Fixing barriers to fish passage

Many instream structures across New Zealand are blocking fish passage and impacting our native freshwater species. Often small, low-cost changes can be made to barriers in streams to improve fish passage. 

The Freshwater Fisheries Regulations require all culverts and fords to be maintained to prevent them impeding fish passage. Dams and diversions may be required to have a fish facility to allow fish to pass. (A fish facility is any structure or device, such as a fish pass or fish screen that is inserted in or by any waterway, to stop, allow or control the passage of fish through, around, or past any instream structure.) Approval is required if any structural change to an existing fish facility is proposed.

See Fish passage authorisations.

The process to fix, replace or remove a structure in a waterway begins with confirming which native fish may use this waterway and checking the New Zealand Fish Passage Guidelines.

The site should then be assessed, changes planned and appropriate approvals gained – the use of specialist fish ecologists during this process is important. Careful consideration needs to be given to which option is best for each structure. In some limited situations, it might be best to retain the barrier if it is protecting threatened species or biodiversity hotspots.

It is important that best practice guidance is followed, and if new or unproven remediation options are attempted or proposed, monitoring is critical to ensure success or find out if changes need to be made.

Removing or replacing a barrier

The best way to restore fish passage is to remove the instream structure whenever possible. Contact your local regional council to check who owns the structure and if any approval is required.

Modifying a barrier

If a barrier cannot be removed or replaced, it can usually be modified to allow fish to pass. Deciding on the best modification depends on the existing structure, the cost, accessibility, how it is impeding fish passage and the ecological objectives for the site. Some fixes will only improve passage for some species, not others. It is also important to choose a proven method where possible to fix fish passage and install it correctly. Remediation inevitably requires compromise and trade-offs between competing values. Achieving unimpeded passage as required by the Freshwater Fisheries Regulations. This will likely only be achievable through structure removal or replacement. In the short- to medium-term, there are a range of remediation options available that can improve fish passage.

Raising the water level to reconnect structures (backwatering)

Structures in streams can have water velocities that are too high and impede fish passage. They can also become overhanging (perched) over time, which creates higher water velocities and vertical drops.

One way to fix these problems is by backwatering: moving or adding rocks downstream (usually in a V-shape) to raise the water level and reconnect it with the water upstream. Backwatering can also reduce or remove the vertical drop and slow the water velocity to help reinstate free passage up and downstream.

Ramp fishways

Ramp fishways are a preferred solution to fix vertical drops that impede fish passage. River wide rock-ramp fishways are the best design for overcoming small vertical barriers.

Nature-like rock ramps usually have rocky ridges across the flow that create pools where fish can rest as they move upstream.

Artificial ramps made from rocks or concrete with a roughened surface can also be built on the downstream face of a barrier.

Baffles

Baffles can be used on the base of culverts or the face of weirs to reduce the water velocity, which allows fish to pass. 

There are a range of baffle designs available. Spoiler and vertical baffles are the recommended option for enhancing fish passage in culverts. These baffles break up the flow and create places where fish can rest on their way upstream. If these and other types of baffle are used, we recommend that monitoring is carried out to inform maintenance and provide more information about their use.

Mussel spat ropes

Mussel spat ropes can be used to enable fish passage in culverts with a diameter of less than 1.2 m. A minimum of two ropes should be used, to make ‘swimming lanes’ between them – more ropes will be needed for larger culverts. The ropes should be secured upstream and downstream to ensure they are tight and flush with the base of the culvert along its entire length, not loose at one end or out of the water.

Non-looped ropes are recommended to reduce the likelihood of debris becoming snagged. Knots (half hitches) can be tied along the sections of rope to break up the flow and create rest areas for fish.

Bypass structures

Bypass structures may be useful when fish passage cannot be restored by making structural changes to a barrier. There are two main types of bypass structure:

• technical fishways, like vertical slot fishways and pool and weir fishways. These structures have been widely used internationally but are not yet common in New Zealand.
• nature-like fishways mimic natural stream characteristics in a channel that bypasses a barrier. They can be used for all types of instream structure types, but generally need more space than technical fishways. Because they mimic natural stream conditions these structures usually enable a wide range of fish species and life stages to pass.

Fish friendly flood or tide gates

Flood or tide gates that are automated or self-regulating and stay open for as long as possible are recommended to allow fish to pass through. Fixes like adding ‘fish- friendly’ flood gate attachments (stiffeners) should be considered to improve passage at flood and tide gates that are not automated or are impeding passage.

Flood pumping stations

Making a pump station ‘fish friendly’ does not always require a full rebuild. Nowadays, a range of fish-friendly pumps are available that can replace existing pumps.