Otago Fish & Game chief executive Ian Hadland said his staff were monitoring wetlands and estuaries near the Waikouaiti wastewater treatment plant — including the Waikouaiti Estuary, Hawkesbury Lagoon and Merton Arm — for further signs of the disease that killed about 1200 birds at the treatment plant this month, but so far it seemed to have been contained.
Fish & Game had called on the Dunedin City Council (DCC) to provide information about the monitoring under way at the plant and the functioning of the facility to see whether its operation may have contributed to the outbreak, Mr Hadland said.
And the organisation would be asking district councils across the region to look at their management plans for wastewater treatment plants to ensure everything was being done to minimise the risk and severity of harm to wildlife caused by the disease, he said.
A northern counterpart, who has dealt with the disease in the Auckland-Waikato region for years, said he believed the disease would become more prevalent in the South and councils had a role to play to mitigate its effects.
"Unfortunately, what we find is that once you’ve had an outbreak in an environment, there’s a much greater likelihood that you’ll get a repeat outbreak in future years," Auckland Waikato Fish & Game chief executive David Klee said.
"I think what happens is that the sediment just gets saturated with spores, and then ... it becomes way more likely that you’re going to have future events."
A Dunedin City Council spokesman told the Otago Daily Times yesterday, the council remained confident the outbreak at the Waikouaiti wastewater treatment plant was contained.
He said about 1200 birds had died so far, but the number was expected to rise over the coming days.
"The DCC is working closely with Otago Fish & Game, MPI, the Department of Conservation [Doc] and Otago Regional Council to respond to the outbreak.
"Deceased birds are being recovered and buried as part of a management plan designed to help stop the spread and break the chain of infection in and around the plant’s oxidation ponds."
He said the DCC was also contacting other councils in the region to discuss the outbreak, and Otago Fish & Game and Doc were conducting further surveillance of birds for signs of any further spread.
"At this stage, although a small number of dead birds have been found in the area immediately around the oxidation ponds, within a few hundred metres, and the outbreak remains contained to that site."
He said the Waikouaiti wastewater treatment plant provided a natural wastewater treatment process, and oxygen levels within the ponds routinely fluctuated each day as part of the plant’s natural system, depending on weather, the time of day and other factors.
"Monitoring results show oxygen levels were normal before the outbreak was first detected, then dropped at the time the first deceased birds were found, which may be consistent with deceased birds disrupting the treatment process and the large number of birds present.
"There were no specific operational issues identified prior to the initial bird deaths."
Mr Hadland said Fish & Game was pleased the council was taking a proactive role in removing dead birds from the area to avoid further spread of the disease.
Dead birds had been found within 80m of the perimeter of the treatment plant, "which was to be expected", and there had been a few individual dead birds found in the wider area.
But the wider monitoring under way had yet to uncover evidence of an outbreak beyond the treatment plant.
"We will be keeping a close eye on the situation and monitoring for further dead wildlife.
"This is important because it could impact on game bird hunting in the North Otago area, and the potential threat to other bird populations and the commercial poultry industry can’t be understated."
Mr Klee said he "had the unfortunate displeasure of having to deal with [the disease] for a long, long time" in the North Island.
"Unfortunately, wastewater treatment plants do tend to provide the perfect microclimate for the bacteria to flourish and to become available for uptake in birds."
In his patch, it was "really prevalent in a whole lot of different environments", which included natural environments, but wastewater treatment plants were definitely "still the hot spot for us up here".
Low oxygen levels or warm water temperatures in ponds on treatment plant sites could create the conditions required for the naturally occurring bacteria behind the disease, Clostridium botulinum, to reach toxic levels.
And desludging, or aeration at plants could help to mitigate the risk, Mr Klee said.
The toxin found in spores in the water would be picked up by birds but the issue was exacerbated by maggots on birds that died from the disease.
"Two maggots basically have enough toxin to kill the next bird," Mr Klee said.
"And especially waterfowl tend to be quite indiscriminate feeders, so they’ll just pick up these maggots as they’re feeding."
In this case, the majority of the birds that died, paradise shelducks, were at the height of their seasonal moult and so would have been at the plant, on the pond, while flightless, which would have contributed to the number of deaths.
Mr Klee said while preventive measures could be expensive, it was often a more economical option than dealing with the aftermath of an outbreak.
