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Protecting freshwater fish with instream culverts

Protecting freshwater fish with instream culverts

Protecting freshwater fish with instream culverts

Culverts, weirs and flood gates are common throughout New Zealand streams and rivers, but if these structures are not designed, installed and maintained correctly, they can create barriers for freshwater fish.  

About 70 percent of New Zealand’s native fish are threatened or at risk, and one of the main threats is habitat changes. Culverts, weirs and flood gates can create obstruction to fish passage and prevent  fish moving up and downstream, and to the sea. 

A growing focus on protecting the future of New Zealand’s more than 50 species of freshwater fish has seen the release of the New Zealand Fish Passage Guidelines and, late last year, the Conservation (Indigenous Freshwater Fish) Amendment Bill was passed to give the Department of Conservation (DOC) and regional councils a ‘better legislative toolbox to improve fisheries management’. 

Minister of Conservation, Eugenie Sage said prior to the bill, New Zealand “lacked effective tools to look after fish spawning areas, ensure that culverts and drains don’t block fish passage in rivers and streams, and manage fishing on conservation lands”. 

A simple solution 

Anyone looking to install or modify a culvert or ford that may impede fish passage in a natural river, stream or waterway needs to seek ‘fish passage authorisation’ from DOC. Approval is also required if anyone is proposing to structurally change a fish facility or build a dam or diversion structure.  

P&F Global Business Development Manager, Joseph McLean, says that spending time planning the design of instream structures helps make the authorisation process go smoothly.  

“To ensure the design meets New Zealand’s Fish Passage Guidelines, people should be looking to avoid vertical drops, constriction of flows, water that is too shallow, high water velocities, excessive turbulence, sharp corners, overhanging edges, and smooth substrates,” Mclean explains. 

One of the simplest solutions is the installation of a large culvert buried to a sufficient depth so that it simulates the stream environment around it in terms of the channel width, depth and slope. “With a large culvert that is buried to the right depth, you can retain the bank line inside it and ensure you have the natural substrate (such as pebbles, gravel, silt or plants) present throughout the culvert too.  

“You’re aiming to offer the same water depths, resting areas and basic habitat that is present in the rest of the river or stream to enable fish species to easily pass and complete their lifecycle,” says McLean.  

Another principle of good fish passage design, outlined in the Fish Passage Guidelines, is that instream structures should be durable and have minimal maintenance requirements. 

P&F Global’s EUROFLO pipes are one option for instream structures. Made from high-density polyethelene, the pipes are maintenance and rust-free, robust and come in a range of sizes up to 2,100 mm in diameter. They can last up to 100 years.  

To learn more about the New Zealand Fish Passage Guidelines and the range of culvert pipes available, contact your local PGG Wrightson Technical Field Representative or store.