Waiting for all-clear at lagoon

Tomahawk resident Bert Scott is waiting for the potentially toxic algal bloom to disappear before he fishes in the lagoon.
Tomahawk resident Bert Scott is waiting for the potentially toxic algal bloom to disappear before he fishes in the lagoon.
Bert Scott (84) says he was born at Tomahawk and knows ''every inch'' of the Dunedin lagoon.

He regularly fished for trout and perch there - but not when there was potentially toxic algae.

The first time he saw the blue-green bloom in the lagoon was in the 1970s, after then prime minister Rob Muldoon subsidised fertiliser for farmers.

Helicopters ''plastered all the paddocks'' with fertiliser and much of it hit the water directly, he said.

''Not long after that, the first bloom of algae appeared. It's a shame.''

Warning signs have been put up. Photos by Jane Dawber.
Warning signs have been put up. Photos by Jane Dawber.
A ''young fella'' caught six trout in the lagoon on Thursday but threw them all back, he said.

The Otago Regional Council and Public Health South recently reissued a warning about the potentially toxic blue-green algae, Anabaena lemmermannii in Lake Waihola, the upper Tomahawk lagoon, and the lower Taieri River near Henley, and warned it could be in other Otago waterways.

Council director of environmental information and science John Threlfall said the reason for the bloom was not to do with fertiliser, even though algae needed nutrients to grow.

The nutrient levels were the same at Lake Waihola last year and the blue-green algae did not bloom there, he said.

''Something else triggers it.''

There was ''no direct, easy science'' and experts were baffled.

The algae could produce toxins that could be fatal to dogs and cause illness in people, he said.

A sample taken from Lake Waihola showed no toxins. However, the algal bloom could change quickly from being toxic to non-toxic, or vice versa.

Southern District Health Board medical officer of health Dr Marion Poore said people exposed to the bloom could develop allergic reactions, such as asthma, eye irritations, rashes, blistering around the mouth and nose and gastrointestinal disorders, including abdominal pain, cramps and diarrhoea.

Symptoms of poisoning in animals included lethargy, muscle tremors, fast breathing, twitching, paralysis or convulsions. In extreme cases, death could occur within 30 minutes after the first signs appeared.

People should seek medical advice if they suspected illness. If an animal becomes sick, a veterinarian should be contacted immediately, Dr Poore said.

- shawn.mcavinue@odt.co.nz

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