The Christchurch City Council has changed a plan to upgrade the Intersection of Lower Styx, Marshland and Hawkins roads to protect lamprey.

An extra $3 million will be spent and a home demolished so a threatened fish species will be protected when a Christchurch intersection is redeveloped.

A drain running parallel to Marshland Rd is a little-known spawning and rearing site for the threatened and nationally vulnerable lamprey.

The native fish has no bones and is described as the vampire of the seas because they live by attaching themselves to other fish including whales, with their circular mouths, to suck blood and bodily fluids. The adults then swim up rivers to spawn in freshwater

RM MCDOWALL/NIWA The threatened native lamprey, which the Christchurch City Council is protecting by changing the layout of an intersection realignment.

The Christchurch City Council wants to put traffic lights at the Lower Styx, Marshland and Hawkins roads intersection and widen a bridge about 50 metres north of the intersection.

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The original design, adopted by the council in 2015, would have damaged the drain and impacted on the lamprey.

DEREK FLYNN/STUFF The distinctive mouth of a lamprey.

The council was unaware of the presence of the lamprey until an ecological survey was completed during the detailed design phase.

Council planning and delivery transport manager Lynette Ellis said the Department of Conservation (DOC) and Environment Canterbury (ECan) told the council they would not support any resource consent that impacted the spawning ground.

The lamprey could not be moved because of their threatened status, council project manager Adrian Thein said.

JOSEPH JOHNSON/STUFF Niwa principal scientist Dr Don Jellyman completes research at a Marshland Rd drain where lamprey have been found.

"If we do any physical work that impedes the lamprey then the council will be liable."

Staff "exhausted hundreds of options" to find a design that worked.

The bridge would now be widened on one side only and the intersection orientation had moved 10 metres southeast. This option was supported by DOC, ECan, the National Institute of Water and Atmospheric Research (Niwa), and Mahaanui Kurataiao, a company acting on behalf of the six runanga within Christchurch.

JANE KITSON/STUFF The jawless lamprey uses suction to carry itself over rocks in a Southland river.

That option would cost $10.6m and require more private land to be acquired. The original design, which cost $7.4m, needed 1946 square metres of land from two private land owners and the council's Janet Stewart Reserve. The new plan needed 3585sqm from five private land owners and the reserve.

The council's Infrastructure, Transport and Environment Committee on Wednesday recommended the council approve the new design.

Committee chairwoman Cr Pauline Cotter said the move showed how far the council had come when it came to considering the environment.

"In the old days we would have gone straight through."

She said residents had been asking for years for improvements to be made at the intersection.

Cr Aaron Keown said the spawning and rearing site should be celebrated and named as an ecological site and maybe the drain should be renamed Lamprey Creek.

He acknowledged the species' unique appearance.

"I'm surprised there's not a gang called the Lampreys because no-one would mess with them when they saw them."

The landowner, whose home was proposed to be demolished, said she was negotiating with the council in good faith for part of her property and could not comment further because it could potentially compromise those negotiations.

Another affected landowner, Merv Walker, said he was happy to sell to the council and just wished it would make a final decision. It also needed to agree on a price after his valuer and the council's valuer came up with vastly different land values.

Niwa principal scientist Dr Don Jellyman, who has researched lamprey, said he was pleased to hear the council had taken steps to protect the area, which was a critical rearing area for the species.

LAMPREY FACTS​

The lamprey geotria australis is native to New Zealand.

The lamprey is threatened and nationally vulnerable under the Conservation Act

They have no bones, just cartilage, which makes it difficult to estimate their age.

The lamprey grows to an average of 50 centimetres.

After the eggs hatch, they stay in fresh water for up to three years before going to sea.

After three or four years at sea, they return to spawn and then die.