation (DOC) oversaw or supported black-backed gull control work on the Hurunui, Waiau Uwha, Rakaia and Waimakariri rivers.

This work was supported by the local water zone committees, 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.

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 is an anaesthetic compound registered for control of southern black-backed gulls. It is mixed with margarine, applied to a bread bait and hand laid directly within gull colonies. Pre-feeding is carried out 4 – 6 times before the operation to make sure baits are readily taken up within minutes. 

Contractors remove, as far as possible, any uneaten baits and gull carcasses for disposal within 24 hours of the control operation. However, when thousands of birds are culled, a small number of carcasses may be missed.

Alpha-chloralose in bait form (baited bread) is toxic to people if eaten directly.

Symptoms of alpha-chloralose poisoning include drowsiness, loss of co-ordination and, if large quantities are ingested, a reduction in body temperature.

Anyone suspecting ingestion of toxic baits should seek immediate medical help.

If medical assistance is sought immediately, there are effective treatments for accidental poisoning by alpha-chloralose.                  

Alpha-chloralose in bait form (baited bread) is toxic to pets if eaten directly. Poisoning could also occur by eating poisoned gull carcasses..

Symptoms of alpha-chloralose poisoning include drowsiness, loss of co-ordination and, if large quantities are ingested, a reduction in body temperature.

Anyone suspecting ingestion of toxic baits by their pet should seek immediate veterinary help.

If veterinary assistance is sought immediately, there are effective treatments for accidental poisoning by alpha-chloralose.          

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 river banks. Further, pre-feeding is carried out with non-toxic bread baits (4 – 6 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 being washed into the river following rainfall.

In the Hurunui Waiau Zone, for example, the local Water Zone Committee has long recognised the unique values of the Hurunui and Waiau Uwha rivers and has been working on a package of actions designed to ensure that these values are protected. Good management practice and nutrient limits for farms is one component of this package. Managing populations of black-backed gulls is another. An important part of its decision to proceed with the programme was to protect popular swimming sites from E.coli that was traced directly to black-backed gull colonies.

The Zone Committee is also considering improvements to the regulatory framework and further supporting the community to undertake a range of on-the ground-actions to enhance the values of the rivers and wetlands in the zone. 

Control operations will continue if appropriate. Environment Canterbury has contracted Wildlife Management International Ltd (WMIL) to develop an issues and options report with a view to a region-wide southern black-backed gull control strategy. In consultation with DOC and with input from a range of stakeholders, WMIL is identifying the causes of the increase of black-backed gulls in the region and options to address them. The resulting strategy will aim to better coordinate gull control operations throughout the region.