Kākahi (freshwater mussel)
Department of Conservation has undertaken a kākahi survey to better understand their distribution throughout the catchment. Pictured above, were kākahi found in Opouri, Rai Stream, and Tunakino. Juvenile kākahi (20 cm) were recorded at Tunakino River, indicating new recruitment in the local population.
DOC survey sets new baselines
Kākahi (freshwater mussels) are endemic to New Zealand, preferring muddy and sandy river and lake shores. While little is known about their distribution across New Zealand, they are known for their extraordinary life cycle. Kākahi have a dependency on native fish to ‘host’ their parasitic larval stage (the glochidia) in the recruitment and movement of the larvae to new soft sandy and muddy habitats. The overall national decline of kākahi can be attributed to many causes, including habitat destruction and amendment, poor water quality and the decline of native fish.
In early 2024, Department of Conservation undertook a survey across the catchment to understand this native species of ecological and cultural importance. This work undertaken by DOC aligns with the Project objective to better understand freshwater fish and macroinvertebrate habitats and populations. For Ngāti Kuia, tangata whenua of Te Hoiere, kākahi are a tohu indicator for the mauri and health of Te Hoiere's waterways. For ecologists, the presence of juvenile mussels are especially important because young mussels are more vulnerable to contaminants and require a healthy environment that won’t clog their gills.
While DOC’s survey was the largest to date in the catchment and considered our best baseline knowledge so far, kākahi have also been spotted during the course of Te Hoiere Project work over the past couple of years. They were found on a dairy run off block in Rai Valley and, in April 2022, they were found on the edge of Ruapaka Wetland. While tangata whenua and local ecologists already knew that kākahi were present, the DOC survey strengthened our collective knowledge of the current populations with three takeaways:
- Distribution has been found throughout Te Hoiere catchment, showing some resilience within the population.
- There are large populations persisting in pockets, with over 400 found near the confluence of Opouri and Tunakino Rivers and 180 in Rai Stream.
- The presence of juveniles show that new recruitment is happening, as kākahi can live up to 50-60 years.
The idea to survey kākahi in the catchment was sparked at a wananga in May 2023 by Ruihana Smith of Ngāti Kuia. So, when DOC rangers were undertaking similar work to better understand another tohu species in the catchment, kanakana (lamprey), kākahi were already top of mind. DOC Ranger Hana Mason found a large population of kākahi on the lower Tunakino River while placing kanakana pheromone samplers. DOC's Ngā Awa Ranger Greg Smith then took on investigating the distribution of kākahi across Te Hoiere catchment. Because a local investigation of kākahi has never been undertaken to this extent, Greg borrowed Waikato Regional Council’s freshwater mussel ecological assessment protocol, along with its field sheets.
At a wananga in May 2023, Department of Conservation, Te Hoiere Project and Ngāti Kuia discussed tohu species in the catchment, planting the seed for the kākahi survey.
The survey consisted of a 30-minute search, habitat assessment and mussel count if kākahi were discovered on the site, including measuring individuals to gain an understanding of the population structure and new recruitment. Kākahi were found in slower waters where sediment could deposit. There are three species of freshwater mussels in New Zealand, and all that were found in the survey were echyridella menziesii, which can grow up to 11cm long and are often found in fine sand or silt.
The survey sampled 15 sites across the catchment, with kākahi present at eight of those sites. While seven of the sites surveyed didn’t show kākahi, habitat data was still collected to better understand habitats where they are not living. Kākahi may still be present on the same river, just not present at the specific location surveyed. The locations for the survey were all in DOC areas or public places with Fish & Game access.
Overall, kākahi were discovered in opportunistic locations, such as small backwaters along raging main stem rivers, the base of fallen trees with sediment accumulation, and under eroded riverbank edges where sediment accumulates. Perhaps the most concerning, the largest population was discovered at the confluence of Tunakino and Opouri Rivers, concentrated in a deep backwater. This habitat is likely to erode through to the Opouri River in the near future, which will impact the population negatively as the backwater will be flushed clean of soft sediment.
This work to survey kākahi was funded through DOC’s Ngā Awa River Restoration Programme.
Taonga species
For Ngāti Kuia, tangata whenua of Te Hoiere, kākahi are a tohu indicator for the mauri and health of Te Hoiere's waterways.
Read more about kākahi from NIWA's Taonga Species Series.
Read more about tohu indicators and Ngāti Kuia's work to monitor freshwater here.
Kākahi rely on native fish for its life cycle.
