Didymo, or rock snot, is a gross freshwater algae that has taken over many rivers on the South island of New Zealand. Learn all about it here.
I get out hiking and fly fishing as often as I can in New Zealand. Recently I was lucky enough to take a week long fly fishing trip to the top of the South island. I saw some big trout, crystal clear rivers, and other rivers that were disgusting! A couple, including the Buller River, were covered with didymo and we saw zero fish in them.
What Is Didymo?
It’s a freshwater algae that was introduced to New Zealand by people.
Why Is Didymo A Problem?
Fortunately, it is not considered to be harmful to human health, but it does cause problems to healthy rivers. It grows in such abundance that it covers all the rocks in a river.
A 2016 study found that rivers with a high concentration of didymo had a 90% reduction in aquatic invertebrates (insects, which are a food source for fish) and fish life!
This means that not only does it look gross, but it does in fact negatively impact rivers.
Where Did Didymo Come From?
Originally it came from cold, nutrient poor, clear waters in the northern hemisphere, including Canada, Europe, and Asia. It has now spread throughout the world, including many rivers and lakes on the South island of New Zealand. Many of the rivers and lakes on the south island are also nutrient poor and crystal clear.
Where Is Didymo In New Zealand?
It was first found in the Waiau River in the southern part of the south island in 2004. Since then it has spread throughout the south island, but has not yet been found in the North island.
New Zealand has been trying to keep it on the south island and so far it seems to have succeeded. Why it has not yet spread to the north island though is a good question. There is a good campaign to educate people, but it may also have something to do with phosphorous (see below).
What Should You Do About Didymo?
It is most likely spread by people in their gear, such as boots, boats, and fishing gear. Didymo can survive for a long time in water, even only a drop of water. It can survive in wet gear and is a problem if people take their gear from one river to another without treating if first.
Luckily it is not difficult to clean your gear properly. It only takes a small amount of soap or bleach to kill it. Also, if it becomes dry it dies.
Thus, the motto that all anglers and visitors to the South island know or should know – Check, Clean, Dry.
Check – check to make sure there is no visible algae on your gear.
Clean – clean your gear in soap or bleach.
Dry – dry your gear for at least 48 hours before entering a clean waterway.
What Does Phosphorous Have To Do With Didymo?
Recent research has found a correlation between rivers with low concentrations of phosphorous and didymo. In other words when phosphorous is at a very small amount (less than 2 ppb) didymo grows well.
Likewise, in those rivers with higher concentrations of phosphorous (greater than 2 ppb) didymo didn’t grow. This research seems to imply that the amount of phosphorous in a river may be the key to whether or not didymo grows and takes over the river.
Many of the rivers and lake on the south island have low amounts of phosphorous and most of the rivers on the north island have high amounts of phosphorous.
Scientists are still conducting experiments to see how to apply this and what it may mean for the future spread and control of this freshwater algae. Let’s keep our fingers crossed that this new knowledge may help.