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Uncertain Future For Native NZ Mussel, Kākahi

Tuesday 11 February, 2019

When your very survival is dependent on hitching a ride, it is critical that one is available. Sadly for the native New Zealand mussel, kākahi, in Lake Wairarapa there just aren’t many cabs waiting on the rank.

Subsequently, this small, but important little creature faces an increasingly uncertain future. Evidence that this is the case was confirmed during the annual kākahi monitoring survey day at Wairarapa Lake Shore Scenic Reserve on Sunday. Although plenty of adult kākahi were located in quick time, less than a handful of the juvenile mollusc were discovered.

Greater Wellington Regional Council contracts Freshwater ecologist Amber McEwan to lead the research into the secretive little creatures. Amber says although the results were not surprising, it is deeply troubling for the long term future of the species in the region.

According to Amber, it is likely that the main problem facing kākahi populations is a lack of native host fish species - their favourite being the 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.

An explosion of pest fish in the lake such as perch, rudd and brown trout have decimated native host fish populations, hence the scarcity of juvenile kākahi, according to Amber.

“In order to replace the existing population there needs to be a lot more juveniles. It is a big concern, but it is not a new thing, we have known that ever since we started this programme,” Amber says.

Sunday’s monitoring involved about 35 mainly volunteers, 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.

The kākahi measured ranged in size of between 4-6cm, which puts them at about 20-30 years old, Amber says. Juvenile kākahi are much smaller so volunteers scooped up substrate and sieved it, looking for the tiny shells. Only four juveniles were found.

Paul Curry, a veteran of five kākahi counts, says the search for juveniles was “like looking for gold”. Compared to the other volunteers he was richly rewarded, finding two of the four juveniles.

Historically, kākahi have been very important to Māori. They were a source of food and their shells were used as tools.

Kākahi also play an important ecosystem role as they filter water. 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, Amber says.

“The future [for kākahi] is looking really bleak at the moment, they really need some help. We don’t have much in the way of knowledge nor technology in order to help them. But in the last few years there has been a lot more funding directed towards kākahi research in a few universities so we should know a lot more in another couple of years.”

While there are other significant factors contributing to the population decline of kākahi in Lake Wairarapa such as sedimentation and high nutrient levels in the lake water, reducing pest fish numbers could help.

Amber says test cases carried out in Bartons Lagoon to remove pest fish with nets have successfully eradicated rudd from the area. However, it is not yet clear whether this will result in an increase in kākahi.

“Because these pest fish are pelagic, (live in the water column), they can be captured by netting without disrupting life on the lake bed.”

The kākahi monitoring programme is one of a number of projects underway at Lake Wairarapa as part of the Wairarapa Moana Wetlands Project, a joint initiative of Greater Wellington Regional Council, the Department of Conservation, South Wairarapa District Council, Kahungunu ki Wairarapa and Rangitāne o Wairarapa Inc.

The 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 email: biodiversity@gw.govt.nz.


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