From above, Lake Rototoa’s waters look blue and inviting – but it’s what’s below the surface of the Auckland lake that’s causing concern.
Native freshwater mussels, or kākahi, are essential to maintaining lakes’ health. The kākahi help filter water, oxygenate sediment, and create habitats for other native species.
But a recent survey carried out by Aotearoa Lakes divers have revealed the population is in a sharp decline.
“A large portion of the mussels are dead,” Auckland Council’s freshwater biosecurity advisor Belinda Studholme told 1News. “Like 80% of the mussels have died.”
Of the 2238 mussels counted, an alarming 1894 were found dead. There was no evidence of younger size classes, indicating there has been no recent recruitment to the population.
Auckland Council senior healthy water specialist Madison Jones said the mussels can filter a litre of water an hour.
“Across Auckland, we’ve lost all of these mussel species from every lake except for two, so Rototoa being one of the lakes where we still have a population,” she said.
“It’s definitely a similar picture happening across all of New Zealand.”
Pollution, pest fish, and declining water quality are believed to be the primary drivers of the kākahi decline.
At Lake Rototoa, it’s believed perch – a predatory fish – are to blame.
Special enclosures have been set up on the lake by a team of divers to keep the perch away from the mussels and their larvae.
“It’s quite depressing when you see the decline in different lakes,” said Global Dive’s Andrew Simpson, who is taking part in the project.
“We dive professionally and so we’ve done stuff for over 20 years, and yeah, it is quite a challenge to see the decline elsewhere.”
Also living in the enclosure are native fish called bullies.
They’re crucial to the breeding of the mussels, transporting their larvae to different areas.
So far, two enclosures have been set up and hopes are high, with juvenile kākahi already successfully developing without issue.
Studholme said the trial could pave the way for improved lake health in other areas.
“If we could have kākahi on demand that would filter lake water, that would be a fantastic outcome,” she said.
Jones added: “My hope for this trial is that we’re able to safeguard kākahi in Lake Rototoa and that other people can use the same method to do the same in their lakes.”
