Dog owners are being cautioned to keep an eye out for a potential toxic algae which thrives in local rivers during the summer.

Phormidium is a naturally occurring algae common to the Cardrona and Hāwea rivers and the Otago Regional Council (ORC) is encouraging dog owners to be able to identify this potentially toxic algae and keep their dogs away from it.

Phormidium forms thick dark brown mats on rocks in the riverbed and can sometimes look like black tar. 

In the river flow, phormidium mats can detach from the riverbed and wash up on riverbanks and dogs are particularly susceptible to poisoning as they’re attracted to the phormidium’s musty smell.

If eaten, phormidium can result in severe poisoning or death.

In December 2018 Phormidium was confirmed in the Hāwea River, upstream of the Camphill Road bridge and warning signs were erected. 

Mats of phormidium can be washed onto the shores of rivers and it’s musty smell is especially attractive to dogs.

“We haven’t had any reported sightings of phormidium so far this season, but we want to remind dog owners to stay vigilant whenever their dog is near water,” ORC’s science team leader (water) Helen Manly said.

“The warm conditions we’re expecting to see with this being a La Niña summer will provide ideal conditions for potentially toxic algae blooms. 

“Don’t wait for a warning sign – if you’re taking dogs anywhere near a riverbank, keep them on a lead and well clear of washed up phormidium mats along the riverbed,” she said.

Any person – or dog – who may have had contact with toxic algae, should rinse off with fresh water as soon as possible, Helen said.

Signs a dog has been poisoned by toxic algae include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsion and any such indicators should be treated as an emergency and referred to a vet immediately, she said.

Any person experiencing a reaction caused by contact with toxic algae, including symptoms such as skin rashes, nausea, tummy upset, and tingling and numbness around the mouth or tips of fingers, should also seek urgent medical attention.

Helen said the ORC monitors bacteria levels at swimming spots but potentially toxic algae can emerge anywhere along rivers so it’s not part of an active monitoring programme.

“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,” she said.

To learn more about potentially toxic algae, including how to identify algal blooms in rivers and lakes, see the Land, Air, Water Aotearoa (LAWA) factsheet.

To report algal blooms in Otago lakes and rivers, contact the pollution hotline: 0800 800 033.

PHOTOS: Otago Regional Council