Fixing barriers to fish passage
IntroductionModifying or removing barriers in streams can restore fish passage and provide access to habitat upstream and downstream.
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 restore fish passage. Mussel spat rope can fix a small perched culvert, for example, and a small fish pass beside a weir can allow fish to move up and downstream.
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.
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 recommended. 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 remediation options are attempted or proposed, monitoring is critical to ensure success or find out if changes need to be made.
See section 5 of the New Zealand Fish Passage Guidelines and the summary factsheet How to fix fish passage barriers at existing instream structures (PDF, 754K).
Removing 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 or replacing 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. It is also important to choose a proven method to fix fish passage and install it correctly.
|Common problems||Possible fixes|
|Removal||Replacement||Backwatering||Ramp fishway||Baffles||Mussel spat ropes||Bypass structures||Fish friendly flap gate|
|Excessive fall height||Y||Y||Y||Y||?||Y|
|High water velocities||Y||Y||Y||Y||Y||?|
|Insufficient water depth||Y||Y||Y||Y||?|
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.
See the New Zealand Fish Passage Guidelines.
See also Lessons Learnt case study 3: Placement of downstream rock weir and installation of culvert baffles to enable salmonid passage.
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.
See section 5.2.3 of the New Zealand Fish Passage Guidelines.
See also Lessons Learnt case studies 1, 7 and 11:
- retrofitting weirs to create fish ramps
- installing floating fish ramps to provide passage
- installation of a rock ramp and baffles to restore fish passage at a perched culvert.
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 baffles are the recommended option for enhancing fish passage in culverts with a diameter greater than 1.2 m. These baffles break up the flow and create places where fish can rest on their way upstream. If other types of baffle are used, we recommend that monitoring is carried out until there is more information about their use, their methodology is proven and appropriate guidance has been developed.
See section 5.3.3 of the New Zealand Fish Passage Guidelines.
- installation of a rock ramp and baffles to restore fish passage at a perched culvert
- placement of downstream rock weir and installation of culvert baffles to enable salmonid passage
- installation of a fish pass and baffles to promote trout passage through long perched culverts
- Installation of flexible baffles to restore a very long culvert.
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.
See Section 5.3.3 of the New Zealand Fish Passage Guidelines.
For more information see:
- Appropriate use of mussel spat ropes to facilitate passage for stream organisms (specific guidance about installing mussel spat ropes).
- Lessons Learnt case study 2, 6 and 10:
- installation of mussel spat rope to retrofit a perched culvert.
- fish passage facilitation by increasing flow and installing mussel spat rope on a dam spillway.
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.
See section 5.3.4 of the New Zealand Fish Passage Guidelines.
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 should be considered to improve passage at flood and tide gates that are not automated or are impeding passage.
See section 5.3.5 of the the New Zealand Fish Passage Guidelines.
See also Lessons Learnt case study 5: Fish friendly gate installation facilitates upstream fish passage.
Improving fish passage at pump 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.
See Improving fish passage at pump stations (PDF, 543K) for advice.
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.