Scientists are finally about to discover where a slimy swimmer of our streams and rivers winds up after making an epic, once-in-a-lifetime Pacific odyssey.

Having tagged a group of native longfin eels, Niwa's Dr Paul Franklin and his colleagues hope to crack an age-old mystery: where they go to spawn, and then die.

Longfin eels, found in almost all types of waters throughout the country, are extraordinary creatures for several reasons.

Young eels, called elvers, are capable of climbing 20m waterfalls and even dams.

Advertisement

Older eels have been recorded living to at least 60 years old, and growing to as heavy as 40kg – or the equivalent weight of a five-year-old boy.

But yet more intriguing is what these threatened, secretive and largely nocturnal fish do at the end of their lives – complete a massive journey out into the Pacific Ocean to find their spawning grounds.

Franklin, a freshwater ecologist, said no one really knew where the eels spawned, which had made for a big gap in knowledge about their life cycles.

His team recently attached electronic tags to 10 female eels caught in the Waikato River near Te Kauwhata.

Eels about to migrate developed a blue ring around the eye as well as other characteristics, making it easier to sort the travellers from the stayers.

The tags - recording temperature, depth and light, and weighing about 40g and 12cm in length – were programmed to be released from the eels at different times from five to eight months after being attached.

Scientists have attached electronic tags to 10 female eels recently caught in the Waikato River near Te Kauwhata. Photo / Stuart Mackay, NIWA
Scientists have attached electronic tags to 10 female eels recently caught in the Waikato River near Te Kauwhata. Photo / Stuart Mackay, NIWA

They would then pop up to the ocean surface and transmit their data via satellite, hopefully providing scientists with enough information to determine their migration route.

"We know from previous Niwa research that the most likely breeding zone is a large area between Tonga and New Caledonia but we are hoping this project will give us a much more precise location," Franklin said.

Advertisement

If scientists were right about the likely breeding area, it would take several months for the eels to get there.

They stopped feeding once they began migrating and travelled up and down in the water column as they migrated – spending daytime at depths of up to 800m and coming back near the surface at night.

Once the eels arrived, they spawned and then died, leaving their larvae to be transported back to New Zealand on ocean currents.

"Knowing the location of eel spawning grounds has been a challenge worldwide," Franklin said.

"For our New Zealand longfin eel, we just need to find that final bit of information to nail down where they go. Hopefully we may be close to solving that by Christmas."

The tags - recording temperature, depth and light, and weighing about 40g and 12cm in length - were programmed to be released from the eels at different times. Photo / Stuart Mackay, NIWA
The tags - recording temperature, depth and light, and weighing about 40g and 12cm in length - were programmed to be released from the eels at different times. Photo / Stuart Mackay, NIWA

Niwa scientists were also trying to find out more about the eels' early life history, including larval migration routes.

Advertisement

They were trying to find out whether the numbers making it to New Zealand were being affected by processes happening during their marine life.

One project involved studying the ear bones of the glass eels – the stage when they are about 7cm long.

The ear bones added a layer of calcium carbonate each day, like ring tree rings, which could provide information on growth, diet and movement.