High levels of E.coli in a popular North Otago swimming hole have been caused by a large colony of nesting gulls.
Otago Regional Council staff have been baffled for the past three years about what was causing the high levels of bacteria. Sending a helicopter into the gorge, gave them an aerial perspective of the problem.
Matt Hickey, the council's manager of resource science, said birds are bad for carrying nasty bugs.
"It's not dangerous to swimmers but there is a risk of falling sick," he said.
The gulls were found about 5km above the Clifton Falls bridge, in rugged terrain.
Local farmers have been assisting council staff by providing access to their properties and the nearby river for closer inspection.
Once the source of the bacteria was identified, staff took water quality samples immediately above and below the colony, with widely divergent results.
Upstream of the colony, the bacteria concentrations were 214 E.coli/100ml, whereas immediately downstream the concentration was far greater at 1300 E.coli/100ml.
Mr Hickey said according to government water quality guidelines for recreational swimming areas, those with less than 260 E.coli/100ml should be safe, whereas water with more than 550 E.coli/100ml could pose a health risk.
Six colonies of gulls were counted in the gorge and it is likely the gulls have been returning each year to nest for the past 20 years.
"Unfortunately, these nesting gull colonies are likely to continue to cause high E.coli concentrations in the upper Kakanui River, particularly during the breeding season," he said.
The gulls are present in the gorge from about December through to February.
Mr Hickey said the council is not urging swimmers to stay out of the water, but is warning those entering the swimming hole not to swallow any water.