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Introduced Weeds and Fish

Threats to Ecology
Cultural Importance

Introduced species are not native to a place. They often cause damage to the ecosystem.

Pest fish

Introduced fish can harm freshwater environments and native species. Introduced fish can prey on native fish, change the habitat, spread disease, and take over native fish communities.

You can help

  • Don’t put any fish into freshwater without first getting the proper permission.
  • Wash all your gear down at the end of the day and before you go to a new place - including trailers, rods, nets and waders. (See Check, Clean, Dry below).
  • If you see people releasing fish, contact your local Fish & Game New Zealand office or Department of Conservation office as soon as possible.
  • Empty your aquariums into the garden, not into drains or streams.


Didymo is also known as 'rock snot'. It was recently found in New Zealand and so far has been found in several rivers in the South Island.

Where does it grow?

Didymo lives in flowing, clear, cool rivers.

What damage could it do?

Very large blooms could change the trout habitat by covering the stones and gravels in riverbeds. This would affect insects (mayflies, caddisflies, and stoneflies) that trout and other animals like whio feed on.

Didymo would also make angling difficult because it creates dirty water.

Stopping the spread

Getting rid of pest fish and plants is very hard. The only solution is to keep them out. Whenever you go into a new lake or river, always do the following:

  • Check for wet gear that has been used in the water.
  • Clean all gear that has been used in the water.
  • Dry all gear that has been used in the water.

Audio Māori keywords: 

Find out more: DOC's role in managing threats to freshwater ecosystems.

Some intoduced fish species such as koi carp can degrade water quality. Image: LEARNZ.

Didymo is known as 'rock snot'. Didymo can harm the river habitat. Image: LEARNZ.

Check and remove any pieces of weed or fish eggs from gear that has been used in the water. Image: LEARNZ.

Always clean your gear properly after being in freshwater environments. Image: LEARNZ.

Make sure all your gear is dried well. Image: LEARNZ.

Threats to Ecology
Cultural Importance