There are many lamprey species in the northern hemisphere, fewer in the southern hemisphere and here in Aotearoa we have just the one – piharau, also known as kanakana in the South Island, and pipiharau, pihapiharau, korokoro and nganangana besides. Its western names are Geotria Australis and the pouched lamprey – the latter a reference, says Dr Jane Kitson, to the "floppy sack" under the chin of the adult males. "They're not very attractive at that stage," she laughs.

A very secretive fish, there is a lot that still isn't known about them, and this is hindering the ability of iwi and state agencies to protect dwindling numbers. Their life stages are so different from one another that scientists in the past have mistaken them for different species.

Piharau/kanakana can be found far and wide, although their abundance declines the further north you go, with the highest numbers centred around Te Pātaka o Rākaihautū, Otago and Murihiku. They do however behave the same way across the country. "The babies leave the spawning site and head downriver, hiding in the sediment for about four or five years. They're just filter-feeders, filtering out the water and the bits and pieces in there," says Kitson.

Once they have matured into "mini-adults" they make their way out to sea. There they start the parasitic stage of their life-cycle, where they attach to larger fish using their disc-shaped mouths, and drain fluids from their host. Inside their mouths is a tongue that's stranger still – a raspy razor sharp blade that helps them scrape away tissue and scales. "They use their mouths for climbing too, rocks and waterfalls," Kitson says. "They're good at climbing, I know this from having them in my wash house where they climbed up the walls."

While at sea they transform from a kind of muddy brown to a bright blue for better camouflage. "Five odd years out at sea, and then they come back."

Sam Tamarapa (Ngāti Ruahine, Ngā Rauri, Te Āti Haunui a Pāpārangi) has been involved in efforts to manage and protect piharau in Taranaki for many years, but his interest in them started as a young boy tagging along with uncles in Whanganui.

He reconnected with the practice after moving home to Waitara in Taranaki, and meeting some piharau fishermen. Then one day, a man who he would often fish with told him he was moving away – but the spot where they fished together was designated for his whānau only. Tamarapa went to the man's father and asked if he could continue fishing there. "He said: 'Of course you can, as long as when you catch the fish, you share it out.' So that started my journey in terms of my hands on involvement," Tamarapa says.

He's since become an expert in the ways of the piharau. "The species is quite shy. It travels at night; it travels when the rivers are in flood, and they seem to like the coldest, darkest, wettest nights. They shy away from the daylight. There's probably a link with the moon as well because the tides can push them upriver."

He too is aware of the gaps in knowledge held about piharau. "When they're moving and migrating up the rivers, their sole purpose is to spawn. They're like salmon, they'll swim upstream, find a spot and spawn there."

Although different iwi have their own methods for catching piharau and kanakana, one thing they share is that those catching them today are using essentially the same methods as their ancestors. On the Waitara river, Tamarapa says, the whakaparu is still a common trapping technique.

The lamprey is considered a delicacy all over the world – especially in South America, Japan, and among First Nations peoples in Canada. King Henry I of England was known for his love of them and is reported to have died from a "surfeit of lampreys".

"It's quite fatty, quite oily. The old people used to say if you can eat two or three of the things in one sitting you're doing well," Tamarapa says. "You eat the whole thing. They have no bones, just cartilage. In Taranaki here we grill them on a fire outside. Put some netting down on embers – we have different ways of tying them together – and we lay them down on the netting and grill them. I'm getting hungry now thinking about it!"