Council says.
Phormidium, a naturally occurring algae, forms thick dark brown mats on rocks in the riverbed and can sometimes look like black tar.
It thrives in Otago rivers during summer - particularly the Cardrona, Hawea and Manuherekia Rivers - but has also been spotted in other parts of the region, such as the Waianakarua River and Silver Stream.
Dogs are particularly susceptible to poisoning from phormidium as they are attracted to its musty odour.
It eaten, it can result in severe poisoning or death.
In the flow of water, phormidium mats detach from the riverbed and wash up on riverbanks.
Regional council science team water leader Helen Manly said there had not been any reported sightings of the alae this season so far, but it
was important for dog owners to stay vigilant.
“The warm conditions we’re expecting to see with this being a La Nina summer will provide ideal conditions for potentially toxic algae blooms.’’
When taking dogs anywhere near a riverbank, owner should keep them on a lead and well clear of washed-up phormidium mats along the riverbed, she said.
Though the regional council monitored bacteria levels at swimming spots, potentially toxic algae could emerge anywhere along rivers, including areas where active monitoring was not carried out.
Any person or dog who may have had contact with toxic algae, should rinse off with fresh water as soon as possible.
Anyone experiencing a reaction from contact with toxic algae should seek urgent medical attention, and any signs a dog has been poisoned by toxic algae - including lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsion - should be treated as an emergency and referred to a vet immediately.
“The best thing to do is be aware of your surroundings, learn how to identify potentially toxic algae, steer well clear of it and keep your dog on a lead and out of the water,” Ms Manly said.
• People can report algal blooms in Otago lakes and rivers by contacting the pollution hotline on 0800 800 033.
