Protect our waterways from freshwater pests

Pānui Taiao - July 2021

Fresh waterways around Tairāwhiti are treasured by us all, and it’s up to us to protect them from freshwater pests.

While we're out enjoying the beautiful rivers and lakes throughout our region, we could be unknowingly spreading plant and fish pests between our waterways.

All it takes is one tiny fragment of weed, one droplet of water, or a single fish egg. If aquatic pests hitch a ride on your gear they can spread to previously unaffected waterways – choking our lakes and rivers, making them murky, and out-competing native wildlife.

We can stop the spread by using the Check, Clean, Dry method whenever we travel between waterways. Here is some information on common  freshwater plant and fish pests, including didymo, and about the Check, Clean, Dry method.

What are the threats?


Tairāwhiti along with the whole North Island is free from didymo – or as it is commonly known, ‘rock snot’. And we need to keep it that way.

However, didymo is widespread throughout the South Island where it continues to decimate fresh waterways.

When it blooms, didymo creates a thick brown ‘snotty’ layer that smothers everything. This bloom devastates the aesthetic and recreational value of rivers. It could cause significant damage to fresh water native fish, trout, plants and insects in our region. Once didymo has established itself in a waterway, it is nearly impossible to manage or eradicate.

Didymo can be spread by just one microscopic algal cell in a single drop of water: which is why we all need to be extremely vigilant with Check, Clean, and Dry when going from one waterway to another. Water users are legally obliged to prevent the spreading of didymo.


Lagarosiphon or Lake Weed, was introduced as aquarium plant, but when it got into Aotearoa’s waterways, it wreaked havoc.

Lagarosiphon is widespread throughout the North Island and is actively controlled in Lake Waikaremoana. Leaves are dark green and curl downwards back towards the stem and are arranged spirally around the stem.

It grows quickly to depths of up to 8m, forming dense mats that block water flow and displace native plants. It also impacts water quality and oxygen levels, and inhibits recreational use and hydroelectricity generation.

Left uncontrolled, large beds of weed form and drift onto shore, leaving ugly heaps staining our waterways.  Lagarosiphon is spread through tiny plant fragments that easily break off and are carried through boating and other activities to establish in new areas.

This is why we must check all of our gear for plant fragments and remove them before when we leave a waterway.
hogwart 2


Hornwort is ranked as Aotearoa’s worst aquatic weed by NIWA. It’s found in over 30 different lakes in the North Island, and is believed to have been eradicated from the South Island.

It’s currently contained in Tairāwhiti.

Its leaves are finely divided and feel rough to the touch. Hornwort rapidly invades waterways and its dense growth crowds out native species. It's the tallest growing (up to 10m) and deepest growing (to depths of 12m) weed in New Zealand.

Because it doesn’t have roots, weed maths can float to further depths. It's extremely tolerant to temperature, light, nutrient and quality variations. It crowds out all other species in its wake, including native species and other introduced pests such as lagarosiphon.

Other impacts include blocking waterways, impeding drainage, and choking hydro turbines.

Hornwort can spread through a single piece of broken stem, making it great at invading new areas. We must Check, Clean, and Dry all our gear to prevent unknowingly spreading hornwort.


Gambusia or mosquito fish, are a pest fish found in fresh waterways. They are widespread through New Zealand are found in Tairāwhiti waterways, but are under a program of progressive containment by Council’s biosecurity team. 

Gambusia are tiny little fish about the size and look of whitebait, only thicker and fatter. They were intentionally introduced for mosquito control – however that did not work here. 

These fish are prolific reproducers and can easily take over a waterway.

They're aggressive to native fish, as well as out-competing them for food and have been known to eat native fish eggs. Their high tolerance to poor water quality, high salinity and a range of water temperatures make them especially dangerous.

A pregnant fish is needed to spread the population as they give birth to live young. Using the Check, Clean, Dry method between waterways stops the spread of these pest fish.


Catfish are a nasty freshwater pest fish. They were intentionally introduced and are now widespread throughout North Island lakes and slow-flowing streams.

Catfish are not found in Tairāwhiti and it’s up to us to keep them out. 

Catfish can survive in a range of conditions and for a long time out of water, making them difficult to kill. They are best known for their distinctive whisker like barbells around the mouth, with a dark brown to green back and pail underside.

They have a number of impacts, many of which are still being investigated: including reducing water quality, preying on native fish as well as eating and outcompeting the native koura (freshwater crayfish).

Catfish can spread to new waterways through fish eggs unknowingly carried by us in our freshwater activities, so we must make sure to Check, Clean, and Dry all our gear.


We are required by law to kill catfish upon capture and not return them to the water. To report a suspected sighting of Catfish in Tairāwhiti, contact Council.

Our responsibility

It's up to us to stop the spread of these freshwater pests into Tairāwhiti. The solution is simple Check, Clean, Dry.

Check: Remove any plant matter. Check anything’s that’s been in contact with the water – especially things like the tread of your shoe.
Remember - if it’s wet, it’s a threat.  Leave any plant fragments far away from the water’s edge.

Clean: soak or scrub your all gear and equipment with detergent for at least a minute.
Make sure the item is fully wet – without air-pockets or bits the water can’t get into. Soak absorbent material for longer to make sure it’s wet right through. An alternative to cleaning in freezing until solid.

Dry: wait until the gear and equipment is dry to the touch, then leave to dry for at least a further 48 hours to make sure any invisible pests are completely dead.
Using quick-dry or non-absorbent equipment where you can will save on drying time.

When you do need to Check, Clean, Dry?

One waterway

If you’re only using your gear and equipment in one waterway, you don’t’ need to Check, Clean, and Dry.

Moving between waterways after several days. If you wait several days after using one waterway before moving into another, you need to Check and Dry. Check your gear is clear of plant debris and Dry to the touch for at least 48 hours.

Moving between waterways within a few days: This is the high-risk time for spreading pests. Check, Clean, and Dry every item that has had contact with the water.

With your help, we can stop freshwater pests from spreading.

Further Links

Ministry for Primary Industries - Check, Clean, Dry

Fish & Game - Check, Clean, Dry

Department of Conservation - Biosecurity

To find out more about pests in our region