Innovative solution gives hope for migratory fish at Wairewa/Lake Forsyth
Environment Canterbury media release: 8th October 2024
An innovative new concept to create a permanent outlet to the sea for migratory ika/fish at Te Roto o Wairewa/Lake Forsyth is underway after previously receiving funding towards planning and development.
Wairewa is a shallow coastal lake on the south-western side of Banks Peninsula that connects to the sea through a gravel bank at the small settlement of Birdlings Flat.
Traditionally, Wairewa and its wider catchment were highly regarded by Ngāi Tahu as an important source of mahinga kai/mahika kai - offering a bounty of food and resources. Historic forest clearance, wetland drainage and agricultural land intensification means many of the lake’s taonga species are now in critical decline.
Find out more about mahinga kai in Canterbury.
In recent years, unique Wairewa Rūnanga infrastructure has made significant improvements to the catchment. The next stage of works is now underway, after previous funding through our Fish Habitat Fund supported the investigation and design of a permanent outlet to the sea to support the lifecycles of migratory fish.
Declining lake conditions needed urgent solutions
Deforestation of the Wairewa catchment increased gravel and sediment washing down to the low-lying lake, blocking its natural opening to the sea. For the last century, the lake has been opened periodically using machinery, to prevent widescale flooding.
“When that happened, the ecosystem went into hyper shock,” explained Robin Wybrow (Waitaha/Kāti Māmoe/Kāti Kuri/Ngāi Tahu) of Wairewa Rūnanga.
“The lake might have been over two metres and drop below 500cm when it was opened to the sea. Because there was no control over water flow, you would get saltwater incursions of up to a kilometre into the lake, creating a breeding ground for some of the least-desirable algaes.”
With the health of the lake declining, so too was the population of tuna (eel) and other taonga species, such as inaka/whitebait and pātiki/flounder, with their lifecycles dependent on quality habitat and the lake being open at the right times.
“The most alarming factor was there weren’t enough elva (young eels) getting into the lake to sustain the population. It was critical for us to address the water quality and provide permanent access for migratory species.”
Out of the box thinking drives powerful change
Ko Awa Ika o Wairewa is an innovative take on a conventional fish pass; an apparatus designed principally by Wayne Alexander to connect the lake to the Pacific Ocean 24/7. Effectively a long box anchored to the seabed, this highly engineered equipment represents several years of meticulous planning.
With designs now completed, the next major steps will be consenting and securing funding for construction. The apparatus can be turned off and on and could be replicated in other areas with similar problems.
“We’ve already lost 80-90 per cent of the tuna population in the northern hemisphere,” Robin said.
“In Aotearoa, we could be facing the same situation. Losing these taonga species is not something we’re willing to accept.
“We’re extremely grateful for the commitment of Canterbury Regional Council (Environment Canterbury) and Christchurch City Council for their support and for considering unique and innovative solutions to crucial issues.”
History of success in iwi-led innovation
Civil engineering innovation is no stranger to Wairewa, with the Rūnanga already investing significantly in the development of an outlet canal, bridge and groyne as a more effective method of lake-level management and to prevent widescale flooding.
Compared to the traditional method of a direct cut opening the lake to the ocean, the infrastructure enables control over the lake levels and prevents saltwater from entering the lake when it is open, mitigating issues like algal blooms.
Since it was put in place 15 years ago, water quality continues to track in a positive direction. Cyno-toxic algal blooms have decreased by 99.6 per cent and macrophyte beds (aquatic plants) are flourishing, providing habitat, oxygen and food sources for fish, invertebrates and wildlife.
“We’re always trying to be on the front of the waka in terms of our lake knowledge,” Robin said. “We believe Te Roto o Wairewa is the only low-land lake in the country that has been able to reverse this level of decline.”
Funding support available for fish passage initiatives
The Fish Habitat Fund is an initiative to address site-specific issues that threaten native fish populations and provide financial support towards remediating these issues. Up to $100,000 per year is available for projects that support general fish passage, any threatened/at risk freshwater fish species, increasing resilience of native fish strongholds in the face of climate change.
Find out more about managing instream structures on your land.