How to spot a native: Crayfish

Although crayfish numbers are falling even in marine reserves, it's definitely worth the experience of snorkelling to see them in their world. Photo / Anna Barnett

I remember my granddad heading down to the rocks in a burnt orange flannelette shirt to pull up the craypots at low tide. Little kids' eyes were huge as the feelers pierced the surface and the pot of crayfish burst out of the water.

To my childish mind, those armoured koura looked like monsters from nightmares with their vicious spikes, eyes on stalks and the scary spontaneous way they flapped about.

Maybe that's why we haven't taken as much care of this native species as we have others.

By day, koura hide in underwater caves and cracks in rocks. Here they're protected from storms, predators and light. Koura are creatures of the night that hunt small shellfish, crabs, starfish and kina to return to safety before the sun rises.

It has taken scientists decades to figure out their mysterious life cycle. Our thorny friends court for several minutes or a few days before mating. During a spring sunrise, female koura will climb to the top of a tall underwater rock, hold on tight and release all their little ones from under their tails into the current.

Small female koura can have over 20,000 eggs but large females have over half a million. Eggs from large female koura are bigger too, meaning their offspring have more resilience against starving in the incredible ordeal they are about to undergo. Miracles of nature take those eggs up to 1000km out to sea as they transform into larvae and other strange life stages - a big OE over a few years.

Strong eddies, like undersea cyclones, spin off Wairarapa and send some of the larvae back to the mainland coastline. Some adult koura then stay in their home reefs, while some march great distances together against the current in a huge migration.

Ngati Toa still share memories of being able to watch from the hills as koura in their thousands marched out of Porirua Harbour.

But much has changed. Human hunting has caused waves of depletion and overfishing. Since the 1950s, commercial fishing has taken the vast majority of koura.

Along the coastline from East Cape, around Bay of Plenty, up around Coromandel, the Hauraki Gulf and up to Waipu (crayfish management area 2/CR2), marine biologists have been ringing the alarm bells that crayfish populations have collapsed.

"Functionally extinct" is how experts describe this catastrophe. Because humans have so heavily hunted the biggest koura and snapper, which are the natural predators of kina, the kina populations have exploded. Kina have scoffed the seaweed forests around coastal rocks. But the young koura need seaweed to help hide from the fish larger than themselves, big starfish, small sharks and octopus that are hunting them down.

Within a human lifetime we have killed off the best breeding crayfish and set off a chain of events where their seaweedy reef homes have been wrecked. This can be turned around by creating more and bigger marine reserves and completely closing the CR2 fishery until it recovers, then very cautiously allowing reduced harvesting in future so this never happens again.

Where you can see crayfish

Although crayfish numbers are falling even in marine reserves, it's definitely worth the wonderful experience of snorkelling to see them in their world to show how it could be once again at:

•The marine reserve near Leigh
•Tāwharanui Marine Reserve, north of Auckland
•Te Tapuwae o Rongokako Marine Reserve, north of Gisborne
•Taputeranga Marine Reserve, Wellington