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Scientists hoping to solve great eel mystery

NIWA freshwater scientists are pinning their hopes of solving an age-old mystery on 10 female longfin eels who are about to begin an epic journey to their spawning grounds somewhere in the Pacific Ocean.

NIWA freshwater ecologist Dr Paul Franklin says no one really knows where the eels spawn and this has long been a gap in knowledge about their life cycle.

“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.”

Each of the eels, which are endemic to New Zealand and can grow up to two metres long and weigh up to 25kg, have been fitted with an electronic tag that will record temperature, depth and light as they migrate. It is a once-in-a-lifetime journey for the eels, who die after spawning.

The tags, weighing about 40 g and 12 cm in length, are programmed to be released from the eels at different times from five to eight months after being attached. They will then pop up to the ocean surface and transmit their data via satellite, hopefully providing scientists with enough information to determine their migration route.

The eels, which live for about 60 years but can get to as old as 100, were caught in the Waikato River and fitted with the tags at Te Kauwhata last week. Eels about to migrate develop a blue ring around the eye as well as other characteristics, making it easier to sort the travellers from the stayers.

If scientists are right about the likely breeding area, it will take several months for the eels to get there. They stop feeding once they begin migrating and travel up and down in the water column as they migrate – spending daytime at depths of up to 800 m and coming back near the surface at night.

Once the eels arrive they spawn and then die, 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. 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,” Dr Franklin said.

Longfin eels have a complex life cycle. After spawning, the larvae are transported via ocean currents to New Zealand to grow and mature in fresh waters. They are classified as at risk and declining.

NIWA scientists are also trying to find out more about the eels’ early life history, including larval migration routes. They want to learn if the numbers making it to New Zealand are affected by processes happening during their marine life.

One project involves studying the ear bones of the glass eels – the stage when they are about 7cm long. The ear bones add a layer of calcium carbonate each day, like ring tree rings, which can provide information on growth, diet and movement.

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