Commercial fishers still use traps with the same form - but made of wire or nylon. On the Whanganui, Ngati Hau people at Jerusalem make the best traps.
The class began with an eeling karakia the children had learned. Mr Potaka said he learned it himself, from Te Wera Firmin, and saw it work on a day out on the river at Atene.
"Within an hour we had a big bag of eels. We took them all back to a tangi at Parikino."
Pa tuna, large constructions, were used to direct the river's water to flow to one point, where a trap held the eels.
All the eels in the Whanganui River are long-finned, Mr Potaka said, and they can be found right up to the snow line. Local lakes, such as Wiritoa and Kaitoke, have smaller short-finned eels.
Eels that become landlocked and can't get to sea to breed are called tuna kuia.
All New Zealand's eels migrate to a deep trench near Tonga to breed, although no one knows exactly where it is.
"They have tried all sorts of things to track them but the eels go so deep they can't track them."
The smaller, male, eels leave first, in February. By July all have them have gone.
Young tuna, called glass eels, are transparent and leaf shaped. They swim back to New Zealand and colour up as they go inland. They can end up in any river and Mr Potaka catches a bucketful at Pungarehu, then releases them.
"They're not good to eat but I go every year to prove that I can still catch them."
He'd like a "Polynesian triangle" on the seas from New Zealand to Tonga and Easter Island to protect young tuna.
"There are people from other countries that catch those little eels before they get home."
He'd also like people catching eels in the Whanganui to release any over 2.5kg, to give the larger females a chance to breed.
"If we all eat the big ones we would have none going out to sea to make the babies."
Ngai Tahu has a similar ban, on taking eels over 2kg, in the South Island.
He told the children he came from a hunting and fishing family. He grew up at Parikino when there was no electricity and successful fishing was a matter of survival.
"All we had was pigs, sheep and cattle and fish to eat. The river was full of fish, full of eels.
"We had to catch eels to stop going hungry."
Kura principal Miriama Harmer asked what can be done to keep eel populations strong.
Mr Potaka says wetlands protect small eels, and there used to be many more of them.
Looking after eels was more about looking after the environment, rather than banning commercial fishing.
Run-off from nitrogen fertiliser isn't good for eel populations and he suggests waterside plantings to soak up run-off before it reaches the river.