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The cause of death of black swans at a Dunedin lagoon this winter is still unknown after an Otago Regional Council investigation into the matter.
Council regulatory and communications general manager Richard Saunders said yesterday it did not know the cause of death of the swans found at Tomahawk Lagoon at the end of last month.
An analysis of two of the dead birds did not identify a cause but found they were healthy birds that died ‘‘very acutely’’, he said.
The results of water samples taken by the council’s investigation team did not indicate water quality was likely responsible, he said.
‘‘We have closed our pollution investigation as no evidence of pollution was uncovered from testing.’’
After complaints from the public last month, an Otago Fish & Game field officer found the carcasses of six black swans, a mallard duck, and a New Zealand scaup at the upper lagoon.
They were in various states of decay, some on a track and others were in the water.
Citizen science water quality monitoring by Ecotago has shown the lagoon water regularly surpasses national guidelines for turbidity, E. coli, nitrates, and phosphate levels.