Museum Notebook: Kākahi an important traditional food source for Whanganui iwi

Kākahi shells. Whanganui Regional Museum Collection ref: 1802.5775.8

On display in the Whanganui Regional Museum exhibition Ngā Wai Honohono is an interesting tool that looks like a giant comb.

It's an intricately carved wooden bow-shape, with a row of long spiky wooden teeth attached. Next to it is a pile of black mussel shells. They are the empty shells of kākahi, the endemic New Zealand freshwater mussel. The tool is a rou kākahi - a rake for gathering kākahi from the sandy bottoms of rivers and lakes.

Kākahi are an important traditional food source for Whanganui iwi. Formerly abundant throughout the Whanganui River and its tributaries, they were usually air-dried for use as a chewy, nutritious and portable snack.

Kākahi begin their lives as eggs. Unusually for a mollusc, the mother kākahi keeps her eggs inside her gills until they hatch into little wiggly larva that can swim freely. Next, they find a freshwater fish to attach themselves to, and hitch a ride.

A favourite host is the kōaro, one of our endemic and endangered freshwater fish, which begins its life near the sea and migrates inland as "whitebait". Kōaro that escape the whitebait nets will journey upstream to their eventual homes, sometimes with a little hitchhiker attached - the larva of a kākahi. When they arrive at a suitable habitat, the larvae drop off and burrow down into the sandy bottom of a nice clean stream or river. They hide buried under the sand for the first five years. Eventually, the young kākahi makes its way up through the sand to live, still partly buried, filtering food such as algae out of the water and growing very slowly for the next 50 years.

Sadly, all three endemic species of Echyridella menziesii - kākahi - are now declining, and are classified as threatened or at risk of extinction. As a species that relies on clear water, kākahi are unable to thrive in heavily silted water. Juvenile kākahi are sensitive not only to silt, but to water-soluble contaminants such as pesticides and fertilisers. Being filter-feeders, all shellfish are gradually accumulating bacteria, toxins and other pollutants from the surrounding water into their bodies. That may help the water become cleaner, but the shellfish, as a result, become unsuitable for food.

One of the most pressing environmental concerns, not only locally but nationally, is what we can do to clean up our waterways and restore them to the clarity and health they had in the past. By cleaning up the waterways, we also protect all the creatures that inhabit freshwater environments.

The Museum's exhibition, He Awa Ora: Living River, features some contemporary efforts being made locally to protect and uphold the health of Te Awa Tupua - the Whanganui River.

Ultimately though, the work of cleaning up rivers and lakes is the responsibility of us all. Freshwater native species are taonga that need our care and protection. Perhaps in the future, the now threatened kākahi will become abundant again - if there are any adult kōaro left to hitch a ride with.

• Margie Beautrais is the educator at the Whanganui Regional Museum.