Source: NZ Department of Conservation
About this time last year, a group of DOC rangers and scientists set out from Twizel for a regular check of a population of threatened fish in nearby Corbies Creek. It was a beautiful day. Soon after getting their gear in the water, they realised something was very wrong. Where were all the fish? Only a year ago they’d found more than 100 in a 25m stretch, but there were hardly any there now.
Corbies Creek, along with just a few other small streams in Canterbury, is a refuge for native lowland longjaw galaxias. If we lost them from here, they’d be gone from everywhere. Sleek, pencil-thin and exquisitely camouflaged, their pale-yellow skin is dusted with brown and silver flecks. Adults rarely grow longer than 80 mm.
Longjaws are one of New Zealand’s river-resident galaxiid species that live their entire lives in a single waterway. All river-resident galaxiids are vulnerable to being eaten or displaced by larger fish. Some, including longjaws, can’t share habitat with any bigger fish. To safeguard this population, an exclusion barrier has been built to stop predatory trout and kōaro from swimming up into their habitat.
So how had two brown trout – the cause of the drastic decline at Corbies Creek – got up there? Sjaan Bowie, DOC senior freshwater technical advisor, thinks the trout were carried across a paddock from a nearby waterway, in a particularly high flood event a few months earlier.
Rest assured the trout were quicky removed and the longjaws are bouncing back.
“We’re pleased to report that monitoring in March this year found numbers had risen from just 12 to more than 50 fish, and no more trout have been seen upstream of the barrier.”
Limited tools available – innovations welcome
Sjaan says this near-miss extinction of longjaws in Corbies Creek shows that more management tools will be needed to protect our freshwater fish in the future.
“What we’re doing generally works fine for small streams under current climatic conditions. But with increasing temperatures, we’re seeing trout head further inland looking for cooler water. More severe weather is also causing bigger floods and longer droughts. This combination increases the risk of trout making it past barriers or accidentally getting into threatened fish habitat, as we saw in Corbies Creek.”
She highlights the need for better technology – both for remote monitoring of populations and to protect larger areas.
“We’re looking at remote water level monitoring, so we’d get a warning ‘ping’ and could go and check if a barrier had been breached or there was an overland flow. There’s also a need to protect more and larger areas to prevent individual populations becoming genetically isolated.
“A fish exclusion barrier that works in larger rivers or low gradient streams without backing up the flow and creating a pool, would also make a big difference to the ongoing survival of these species. If anyone has bright ideas about how to build something like that, we’d really love to hear from you.”
Sjaan says the same issues are faced in fish conservation around the world, so any solutions we created here could be used internationally.
Regardless, future work to secure our river-resident galaxiids is likely to include building exclusion barriers in new streams and moving current barriers downstream. Other tools like captive breeding and translocations into protected areas are also likely to be necessary.
Let them through – migratory fish need to move
Managing the other group of New Zealand’s native fish couldn’t be more different. It’s vital for these species to be able to move up and down waterways and get to and from the sea to complete their lifecycles. In this group of migratory species are eels, bullies and the fish we collectively known as whitebait – the juveniles of īnanga, kōaro and banded, giant and shortjaw kōkopu.
The strongest swimmers of the group move the furthest inland. Kōaro stand out as best in class as they can climb near-vertical walls. Īnanga are the most challenged by inclines, jumps, rapids and fast flows, and tend to stay in flatter areas near the coast.
Human-built structures in waterways can present swimming challenges. Conservation work for migratory species therefore includes identifying, fixing or removing barriers like poorly designed or unmaintained culverts, fords, dams and weirs.
As part of her role, Sjaan advocates for better fish passage. She’s helped develop and update fish passage guidelines and resources, given dozens of seminars about best practice, offered advice and support to others, and coordinated the New Zealand Fish Passage Advisory Group.
“We can make a real difference for migratory fish by removing barriers. Yes, we can plant trees and improve habitat but if we can take out something that’s stopping migration, the benefit is immediate. It means the fish aren’t slowed down or stopped in their migration and allows them to get to natural habitat upstream to grow and mature.”
Researching ways to fix impassable culverts
Culverts are a big issue. There are hundreds of thousands of them around the country and some hinder or block fish passage by creating overhangs or impassably fast flows.
Sjaan and her colleagues have been testing different retrofitted baffles and ramps to see how well they help fish move up and through culverts.
“We couldn’t find a lab that was big enough, so we chose some barriers in waterways on the South Island’s West Coast. It has high rainfall, lots of culverts and an abundance of fish.
“It looks like these fixes can be used to improve passage for some species under certain conditions, but not for all species. They may be best considered as a temporary solution. Final results will indicate when they improved passage, and allow us to offer better guidance on installation, monitoring and maintenance of these fixes.”
Sjaan says if there’s a structure that’s restricting fish passage, the best option is always to remove it. “If that’s not possible we need to consider replacing or fixing it permanently.”
Spectacular success at Te Pouaruhe wetland, Wairarapa
Our work with the Wairarapa Moana Wetlands project restored fish passage to Te Pouaruhe wetland in early 2022 – using a large digger.
The area was drained for agriculture in the 1940s and separated from Lake Ōnoke by a stopbank and two culverts. One of the culverts had a flap gate that severely limited fish access to the wetland from the lake and the sea. The digger removed the culverts and made two breaks in the stopbank that now provide free passage up and downstream.
Before and after fish surveys in 2019 and 2023 found huge differences in the number and range of species present. Īnanga and common bullies were found at every sampling site in 2023 and in large numbers at most sites. At one site, the number of īnanga rose from 339 to 1563 after fish passage was restored.
Challenges to fix ford in lower Waipoua River, Northland
This ford across the Waipoua River was built to provide access for mana whenua (local residents) and commercial forestry vehicles.
It’s a significant barrier to fish passage because of a drop off downstream and culverts inside the ford that accelerate the flow. Installing four fish ramps has helped, but a permanent solution is still needed.
“Having a barrier 5 km from the sea restricts or prevents fish access to around 100 km of beautiful stream habitat in kauri forest”, says Sjaan. “Improving fish passage there would make a big difference for many species, including threatened shortjaw kōkopu.”
Fixing the ford is a priority for Te iwi o Te Roroa and DOC and options, including a fish bypass or replacement bridge, are being looked at.
Progress to celebrate and some lessons learned
Reflecting on progress in the last 10 years, Sjaan is pleased to have national guidelines, improved policy and new tools in place.
“The Fish Passage Assessment Tool is one way that anyone can record instream structures and assess the risk they pose to fish passage. The tool has contributed to a database of more than 150,000 structures nationwide that are being prioritised and ticked off.
“It’s been exciting to see councils such as Northland, Taranaki and West Coast, as well as other organisations, taking action to remove barriers and put in some good fixes to open up habitat for fish.”
Sjaan says instream structures always have at least a dual purpose – to transport water and allow fish to move – and both are important to consider.
“One stand-out lesson for me though is the benefit of oversizing and embedding culverts. They will be long-lasting, stand up to floods, and provide good fish passage.”
This article was first published in the New Zealand Water Review.
Read more about our work to secure populations of migratory fish: Ngā Ika e Heke migratory fish workstream: Freshwater restoration
