Opihi River

•  The best thing you can do to keep yourself, your kids and your dog safe is to know what toxic algae looks like and avoid it. 

• If you are not sure, keep your dog on a lead at a stony river, and don’t let them in or near the water.  Bring water from home for your dog so that they don’t need to drink out of the river.

What is toxic algae?

Freshwater algae are naturally occurring in rivers and lakes in New Zealand, including waterways with good water quality. Most algae is harmless, if a bit slimy – but one group of algae, known scientifically as cyanobacteria, can be toxic.  Most of the year it is present at low levels that are not much of a danger.  But it can be a problem during the summer months, when low rainfall, warm temperatures and more sunlight create an environment where it can thrive.  In parts of stony-based rivers it can form extensive mats which can be toxic to humans, dogs, livestock and wildlife. 

Scientists do not yet understand when and why this algae turns toxic, so to be safe, always treat it as toxic.

Dogs are particularly susceptible to poisoning from toxic algae as they are often attracted to it's musty smell, and love to scavenge and play near water.  Even a small amount (size of a 50 cent piece) can be harmful.

What to look for

In parts of stony based rivers toxic algae form soft, leathery-looking mats on rocks in the riverbed.  These mats range in colour from blackish/brown to dark green (it's quite different from normal harmless green algae, which looks bright green and often forms long strings).

These mats can come loose and wash up on the edge of the rivers, or form ‘floating rafts’ in shallow areas.  As they dry out they turn light brown or white and look like dried leaves or cowpats. They produce a strong musty smell, and this is when it poses the biggest risk to our dogs, as they love the smell and many dogs will try to eat it if they get the chance.

Council staff monitor for these mats in parts of rivers where these are known to form at popular swim sites.  Staff are not able to monitor all parts of rivers, so please learn what to look for to keep your dog safe.  Dog deaths have been reported from sites where toxic algae have been present in small amounts, and at levels below the threshold for public health warnings at monitored swim sites.  Please alert your regional or unitary council when you see algal mats so they can warn other people.

What if my dog has eaten toxic algae?

If you suspect that your dog has eaten toxic algae, you should contact your vet as soon as possible.  Signs a dog has been poisoned by toxic algae include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.

Find out more about toxic algae here