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 15,000 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 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.

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.

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. 

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.

Information through our consents and compliance page on fish passage and instream structures is a great place to start. We provide rules and guidance on how to help our native fish population thrive. 

External resources