With all of Lake Wairarapa to play in, it can be a lonely life for a juvenile kākahi, the native New Zealand freshwater mussel. While there are plenty of the adult mollusc living on the lake bed, juveniles are few, and it is cause for concern.
This year’s annual kākahi monitoring survey day at Lake Domain Reserve in the Wairarapa recorded reasonable numbers of adult kākahi, but not a single juvenile.
The kākahi monitoring programme, now in its seventh year, is one of a number of projects underway at Lake Wairarapa as part of the Wairarapa Moana Wetlands Project, a joint initiative of Kahungunu ki Wairarapa and Rangitāne o Wairarapa Inc, Greater Wellington Regional Council, the Department of Conservation, and South Wairarapa District Council.
Freshwater ecologist Amber McEwan, who coordinates the event says the juvenile no-show is troubling, and signals a worrying outlook for the fate of the species in Lake Wairarapa.
Kākahi play an important ecosystem role as they filter water, as one kākahi can filter about one litre of water per hour. In the past, large beds of kākahi probably helped to maintain the clarity and ecological health of New Zealand’s waterways.
Kākahi are a taonga species for Māori; as a traditional source of food and as tools.
NIWA freshwater ecologist Mark Fenwick says kākahi are considered to be a bioindicator in freshwater environments.
“The fact they are still relatively easy to find in Lake Wairarapa is encouraging. Like all species, however, their future will be dependent on having babies, and right now baby kākahi are hard to find,” says Mr Fenwick.
“It is a problem mirrored all around New Zealand. Populations of kākahi are year-by-year getting older which will eventually get to a tipping point where there is no recruitment.
“In some places they are becoming virtually extinct; there are viable adults but they are not producing any babies - Lake Wairarapa is not there yet,” says Mr Fenwick.
Greater Wellington’s Wairarapa Moana Project Lead, Kereana Sims says “The kākahi monitoring programme helps us to build our understanding of what is happening with the ecology of Wairarapa Moana, but it’s also a really important way for us to grow the connection between people and the Lake.”
This year’s monitoring involved about 35 citizen scientists, measuring and recording information about the kākahi and returning them safely to the lake. The survey alternates between two sites at Lake Wairarapa - Lake Domain at the top end of the lake and Wairarapa Lake Shore Scenic Reserve on the Western side.
Mr Fenwick says the two locations are quite different and may help to explain why there were no juveniles found in this year’s survey.
The monitoring site at Lake Domain Reserve is between two waterways flowing into the lake - Tauwharenīkau River and the outlet of Barton’s Lagoon. It makes it more likely that trout and other introduced fish such as rudd and perch are prevalent, a factor which may be contributing to the decline of kākahi.
Kākahi populations rely on host fish species - their favourite being the native kōaro - to facilitate successful reproduction. As host fish they act like a taxi whereby kākahi larva, called glochidia, are sneezed out by their mother and latch onto a passing fish using a hook at the top of their shell.
“This side of the lake probably has less native fish, and more exotic which has a negative effect for the kākahi population because they are not able to transform into the adult form on the exotic species, instead probably dropping off and dying.”
Not only are exotic fish likely making it hard for juveniles to successfully transform, it is also very difficult for baby kākahi to survive in most parts of Wairarapa Moana these days.
This is due to the effects of nutrient and sediment runoff from surrounding land, which creates a hostile lake bottom, rather than the clean, oxygenated habitat that baby kākahi need.
The Wairarapa Moana Wetlands Project began in 2008 with the aim of enhancing the native ecology, recreational and cultural opportunities on public land in the area, and includes restoration work at Ōnoke Spit, Lake Domain Reserve, Donald’s Creek as well as Ōnoke/Ōkorewa Lagoon.
Wairarapa Moana is one of the largest remaining wetland complexes in New Zealand, and has ecological values of national and international significance.
For more information on the Wairarapa Moana Wetlands Project visit the website: www.waiwetlands.org.nz