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Didymo: Is this the answer?


Didymo in a South Island river


Over the past few years several researchers including Cathy Kilroy from NIWA and other researchers from Canada and the USA have been pursuing the didymo problem. Their findings have been very encouraging yet have largely gone un-noticed by the angling community.

This ongoing reseach may even provide answers and, in time, even provide workable solutions to inhibit the spread of didymo.

This article is a brief summary of the major findings to date.


Didymo has has turned up en mass in many South Island streams and rivers and even along many lake shorelines. Consequently it has become the bane of anglers and other water users who have been plagued by the rapid spread of this invasive alagae through many waterways.

These algae “blooms” have also appeared in waters in North America and Europe.

The cause of it's spread has, to a large extent, been blamed on itinerant anglers moving it between the waters they are fishing by not drying and/or cleaning their gear properly.

What is didymo

Didymo or "rock snot" as it is colloquially known, is an algae that can spread quickly to cover the rocks in a river or lake shore. It has several strange features in that it prefers waterways which are nutrient poor and seems to be very selective as to where it can proliferate.


As lakes tend to strip the nutirents from water, didymo blooms are often worst at a river's source where it leaves a still water. The Buller, Gowan and Clutha are all good examples of this.

Didymo can survive for a long period of time in a single drop of water and so is easily transported between waterways. That said it is in fact a reasonably fragile organism and can be killed by quite dilute solutions of products such as bleach. Once it is dry or frozen it also dies.

The North Island

While it is found in many South Island rivers there have been notable exceptions to its spread. It was observed early on that spring creeks could not support didymo and most surprisingly it has never appeared in the North Island of NZ.

There has been much speculation as to why it has not been found in our northern waters despite the fact that there is plenty of evidence that anglers do move freely between the islands and it is doubtful to say the least that all have scrupulously adhered to the "check, clean, dry" message.

The response to date


So far the response to the didymo threat has been rather haphazard. Biosecurity NZ ran a comprehensive response from 2004 to 2008. It terminated in 2008 when it became clear that stopping the spread was not feasible.

Felt soled boots were found to be great ways to spread didymo. Water could stay in the felt for up to two weeks and didymo needed only one drop of infected water to be transferred from one water to another. Consequently felt soled boots were banned by Fish & Game causing many an angler to take an undignified glide down the river they were fishing. (Many other regions around the world have followed Fish and Game's example and banned felt soles).

But this ban on felt only applied to anglers. Other water users such as white-baiters, rafters, kayakers and others could continue to use felt soles if they were not targeting trout or salmon.

Allied with the ban on felt there was a massive campaign to educate the public with a "check, clean, dry" message being touted around the country. Wardens were stationed at the interislander ferry terminals (but not airports) to check that infected tackle was not being moved between the islands.

The new research findings

To briefly summarise several years research, Kilroy et al found that in waterways where the amount of phosphorus in the water was lower than 2 parts per billion, didymo flourished.

In waterways that had higher than 2 parts per billion of phosphorus present, the spread of didymo was suppressed or absent altogether. It appears from the evidence to date that phosphurus in quite minute quantities in waterways will stop didymo from flourishing.

For more details about the research see “Didymo in New Zealand: Ten years on by Cathy Kilroy

What this might explain

If it is proven that phosphorus (or lack of) in a waterway is the trigger for didymo then it will provide an explanation as to why didymo has not established a presence in North Island rivers. Most (though not all) North Island rivers have a high percentage of volcanic material in them which in turn means a higher proportion of phosphorus.

If phosphorus is the trigger, then the vast majority of North Island waters cannot and will not sustain didymo. They are not in danger of becoming infected. This alone will be a huge relief to every angler in the North Island and worth celebrating.

More details about the effects of phosphorus can be found in the article "Geographic pattern of phosphorus in river water"

Further experiments

Kilroy and colleagues carried out a small-scale experiment in the Ohau river which was heavily infected with didymo. Working in a small side braid, they found that cover by didymo was greatly diminished following application of small amounts of phosphorus by diffusion. Yet in untreated sections of the braid, didymo continued to flourish as usual.

This demonstrated that the pattern they observed of didymo blooming only in rivers with low phosphorus was unlikely to be just coincidence.


The Ohau river used for experimentation showing the didymo infestation

Where to from here

As with all such research, no matter how promising the first results, it will be necessary to proceed with caution.

Questions such as what the impact will be of even a minute amount of phosphorus added to a living ecosystem needs to be examined. But as phosphorus naturally occurs in most if not all of the North Island waterways, this is probably not a major issue.

Kilroy also notes that we are still at the early stages of understanding the effects of phosphorus on didymo. We don’t know exactly how increasing phosphorus levels prevents didymo blooms, and we don’t know why didymo is often not found, even in small amounts, in rivers with high phosphorus levels.

The only way to learn more is to carry out repeat experiments in different waterways, to see if adding phosphorus has a general effect of inhibiting didymo blooms.

It is of course more than just emptying a container of phosphorus into a river at source and watching the magic results. There is a lot to do to work out the most effective way of applying phosphorus and to monitor the effects on both didymo, other algae and the eco-system.

The effect on fish Until recently there was little hard evidence as to what the effect of didymo blooms had on aquatic lie and trout in particular. The study by Jellyman and Harding (2016) has shown that didmo blooms have a profound effect on trout and other aquatic life. Read more>>
The role for anglers

Whatever happens from here, this is a major breakthrough and every angler should be happy with what has been discovered to date.

Anglers as a group need to be very supportive of NIWA and the work they are doing as this may be the beginning of reversing the spread of this noxious pest from our waterways and returning rivers and lakes to something we can be proud of. It is probably unrealistic to believe that we will ever rid our waterways of this noxiuos weed, but we may now have a tool that can control it.

It could even be the signal to relook at the felt sole ban in the North Island at least.

Further information

We will of course be following developments with great interest and want to ensure that this important study carries on and does not just languish in an academic library.

To see more detail see "The Didymo Story": Bothwell, Tayor and Kilroy

Important note

While this article has been about didymo and how there may now be cause to think we can control it, anglers must remain vigilant that didymo is only one of a number of invasive species that have entered our waterways.

When moving around we must remain aware that other weeds can also be easily transferred between waters and the Check, Clean, Dry message still should be applied when moving from one fishery to another.

Thanks and acknowledgement I would like to thank Rhys Barrier from Nelson Fish & Game for bringing this important research to my attention and especially to Dr Cathy Kilroy from NIWA for her assistance and patience in checking and amending this article so that it faithfully summarised her findings.
Further reading

If you would like to read more see:







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