Otago’s rivers, streams, and waterways support a remarkable diversity of native fish species, many of which rely on being able to move freely through freshwater habitats to feed, migrate, spawn, and survive.

The term ‘fish passage’ refers to the ability for fish and other aquatic species to move through rivers, streams, wetlands, drains, and other connected freshwater habitats in the way they naturally would without man-made structures and barriers.

Many of Otago’s native fish species need to move through waterways at different stages of their life. Some species migrate long distances between freshwater and the ocean, while others move locally between feeding, spawning, refuge, and seasonal habitat areas within river systems.

Structures such as culverts, weirs, flap gates, fords, and poorly designed stream crossings can sometimes create barriers that prevent fish from moving upstream or downstream. Even relatively small barriers can significantly affect the life cycle of some native fish species.

Maintaining fish passage helps support healthy freshwater ecosystems by:

• allowing fish to access important habitat;
• supporting migration and spawning;
• maintaining population connectivity;
• improving ecosystem resilience;
• helping protect native freshwater biodiversity.

Otago’s waterways support a remarkable diversity of indigenous fish species, 33 in total, including migratory whitebait species, tuna (eels), kōaro, bullies, and a number of rare non-migratory galaxiid species found nowhere else in the world.

Many of these species rely on connected freshwater habitats to complete different stages of their life cycle. Some migrate between freshwater and the ocean, while others move between stream headwaters and large inland lakes, pools, tributaries, wetlands, and seasonal refuge habitats within river systems.

Otago also contains a particularly high diversity of threatened non-migratory galaxiid species. These fish are often isolated within small headwater streams and fragmented habitats, making them especially vulnerable to habitat loss, stream modification, and introduced predatory fish such as trout.

Across Otago, more than 14,500 known instream structures such as culverts, weirs, flap gates, and fords have the potential to affect fish passage. In some catchments, barriers can prevent fish from reaching large areas of upstream habitat.

Improving fish passage helps:

• reconnect fragmented habitats;
• support native fish populations;
• improve freshwater ecosystem health;
• increase resilience during floods and droughts; and
• help protect Otago’s unique freshwater biodiversity.

Fish passage is not always about removing every barrier. In some parts of Otago, carefully managed barriers can help protect Otago’s threatened non-migratory galaxiids from introduced predators such as trout. Understanding where fish should, and should not, be able to move is an important part of freshwater management within the region.

Fish passage requirements for structures installed on or after 3 September 2020 are generally managed differently from structures that already existed before this date.

This distinction exists because many older culverts, weirs, flap gates, and stream crossings were installed before modern fish passage requirements existed. Across Otago and the rest of New Zealand, many of these structures now create barriers that affect fish movement and freshwater habitat connectivity.

Current freshwater management frameworks generally focus on:

• preventing new barriers from being created through fish-friendly design requirements for new structures; and
• progressively improving existing barriers over time where practical and achievable.

New Structures

Key points:

• Fish passage should be considered early during project design.
• Poorly designed structures can create long-term barriers.
• Fish-friendly design is often easier and cheaper to incorporate during initial construction.
• Early planning can help avoid future compliance and remediation costs.

Structures installed on or after 3 September 2020 are generally managed as new structures under the NES-F and should be designed and installed in a way that allows fish and other aquatic species to move naturally through waterways.

Under current freshwater management frameworks, many new structures are expected to incorporate fish-friendly design principles from the outset. This helps prevent new fish passage barriers from being created and reduces the need for future remediation works.

Existing Structures

Key Points

• Existing barriers are often prioritised based on ecological benefit and practicality.
• Small remediation works can sometimes significantly improve fish passage.
• Maintenance issues such as debris build-up and scour can also create barriers.
• Not every existing structure requires immediate replacement.

Structures already existing before 3 September 2020 within Otago waterways were installed before modern fish passage guidance and requirements existed. Some of these structures may now create barriers that affect fish movement and freshwater ecosystem health.

In many cases, existing barriers can be improved over time through remediation, maintenance, replacement, or small design changes that improve fish movement.

Strong visual options for this section include:
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split infographic comparing new vs existing structure pathways;
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examples of compliant and non-compliant culvert designs;
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before-and-after remediation imagery;
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simplified policy flow diagrams;
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catchment-scale examples of legacy barriers; and/or
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examples showing how small remediation works can improve fish movement.

Fish passage barriers can occur in many different forms, ranging from large dams and weirs through to relatively small culverts or stream crossings.

Some barriers completely prevent fish movement, while others may only affect certain species, life stages, or flow conditions. In many cases, barriers are not intentionally created, but instead occur because structures were poorly designed, incorrectly installed, undersized, damaged, or not regularly maintained.

Common fish passage barriers found within Otago waterways include: perched culverts, excessive water velocity within culverts, shallow water depths, flap gates and tide gates, weirs and vertical drops, debris blockages, undersized or damaged structures, poorly designed ford crossings, and outlet scour and erosion.

Suggested Annotated Barrier Types (Part C)

Perched/overhanging Culverts A culvert outlet sitting above the natural stream level can create a vertical drop that many fish species cannot climb.
Excessive Water Velocity Fast, smooth water flowing through steep or undersized culverts can prevent fish from swimming upstream.
Shallow Water Depth Some structures may not maintain enough water depth for fish to move through during low flows.
Flap Gates / Tide Gates Flap gates can create physical barriers and may also alter natural flow conditions and fish behaviour.
Debris Blockages Branches, sediment, vegetation, and rubbish can reduce water depth and create additional barriers.
Weirs and Vertical Drops Vertical structures can prevent fish from moving upstream, particularly smaller native species.
A Good Culvert A fish-friendly culvert is typically wide enough and/or embedded sufficiently to allow natural stream substrate and flow conditions to continue through the structure.

Many fish passage barriers are not obvious at first glance. A structure may appear functional from an engineering or drainage perspective while still creating a significant barrier for native fish species.

In many cases, fish passage issues are caused by relatively simple problems such as outlet drops, shallow water, excessive water velocity, debris build-up, or poor maintenance.

The examples below provide some common warning signs that a structure may be affecting fish passage.

These signs do not replace a formal fish passage assessment, but they can help landowners and contractors identify when further advice may be needed.

In many cases, fish-friendly structures are simply designed to allow water and fish to move through them as naturally as possible.

A good general rule is that a structure should aim to behave like a natural part of the stream rather than a barrier or artificial chute.

Fish passage issues often occur when structures:

• create vertical drops;
• increase water velocity;
• reduce water depth; or
• interrupt the natural stream bed.

Good fish-friendly design generally aims to:

• maintain natural stream conditions;
• maintain water depth;
• reduce excessive water velocity; and
• allow fish to move naturally upstream and downstream.

Incorporating fish passage into the initial design phase is often significantly easier and cheaper than retrofitting structures later.

For more detailed technical guidance, refer to the external resources below. 

Many existing fish passage barriers can be improved through removal, replacement, remediation measures, maintenance works, or upgrades.

Where a structure is no longer required, removal may provide the most effective fish passage outcome. Where a structure remains necessary, replacement with a fish-friendly alternative may be appropriate. In many other situations, existing fish passage barriers can be improved through relatively simple remediation measures, maintenance works, or upgrades.

In some situations, small changes such as reducing outlet drops, improving water depth, removing debris blockages, installing baffles, embedding culverts, or adding rock ramps can significantly improve fish passage outcomes.

The most appropriate remediation approach will depend on the structure type, stream characteristics, fish species present, surrounding habitat, and practical site constraints.

While fish passage is generally an important part of maintaining healthy freshwater ecosystems, there are some situations where carefully managed fish barriers can help protect vulnerable native fish populations.

In parts of Otago, some threatened non-migratory galaxiid species persist within isolated headwater streams and refuge habitats that remain protected from introduced predatory fish such as trout. In some situations, natural or artificial barriers can help prevent trout from accessing these habitats and impacting vulnerable native fish populations.

Because of this, improving fish passage is not always as simple as removing every barrier within a catchment. In some locations, barriers may provide important ecological protection for threatened native species that do not migrate between freshwater and the sea.

Determining whether a barrier should be retained, modified, or removed can involve complex ecological considerations including:

• fish species present;
• upstream and downstream habitat quality;
• the presence of introduced fish species;
• catchment connectivity;
• and long-term ecological outcomes.

Any proposal to modify or remove a structure that may currently limit trout movement into sensitive native fish habitat should generally be undertaken in consultation with ORC, DOC, and suitably qualified freshwater ecology specialists.

Works within rivers, streams, drains, wetlands, and other watercourses may require resource consent or other approvals depending on the activity being undertaken and the potential effects on freshwater habitats and fish passage.

Fish passage requirements are commonly relevant for activities involving:

• culverts;
• bridges and stream crossings;
• flap gates and weirs;
• stream diversion works;
• reclamation or realignment works; and/or
• other instream structures.

If works have the potential to affect fish passage or freshwater habitat, seeking early advice from ORC and/or suitably qualified freshwater specialists may help avoid delays, redesign costs, or future remediation requirements.

Fish passage requirements for new structures installed after 3 September 2020 are generally managed differently from older existing structures. In some situations, structures that do not meet permitted activity standards may require resource consent, fish passage assessments, monitoring, and/or ongoing reporting requirements.

Users should refer to ORC’s existing consenting and regulatory webpages for detailed approval pathways and current requirements.

A range of national, regional, and industry resources are available to help landowners, contractors, consent applicants, and freshwater practitioners better understand fish passage requirements, fish-friendly design approaches, and practical remediation options.

Users should always ensure they are referring to the most current versions of any guidance documents, standards, or regulatory information.