NIWA scientist Eimear Egan is on a mission to find out more about the mysterious lives of eels before they reach New Zealand to start life in fresh water.
"They are very tiny, we can't really attach a device to track their movements and it's very challenging," she said.
"It's kind of the gap in the lifecycle of tuna or freshwater eels; it's the biggest critical knowledge gap that we have in New Zealand," she said.
The first challenge is collecting tiny, translucent shortfin and longfin eels, called glass eels, with modified whitebait nets from the Rangitāiki River mouth near Whakatane, as well as the Grey River and Ashley River mouths in the South Island.
The glass eels become elvers, or juvenile eels, in the river mouth, where they change colour.
"You're standing in the surf at night, there's no lights and you can't see what you're trying to catch," Ms Egan said in a statement.
"I feel like I'm a bit crazy, actually, because it’s quite challenging and it’s quite tedious work, labour intensive."
The majority of glass eels enter the river mouth at nighttime and are most active during full and new moons.
"Fish have an internal kind of a rhythm, they're cued in to their environment," Ms Egan said.
Ms Egan is hoping to find out where in the western Pacific Ocean longfin and shortfin eel larvae spawn, which ocean currents they're using to get to New Zealand and whether changing ocean temperatures are affecting them.
She said investigating the impact of climate change on freshwater New Zealand migratory fish needs to be a focus.
“We want to be able to protect them and conserve them for future generations,” she said.
That’s an aim shared by iwi, who are supporting the project.
“I want to know more so that our people do think, do get to eat it and for our future, for our kids,” Ngati Awa, Tuhoe member Toby Salmon said.
Mr Salmon said Rangitāiki River is a ‘cupboard’ for his iwi and sustainable practices are necessary to protect it and continue benefitting from it.
The glass eel’s ear bone is the key to finding out their birth date and what temperatures they lived in.
“We often refer to them as a fish's diary and the reason is they put down a ring in the ear bone every single day.
“We can look at the patterns of these rings and we can look at the chemistry of the ear bone to try and reconstruct what might have happened to them in the western Pacific Ocean,” Ms Egan said.
The ear bones are half a millimetre in size and have to be painstakingly hand polished to reveal the markings.
The extracted ear bones will be compared with previous collections from throughout New Zealand. Nitrogen and carbon in the glass eel’s body tissue will be analysed to identify what the fish had been eating during migration.
The study is set to be complete in 2021.