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Changing the Current: Fish Passage in Aotearoa

Changing the Current: Fish Passage in Aotearoa

Imagine you’re heading home for the day. There is a lovely stretch of bush to walk through to get there. Even though it is a hot day, there are plenty of shaded places to stop to have a drink or a snack and to escape the sun. As you round the corner, a towering mountain suddenly stands in your way. You’re not equipped to climb it and you didn’t expect such a challenge. But if you can’t climb it, you’ll never get home again.

This is the situation that many of our native fish face. They are quiet and cryptic, and their struggle often goes unnoticed. In New Zealand we have over 50 species of native freshwater fish and over 70% of them are threatened or at risk. Some key reasons for this are pollution, loss of habitat and the challenges these fish face simply to move between habitats.

Many Aotearoa fish species are diadromous, which means they must move between salt and fresh water to complete their lifecycle. This includes some species which climb their way to different parts of a catchment. For example, banded kōkopu have been known to climb up steep 20-metre waterfalls to reach their destination! Over time barriers like dams, weirs and culverts have popped up in our water ways, restricting this vital movement. Sometimes, even just a small lip on a pipe means small fish fall at the top of their climb and can’t finish their journey. This happens across the country but is also happening in our urban areas, such as Wellington.



Aotearoa was once full of open streams and rivers, cutting across the landscape. As humans changed this landscape, many of these were diverted or piped to make way for urban development, leaving them to become out of sight, out of mind. These piped streams become known as ‘stormwater’ which distances them from what they once were and the diversity of species that call them home. In fact, around 95% of Wellington streams are now piped. Little to no light enters these systems and the smooth concrete gives nowhere for fish to hide or rest. In natural, open streams there are a variety of substrates, like rocks and plant roots, that fish can hide under or behind. Riparian plant material provides cover and also falls into the streams, creating places to hide and spots to find a tasty snack. In pipes, fish must battle fast flowing water and disconnected streams, meaning they will sometimes face a drop that means they have no hope of returning, or a dead end to get further upstream, while people carry on with their lives – literally above them.

Thankfully, there are many people and organisations out there working hard on fish passage remediation work and guidelines have been created for building new stream infrastructure to prevent fish passage barriers. Here at Zealandia Te Māra a Tāne, we are leading the Kia Mouriora te Kaiwharawhara/Sanctuary to Sea kaupapa/project which is a collaborative whole-of-catchment restoration initiative with a 100-year vision that the mouri of the Kaiwharawhara is healed.

Fish movement is vital for several reasons, including species survival. Fish need to get to and from the ocean to breed and if they can’t, the species will eventually become extinct. Even if some fish can make it, the overall population will continue to decline over time. This is currently happening to our taonga species, including tuna/eels.

Tuna and their lifecycle are truly impressive! Both longfin and shortfin eels begin their life out at sea in the Pacific Ocean as larvae. They grow into juvenile eels, known as glass eels, and then begin their journey from the ocean into the freshwater of Aotearoa. They enter at the mouth of the river and begin the transition from salt to fresh water. The eels eventually begin to get darker and become elvers, which is the next phase of their lifecycle.  As elvers, they will travel further upstream often facing obstacles such as dams. While there has been remedial work done to try and help them across these, the number of elvers seen migrating has declined since the 60’s. Once the elvers find a suitable spot to bunker down, they will begin to grow into adulthood. This can take many years (30-40) and some female tuna can be as old as 90. Towards the end of the life of a tuna, they become ‘tuna heke’, which is when they migrate out to sea to breed and complete the lifecycle.

Going from freshwater to saltwater is a completely different habitat and the tuna prepare by changing their bodies to adapt to the salt and open water. This is truly incredible and includes a change in colour – their fins become darker and their belly a lighter grey colour. Their head also becomes slender and flatter and their eyes and pectoral fins become bigger. These adaptations mean they can swim and see better in open water and are less visible to oceanic predators.

Fish movement is also important to maintain genetic diversity. Just like you might feel the need to move away from your hometown as a teenager to get a fresh start, if fish have to stick in the same part of a stream, things get stale fast. There will be limited resources available, and fish may begin to inbreed, resulting in an unhealthy population.

If the fish are all in one spot, they also miss out in being involved in the wider food web, eating smaller fish and invertebrates and being kai for other species. It also means nutrients don’t get filtered up or down the system. Simply put, the balance and the mouri/lifeforce of the system is skewed.

At Zealandia Te Māra a Tāne, we are focussing on whole ecosystem restoration which means restoring the environment in such a way that allows these natural processes to occur. We are also looking beyond our fence, further downstream. We can’t restore the water in the sanctuary in isolation, as it travels down the catchment where it faces many other pressures including fish barriers. For example, those young eels will travel upstream from the ocean, find their way into Zealandia, and live here for the next 15-80 years, however we can’t provide a safe place if they never arrive.

While fish barriers can be hard to change on an individual level, one action you can take is to get to know your local stream. Find out where it runs – is it piped or open? Does it need a hand? Planting alongside the water, collecting rubbish or speaking to your local council about the issues your stream faces are all actions you can take to help the awa.

Sometimes, the simple act of reconnecting with your wai/waterways is the biggest action you can take. We can’t care for something if we don’t even know it exists.

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