Didymo is a problem that affects all of us, and is a problem that is here to stay unless NZ finds some miracle cure and it gets eradicated.  In the meantime, we all have to do our best to ensure it doesn’t spread from already infected rivers.

The Party Line

The Biosecurity NZ website recommends (with comments from our resident biochemist):

The boat has to be scrubbed or soaked in a solution

The contact time must be at least one full minute

One of the following solutions will do:

Water at 60 degrees ( 60 degrees is impossibly hot, hotter than any hot water tap, and hotter than you could handle even with thick rubber gloves on)

2% solution of household bleach – (The kind of product referred to here contains 5% sodium hypochloride as the active ingredient. For example, Mitre 10 sells a product called “30 sec”. You would measure 200 millilitres of this in a 10 litre bucket. Remember that once diluted, it becomes worthless within a day or so.)

5% solution of salt (½kg in a 10 litre bucket, long contact time, under full immersion)

5% solution of nappy cleaner (possibly ozone based)

5% solution of hand cleaner

(All of the above agents are relatively mild, but are proven to be very effective)

For these measures to be effective, they have to be followed closely. When it says soak or scrub, when it says a TWO PERCENT solution is required, or when a contact time of at least ONE MINUTE is specified, then be aware that effective control will only be achieved when these instructions are followed closely. If an inspector found Didymo on your boat after you had “cleaned” it, you would find it difficult to prove that you had in fact done so.

Notice also that swimming pool granules do not feature on this list. This compound is designed for “slow release”, and does not give a strong enough chlorine solution.

What do you do about it?

Biosecurity New Zealand has enacted a Didymo Controlled Area on the entire South Island. This legally requires risk items (your kayak and gear) to be cleaned when they leave the South Island, before they are used in another waterway. There is no legal requirement to clean between waterways, but knowingly spreading Didymo is an offence under the Biosecurity Act and carries stiff penalties. Biosecurity New Zealand chose not to have a legal requirement to clean between waterways because it simply cannot enforce the thousands of kilometres of lakes and rivers, and the thousands of individual river activities in New Zealand.

Biosecurity will, however, be heavily promoting the need for all users to clean between waterways – everyone, everywhere, everytime. The onus of water protection is on the individual water user - you.

Because total immersion and a one-minute contact time are difficult to practically achieve with some items, the suggestion is to use a much stronger (at least double the recommended concentration) solution, with a good spray-on coverage and/or scrub to ensure the whole kayak is covered.  Pour a good quantity (e.g. the remainder of your 10 litre bucket) into the boat and swill it around thoroughly.  Pump some through your bilge pump, too.  Don’t forget your gear – paddle, lifejacket, helmet, spraydeck, booties – anything that’s not going straight into the wash when it gets home.  If you’re worried about bleach on your colourful kayaking gear, the salt solution offers a reasonable alternative.

Another alternative is to let everything dry – thoroughly – and then leave it for another 2 days before using it on a different river.  Or, if you can possibly manage it, keep one set of gear for a specific river, and keep it separate from the rest of your stuff.

Other reading

See the Whitewater Club's practical information - it's full of good stuff and well researched.