Baffles and a fish ramp installed in a culvert near Karamea
Image: Nixie Boddy | DOC


Research on the West Coast will help taonga freshwater fish like īnanga get past barriers blocking their travel such as culverts and pipes.

Date:  24 May 2024

Thanks to the proximity of State Highway 6, high rainfall, and an abundance of streams and culverts, West Coast sites are serving as “laboratories” to test how well baffles and floating ramps improve fish passage.

“Many of our freshwater fish species are in trouble, with 76 per cent threatened or at risk of extinction. Barriers to fish passage contribute to this statistic,” says DOC Freshwater Science Advisor Nixie Boddy.

“Barriers in waterways, including culverts and pipes, are in place so streams can pass underneath pathways and roads, but they stop some freshwater fish from swimming upstream to feed and grow into adult fish.”

“When culverts and pipes are narrower than the natural stream bed, the water flows too fast. There’s nowhere for the weaker swimmers like īnanga and bullies to rest and recharge.”

“Weaker swimmers often end up congregating below fish passage barriers where competition for food and resources is high. The fish may starve or get eaten by predators.”

Nixie Boddy says clever solutions such as baffles and floating ramps are designed to help fish make their way upstream.

“Baffles added to the bottom of culverts create eddies and slow flows, so fish can rest before swimming again. They also trap rocks, making a more natural stream bed.”                                   

“Floating ramps help fish climb scoured out “plunge pools” and the small waterfalls that tend to develop at the bottom of culverts.” 

“Problem is, scientific testing in the field has been limited. Our research aims to quantify how well these solutions work in the real world.”

A DOC team, led by Nixie Boddy, has established eight West Coast study sites in 2021, from Karamea in the north to Harihari in the south.

“We’ve counted fish above and below culverts, before and after baffles were inserted and then after ramps were added.”

“We don’t know how effective baffles are on steeper, sloping culverts and culverts of different lengths so we’ve looked into that too.”

“It’s important to keep an eye on modified culverts so we know what ongoing maintenance is required and how long the different types of baffles and ramps last.”

Nixie Boddy says final results are a few months away, but the results so far are exciting.

“Following the installation of ramps and baffles at five test sites known to be home for īnanga, we’re now finding īnanga upstream of the culverts at four of the sites. Previously, īnanga were only found downstream – they simply couldn’t pass the culverts.”

Nixie Boddy says understanding how different fish passage products help the fish will inform the national Fish Passage Guidelines used by landowners and industry.

In the meantime, individuals can do their bit to improve fish passage, and now’s the perfect time with World Fish Migration Day coming up on 25 May.

  • Use the fish passage assessment tool to check and report structures you think might be stopping fish: Fish Passage Assessment Tool | NIWA
  • Check the streams on your land and improve any barriers
  • Learn more about fish friendly culverts and rock ramps


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