Natural waterfalls and instream structures can be used to protect native species and habitats by stopping invasive fish moving up and downstream.

Providing fish passage is beneficial for most New Zealand fish species. However, some natural waterfalls and barriers in key locations can protect native species and biodiversity hotspots by preventing access for invasive fish like koi carp, rudd and gambusia.

For example, non-migratory native fish that live all their life in streams and rivers don’t need to migrate to the sea and back. Invasive species like brown trout, rainbow trout and sometimes kōaro and eels can threaten these species. Instream structures can help protect their populations and the ecosystems where they live.

Built or natural barriers would be most advantageous to prevent extinction of threatened non-migratory species such as lowland longjaw galaxias, Eldon’s galaxias, Dusky galaxias, Clutha flathead galaxias, Teviot galaxias and Nevis galaxias.

See section 6 of the New Zealand Fish Passage Guidelines for more information about species that would benefit from built or natural waterfalls.

Natural barriers like waterfalls and built barriers (designed to keep out some or all species), work well to protect biodiversity hotspots by preventing the target fish species from swimming, jumping or climbing past the structure. Well-placed barriers can stop their spread through a river network or into critical habitats.

Designing an artificial barrier

Planning a built barrier should include the feasibility of removing any unwanted fish that are already present upstream, preventing future invasions, monitoring the populations and maintaining the structure. The design requires specialist advice from fish ecologists and should be undertaken in consultation with DOC. Additional approvals like resource consents may also be required.  

Barriers in waterways with highly erodible soils, steep stream beds or loose substrates should be avoided where possible. These features make it more likely that the barrier will be damaged by flooding or other events and not provide protection to upstream habitats in the longer term.

Regular checks to make sure unwanted fish are being kept out by the structure are also important.

See section 6 of the New Zealand Fish Passage Guidelines for more information and the design features required when building barriers to protect native species and habitats.


Monitoring is the only way to understand how well a structure is working and ensure that any reduction in fish passage is not harming upstream or downstream communities. Monitoring is especially important when:

  • rare or high value fish communities or ecosystems are upstream of the structure
  • more than one structure is in a waterway, and could be causing cumulative effects
  • unproven designs or fixes are being used
  • a proven design is being used in a new situation
  • barriers are being used to manage the movement of invasive species.

See Chapter 7 of the New Zealand Fish Passage Guidelines for more information about monitoring techniques and methods for evaluating the success of fish passage in different circumstances. The fish passage assessment tool can also be used to monitor the structure and the risk to fish passage.

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