The future of an “amazingly long-lived” freshwater mussel – kakahi – in Wairarapa Moana remains uncertain.
The Wairarapa Moana Wetlands Project annual kakahi count – the longest-standing citizen science monitoring programme in New Zealand – found no juveniles in its February survey.
Greater Wellington Regional Council [GWRC], who manages the project, said kakahi age, size, and shell condition were important indicators for lake and ecosystem health.
Adult kakahi remove sediment and algae from the water column, while juveniles require a specific fish host and environmental conditions for their survival.
“Kakahi are amazingly long-lived, and some are thought to be over 50 years old.”
The council said because kakahi were sensitive to changes in water quality and temperature, monitoring its population meant the conditions in the moana could be inferred.
“The future for the New Zealand kakahi is uncertain, as young kakahi are usually absent or very rare, including in Lake Wairarapa.”
GWRC said the ninth annual survey at Wairarapa Moana on February 12, was conducted in ideal kakahi-spotting conditions.
Citizen scientists were split into groups and tasked with finding as many kakahi in an allocated zone in 30 minutes, or a maximum of 50 kakahi.
Citizen scientist John Argue said the sampling method was “very precise” and his team found three kakahi in their zone in half an hour.
“They’re secretive little guys, they don’t jump out at you. But they’re not stationary. They use a little foot to propel themselves along.
“We were told to follow their tracks, with the hope that at the end of it, you’d find a shellfish.
“I don’t think any juveniles were found.”
The National Institute of Water and Atmospheric Research [Niwa] said the kakahi was under threat and was declining worldwide.
It said the decline had been attributed to the loss of habitat, pollution, and possibly through the loss of the host fish, which the complex kakahi life cycle depends upon.
New Zealand kakahi larvae latch onto the gills of native fish, which allows them to filter feed and hitch a ride upstream to areas safe from predation.
Niwa said no single impacting factor had been identified as being consistently important to the kakahi’s decline.
Although the most recent Wairarapa Moana survey is yet to be analysed, last year’s results were sobering.
The survey, which alternates between the northern and western shores of the lake, recorded a sharp drop in kakahi abundance in the 2022 survey.
It said only one kakahi species [E. menziesii] had been found at the western site since 2016.
The fourth survey at the site concluded the population structure continued to be adult-dominated with little sign of juveniles.
The report said eight juveniles were found last year, and the overall drop in kakahi abundance could be attributed to the recent “atrocious” weather.
Wairarapa Wetlands senior biodiversity advisor Ella Buckley said the low numbers of juveniles this year and last, were not anomalies.
“That’s the trend we’ve been seeing over the years.”
Next year, will be the 10th anniversary of the citizen science kakahi count project at Wairarapa Moana.
A report and analysis of this year’s count is expected to be published mid-year.
