Wednesday, February 28, 2024

Christchurch rock riffle to boost fish habitat

Christchurch City Council says a naturalised rock riffle being installed in the Ōtākaro-Avon River near Mona Vale Homestead will help native and sport fish easily move to and from the river’s upper catchment.

The rock riffle has been specifically designed for New Zealand’s ika (fish) species and will be similar to those seen in nature, the Council said.

A naturalised rock riffle is a long ramp with a gentle gradient. Rocks are strategically placed to allow for a wide range of flow conditions to cater to the variety of weak and strong swimming fish species. Larger emergent boulders are also placed in the waterway in clusters to create resting places for fish and spawning habitat for invertebrates.

It will be installed over the existing weir (pictured). Work began last month and is expected to take six months to complete.

“Barriers to movement in waterways have been identified as a significant threat to both native and sports fish in New Zealand,” says Christchurch City Council Three Waters Manager Planning and Delivery, Gavin Hutchison. 

More than 76% of New Zealand’s living native freshwater fish are classified as threatened with or at risk of extinction, Department of Conservation figures show.

“The remediation of this weir will allow access to approximately 9km of waterway upstream of the weir and increase migration, diversity and abundance of ika in the upper river’s catchment.”

“Limited passage of brown trout occurs at this weir but likely not full passage. In addition, the weir is a barrier to many of our native ika who are poor swimmers, in particular inanga (whitebait), bluegill bully and lamprey, which all have conservation status.

“The native fish are unable to swim or jump through the barrier and that’s why we find very few native species in the waterways upstream,” says Mr Hutchison.

Over half of New Zealand’s native fish species travel back and forth from freshwater to the sea to complete their life cycle. The new rock riffle will enable easy access to upstream waterways.

It will be similar to a rock riffle previously installed near the Antigua Boatsheds.

The history of the Mona Vale weir dates back to the 1860’s when the Deans family leased six acres of their land adjoining the Avon River-Ōtākaro loop (including water rights) to William Derisley Wood for the construction of a flour mill.

In November 1860, Mr Wood called for tenders from ditchers and excavators to cut the mill race across the river, and construction began around February 1861.

In order for the mill race to obtain a sufficient flow of water to turn the mill’s wheel, Mr Wood was required to install a weir within the channel of the Avon River loop as a means of controlling the water flow.

Extensive repairs to the original timber weir were carried out in 1939 and the existing concrete weir was renewed by the Christchurch Drainage Board in the mid-1980’s.  

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