Didymo

The invasive alga didymo (Didymosphenia geminata) is being found in more and more South Island rivers since it arrived from somewhere in the northern hemisphere.  At this stage it hasn’t been found in the North Island but it is very easily spread and there is a very real risk of it arriving here.

Didymo is a freshwater diatom (alga).  It attaches itself to rocks and stones in stream and river beds and eventually forms a thick brown mat completely smothering them.  Although it looks slimy, it feels more like wet cotton wool.

What are the effects of Didymo?

The effects on our rivers is likely to be dramatic as it changes the river ecology, affecting native fish, plants and insects.  It is likely to have a big impact on trout fisheries, hydro power schemes and irrigation.  The brown carpet of didymo is also very unsightly and will make swimming an unpleasant experience.

At this stage Biosecurity New Zealand is not able to explain how didymo arrived here or how to get rid of it.  But what is known is that its preferred habitat is clean, fast-flowing, stony rivers i.e. many of the rivers and streams of the Bay of Plenty.

What is also well understood is that didymo is extremely easily transported as a single microscopic cell in a single drop of moisture from one river to another.  It could easily be spread on fishing gear, waders, kayaks, buoyancy aids, boots, boats, trailers and any other equipment that has been in contact with river water.  And remember that all rivers should be treated as if they are infected as in its early stages didymo is invisible to the naked eye.

How can you prevent the spread of Didymo?

Preventing its spread is very simple.  All river users should always practice good river hygiene.  Either restrict use of equipment to a single river or, if this is not possible, check, clean, dry when leaving a river.

1. Check: Remove all visible bits of debris from clothing and equipment.  Leave them at the river.
2. Clean:  Clean all equipment with a detergent solution.
3. Dry:  If cleaning is not practicable (ie. livestock), allow to dry completely and then wait another 48 hours before contact with another river.

Cleaning your gear

Use one cup of ordinary dishwashing liquid in five litres of water (2 cups to a bucketful).  Dunk equipment in the bucket and/or use a small spray bottle to spray solution all over equipment.

Jet boats jet skis, and outboard motor boats 

All boats and trailers should be cleaned thoroughly both inside and out for at least one minute with decontamination solution.
 
In addition, special attention should be given to the following components:
 
Jet boat grate: Manually remove visible clumps of algae from the grate and flush the system with a decontamination solution.
 
Jet unit: Open ball value at bottom of sand trap, remove any residue and flush system with a decontamination solution.
 
Outboard motor: The cooling system should be flushed out with the decontamination solution for the specified time. It may then be flushed again with clean fresh water that has come from a town water supply.
 
Boat interior including anchor recess: Remove excessive water by removing bungs and then wash interior with a decontamination solution.
 
Bilge pump: Flush interior of boat with a decontamination solution and then use the bilge pump to expel residual water before bungs are opened. This ensures that the bilge pump is flushed with the solution, and that residual water within the pump will be free of live didymo cells.
 
Mats, carpet (including carpet on the trailer), anchor rope and other absorbent components: thoroughly soak with decontamination solution allowing extra time for the solution to fully soak through the item.
 
Drying is an acceptable alternative method, provided that all components are completely dry to the touch, inside and out, and then left dry for at least another 48 hours before entering a different waterway.
 
If you do not want to decontaminate your gear, you should restrict use to a single waterway.

    
Other watercraft 

Kayaks, canoes, dinghies and associated equipment and gear should be cleaned thoroughly both inside and out with a decontamination solution. Scrub or spray all exterior parts of the craft with a decontamination solution, ensuring surface contact for at least one minute. Then fill the interior of the craft with the decontamination solution, place all associated equipment, gear and clothing used in the boating activity into the craft, immerse completely and soak for as long as needed to thoroughly penetrate all absorbent items.
 
The equipment may then be rinsed using water that has come from a town water supply.
 
Drying is an acceptable alternative method, provided that all components are completely dry to the touch, inside and out, and then left dry for at least another 48 hours before entering a different waterway.
 
Freezing small items is another option.
 
If you do not want to decontaminate your gear, you should restrict use to a single waterway.

    
Vehicles 

Vehicles should be cleaned thoroughly with a decontamination solution, including spraying the underside of the vehicle and any other parts of the vehicle that have had contact with river or lake water. Commercial carwashes with an underside spray would be suitable. For tyres, see the following tyre section.
 
The decontamination solution should be left on for at least one minute and may then be rinsed off with water that has come from a town water supply.
 
Drying is an acceptable alternative method, provided that all components are completely dry to the touch, inside and out, and then left dry for at least another 48 hours before entering a different waterway.
 
If you do not want to decontaminate your gear, you should restrict use to a single waterway.
    
Tyres 

Tyres which have been backed into streams or used for stream crossings must be carefully checked for clumps of algae and other debris within the treads and then scrubbed, soaked or sprayed and flushed with a cleaning solution for the required treatment time. Tyres may then be rinsed using water that has come from a town water supply.
 
Drying is an acceptable alternative method, provided that all components are completely dry to the touch, inside and out, and then left dry for at least another 48 hours before entering a different waterway.
 
If you do not want to decontaminate your gear, you should restrict use to a single waterway.

Felt-soled waders 

The felt soles of these waders are impossible to clean adequately and it is recommended that they are not used in any river.

If you see anything in a river that you think might be didymo, leave it where it is and call Environment Bay of Plenty on 0800 ENV BOP (368 267) and ask to speak to a pest plant officer or call Biosecurity New Zealand on 0800 80 99 66.