By Sarah Wilcox
Why our native freshwater fish need more help to get to habitat upstream.
It all starts with an egg
In the autumn every year, millions of fish eggs are laid beside rivers and streams up and down the country. They’re tiny translucent balls that diligent female fish stick to or shove into grass, moss or woody debris beside the river when water levels are high. Males squirt sperm-containing milt in their vicinity and fertilisation occurs.
The eggs develop into larvae over a few weeks. It’s a precarious time. Both eggs and larvae can dry out and die, be nibbled by vermin, trampled by stock or gobbled up by other fish.
During the next flood or king tide, the larvae are washed downstream into the sea and grow into juvenile fish. These eggs all belong to whitebait, a collective term for the juveniles of six species of native fish.
By the time spring comes around, they’ve reached about 5 cm in length and move into freshwater again. The tiny fish enter river mouths – some are caught as whitebait and others make it past the nets and go upstream.
They’re headed for cool, secluded places with bugs and insects to nourish them through the summer as they grow into adult fish. The journey takes them through pipes, under roads, past rocks and big rapids. For some fish species it’s a breeze, but for others it’s a perilous and sometimes impossible mission.
Culvert catastrophes
Nixie Boddy is a freshwater science advisor at DOC. She’s been leading a project to find out how to help native fish swim upstream through the concrete pipes and culverts underneath our roads.
“There are culverts everywhere! Hundreds of thousands around the country. It’s by far the easiest way to pass a stream under a pathway or a road – you just pop down a circular or square concrete pipe for the river to flow through and build a road over the top.”
But there’s a problem. Up till now, culverts have been designed to move water efficiently rather than for ecological reasons.
“Culverts are fine if you use a big one – at least 1.2 times the width of the stream. Then the river can behave naturally inside it, create its own riverbed and banks so it’s no harder to swim through than the rest of the river.”
Narrow culverts and pipes constrain rivers and create high-speed flows.
“Fish moving upstream have to swim fast enough for long enough to make it through the whole culvert in one go. That’s usually about 10–14 metres. This much fast-flowing water can prove too much for some of the weaker swimming species.”
An unfriendly culvert can be game over for īnanga, the weakest swimmer amongst the whitebait. It’s a barrier as effective as a brick wall.
After swimming upstream as far as they can, fish trapped by culverts have a high chance of being eaten by predators or running out of resources.
“If they can’t go any further upstream, the fish congregate and get stuck in a small area, so competition for food is high. They can struggle to get enough energy to grow, and many can die.”
A baffling solution
This problem isn’t a new one. One solution has been installing baffles on the bottom of culverts to slow down flows and provide resting places for fish. Two different products are used in New Zealand: spoiler baffles and flexible baffles.
“They’ve been installed all around the country and do a couple of helpful things. Baffles create eddies and areas of slower flow so the fish can rest before swimming fast again, enabling fish to slowly make their way up through the culvert. They also trap rocks, making a more natural stream bed and slower flows.”
But here’s the next problem – we don’t actually know how well they work.
Nixie says some research has been done on the different baffle products, but it hasn’t been comprehensive.
“We wanted to know what percentage of fish can get through the modified culverts, which species, and which size classes. There are also questions about how effective the baffles are on steeper sloping culverts or culverts of different lengths.”
“Understanding how the different products help or hinder the fish will help to inform national guidance, and in turn help the fish!”
And it’s a ‘no thanks’ to plunge pools and waterfalls
Fast-flowing water coming out of a culvert eventually erodes the riverbed downstream and creates a waterfall and a ‘plunge pool’ at the bottom.
“Trout and salmon can generally ump these kinds of obstacles, but our native fish have to climb and swim their way up. Some of the more athletic species can handle it but bullies and īnanga tend to get stuck in the pool.”
Floating ramps are used to solve this problem but they have to be installed correctly with a low-gradient slope that works for all the fish. Maintenance is also essential.
“It’s really important to get it right. We wanted better testing for products used around the country. I see this as urgent knowledge for people who are buying the products with the expectation that they’ll allow all our fish to get through.”
A perfect natural laboratory
So Nixie and her team set about testing some of the products on the market.
“We searched the country for a lab where we could do it, but we couldn’t find one big enough. That meant we had to do it in the field, which was definitely more complicated – and more interesting.”
The South Island’s West Coast turned out to be the perfect place.
“The state highway runs parallel to the coast for hundreds of kilometres. Because of the high rainfall, there are LOTS of streams and culverts at regular intervals the whole way along. Plenty to choose from!”
The team has run intensive studies at six sites from Karamea in the north to Harihari in the south.
“We’ve identified, measured and counted fish in sections of river below and above culverts, before and after baffle installation, then after ramps were added. Further modifications have been made to the culverts and a final survey is coming up this year.”
“It’s also important to keep an eye on the modified culverts so we know what maintenance is required and how long the different types of baffle last.”
How many ways can you catch a fish?
“We’ve thrown everything at it! Spotlighting surveys, eDNA testing the water, trapping fish in nets and electric fishing. Different methods work better for different fish species and size classes – and we wanted to know every fish that’s present.”
Nixie says it was really hard to test the modifications rigorously.
“We did some catch and release trials in the culverts. Fish were released in a modified Southland sock net at the bottom of the ramps and given 24 hours to make it up to a net at the top. We also electric fished through the culvert at the end to find any fish that had made it part way up.”
They were all released unharmed.
Results so far
Nixie is pleased to report that īnanga populations have been found upstream of culverts at 4 out of 5 sites where fish ramps and baffles were installed.
“This is good news because there were previously no īnanga upstream of the culverts in those places. Also, with īnanga being the weakest swimmers of the whitebait species, their ability to get past the modified culvert is a great indication that most other species should be able to get past as well.”
Once all the data has been collated and analysed, Nixie will be making the results available in a final report.
“That’s still a few months away, but what we’ve learned along the way has already been used to help improve the New Zealand Fish Passage guidelines, which contains best practice information for use all around the country.”
“What I find really sad is that so many fish are being stopped just a few hundred metres into their journey. In New Zealand, we often build roads (and therefore put in culverts) close to the coast because it’s a nice flat place to build. Ahead of the fish can be kilometres of beautiful habitat but so many just can’t make it there.”
“There’s a huge opportunity to get this right and make a difference for these species, some of which are as threatened as whio and royal albatross.”
How you can help let fish through
Culverts, pipes, dams and weirs are used to channel rivers and streams throughout the country. And not all of them are good enough to let fish past.
Explore more about how native fish can climb obstacles and why some structures in streams are barriers to their migration – watch these videos.
Here’s how you can make a difference for our native fish:
• Learn more about designing fish-friendly structures
• Use the fish passage assessment tool to check and report structures you think might be stopping fish
• Check the streams on your land and improve any barriers.
Read more about our work to secure and restore populations of native fish:
• Improving fish passage for all our species: Critical Ecosystem Pressures on Freshwater Environments (CRESP) programme
• Four fish that are receiving some extra care: Ngā Ika e Heke migratory fish project: Freshwater restoration
• See what’s happening near you: Regional examples and resources for fish passage.
And look out for our final report on this work later in the year.
World Fish Migration Day
We’re highlighting this important research in support of World Fish Migration Day on 25 May. The day is an international celebration and awareness-raising event about the importance of free-flowing rivers and migratory fish.
Thank you team for your work on a little-known and important problem. Johan’s idea of developing a professional development module for already qualified engineers and contractors to use as a part of their CPD requirements is a good one.
Nice work Team. The extent of fish habitat modification has been massive, but we can progressively address and remediate these barriers. Will be really interesting just to see how much improvement we can get.
RMA team can help with enabling rules in regional plans, giving effect to DG’s S 53 CA role in freshwater.
What a great piece! In my perfect world we’d daylight every underground stream and mitigate culverts as described here. Wonderful work, everyone.
That was super interesting, I definitely learnt something. I can see now why we need to be sure to include fish passage conditions in concessions for culverts.
Very cool , well done team. I look forward to this information being readily available and used by all sectors when dealing with fish passage in culvert situations.
Great work and results team! I’ll look at the updated guidance when needing to install a culvert. 🙂
Nice to see evidence of how well fish passage structures and culvert modifications work in practice. I’ve long thought that teaching what works in our civil engineering schools and developing a Prof Development module for already qualified engineers and contractors to use as part of their CPD requirements would be a way to spread the good practice too. Nice article team!