Public access to Ngāroto has been temporarily closed following a Waikato District Health Board (DHB) escalation of the cyanobacteria public health warning from December 30, 2021, to 'extreme risk' today.
The DHB advised that limiting visitors to the lake will help protect human and animal health from the risk of disease as a result of a large algal bloom that has raised cyanobacteria and E. coli levels to an 'extreme' rating.
The popular walking track around Lake Ngāroto, including the boardwalks will also be closed due to the risk to human health.
Waikato DHB's Medical Officer of Health, Dr Richard Hoskins, agreed with the joint decision by Waikato Regional Council, Mana Whenua and Waipā District Council to close access to Lake Ngāroto.
"The closure of the access into Lake Ngāroto is due to the highly significant rise in both cyanobacteria and E.coli levels. This rise poses a significant risk to all recreational users of the lake, through ingestion of water or inhalation of the cyanobacteria toxin, which could occur to those walking around the lake. The toxin producing species currently detected in the lake are of species that are extremely toxic to all mammals.
"In order to minimise the potential harm to users, we agree that the safest option is to prevent access.
"This closure is consistent with Public Health risk assessment and advice and we fully support closure, it is a hard decision but the right one," Hoskins advised.
Waikato Regional Council's Lakes Scientist Dr Mat Allan said there are a combination of factors that cause cyanobacteria and E.coli levels to rise.
"Shallow lakes with high nutrients encourage high cyanobacteria numbers, especially when temperatures are warm as they have been at Lake Ngāroto this summer.
"But it was a combination of issues that caused oxygen levels to drop, resulting in thousands of fish, including tuna, to die in recent weeks at the lake.
"The lake water stratified, with warm water overlaying the deeper cooler water. On top of this, existing cyanobacteria had collapsed and that rotting biomass sucked the oxygen out of the water, which suffocated the fish. Tolerant fish like carp can suffer through a few hours gasping at the surface, but not right through the day when high temperatures increase the amount of oxygen they need to survive.
"Bacteria then feed on the dead fish and infect ducks. While we have recovered some duck carcasses from the lake with Fish and Game, we haven't yet confirmed they died from botulism, although this is the likely cause," Dr Allan said.
Waipā District Council chief executive Garry Dyet said the council, which is managing the effects of the algal bloom, has stepped up its response to help break the cycle.
"Unfortunately in order to help manage and limit the effects of this algal bloom and to protect lake users, Ngāroto will be temporarily closed until such a time as it is once again safe for visitors."
In addition to daily collections by council contractors and key partners, a boat will be used to access hard-to-reach carcasses and sick birds. Carcasses will be safely disposed of by burial near the lake, taking all the steps to break the maggot transmission cycle.
Bill Harris of Ngāti Apakura, said "Ngāti Apakura are concerned with the loss of fish and waterfowl at Ngāroto – particularly the Tuna (eel) and Kauwau (shag). We are looking forward to continuing our partnership with Waipā District Council to begin putting steps in place to ensure these regular seasonal events cease to occur."
"Ngāti Apakura considered placing a rāhui on Ngāroto, but support the current response work council and their wider support agencies are undertaking at this challenging time."
The most effective way to reduce disease transmission is to collect carcasses in order to reduce the number of spores that persist at a site, so staff are safely collecting carcasses. Council staff are also collecting birds showing signs of botulism. These birds will be taken to local rehabilitation centres for treatment before being released to safe sites within the rohe once it is safe to do so.
Council has been seeking advice from Fish and Game Southern Game Bird manger, David Klee who said judging by the number of carcasses, the botulism outbreak is still in its early stages and is likely to get worse over the next two months.
"The only thing that can be done to mitigate the scale of the outbreak is to try and pick up the dead birds and fish from the lake."
Fish and Game will continue to monitor the wildlife and approved volunteers will pick up birds this weekend.
Council is working with key partners to assist with managing the effects of the algal bloom including Ngāti Apakura, Ngāti Maniapoto, Fish and Game, Department of Conservation, Waikato District Health Board's Public Health Department and Waikato Regional Council.
Work on the boat ramp upgrade at Ngāroto has also been suspended.
For more information head to www.waipadc.govt.nz/ngaroto-algalbloom