Auckland’s trapped dam eels get a human helping hand for epic Pacific migration

July 22, 2022
Nature
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Image Source: https://www.stuff.co.nz/

There’s suspense in the air as Watercare dam technicians Chris Oord​ and Christian Stockle​ drift through the calm waters of the Lower Nihotupu Dam in Auckland’s Waitākere Ranges.

Their small tin boat approaches the nets set up a few days before, to catch and transport eels out of the dam so they can migrate and breed.

Helping the eels is a condition of Watercare’s resource consent to operate the dam, but it’s far more than a formality for Oord and Stockle, who don’t take the responsibility lightly.

“The more I learn about these things, the more respect I have for them,” Oord says.

Each year between February and June, New Zealand’s longfin and shortfin eels embark on a journey to breed.

It is not known exactly where the eel spawning areas are, however scientists believe them to be somewhere between New Caledonia and Fiji, eel expert and NIWA emeritus scientist Dr Don Jellyman​ says.

Once there, the females release their eggs – 1.5 to 3 million for shortfins and 1 to 20 million for longfins – which are fertilized by the males, resulting in “huge numbers of young” – just a few eels could result in a decent “recruitment”.

After they spawn, the eels die.

The larvae then drift back to New Zealand with the ocean currents over a period of six to eight months – the process of which scientists are still learning about. But Dr Jellyman says there is no doubt that the entire migration is “pretty risky” for the eels.

“It’s fascinating. Somehow eels from all over New Zealand manage to arrive sort of at the same time to do this thing.”

Oord says the elvers (baby eels) and adults will “happily” climb in and out of the Lower Nihotupu Dam when it spills, but need some human help when it’s not.

Oord and Stockle begin to haul out their nets to see if they’ve caught any eels. Has Stuff’s visual journalist donned his wetsuit and waded into the dam for nothing?

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Success. Seventeen eels have been secured.

As the men haul the eels into their boat, things start to get “slithery” with slime, secreted by the eels as a defence mechanism. It also keeps them moist, allowing them to breathe through their skin while out of the water.

Back on land, the eels are evaluated for physical signs that they are ready to migrate, which include a blue ring around their eyes to help them see in deep, dark parts of the sea and elongated heads to make them more streamlined.

Their stomachs also change colour from yellow to silver, so that predators looking up mistake the silver as daylight.

The first eel weighs in at 1.8kg and – after a gentle struggle to get the eel to lie straight – measures 72cm in length. It is deemed by Oord to be “definitely” migratory.

Stockle helps put the eel into a large tub, covering it with wet Hessian net while it waits for its mates to be measured.

The next eel weighs a much smaller 250g, like a “pet eel”, Oord says. It doesn’t have any of the telltale signs it’s ready to breed, but is measured anyway, so the data can be given to the Ministry for Primary Industries.

“All the data is good data.”

Back into the dam it goes.

And then, a brief moment of panic – at least for this squeamish reporter – as several of the yet-to-be measured eels wiggle their way out of their tub and try to follow it to freedom. One succeeds.

“Things never go perfect when dealing with animals. You’re fooling yourself if you think you’re holding one in your hands,” Oord says of the slippery attempt to catch the escaped eels.

By the time Oord and Stockle have looked at them all, eight migratory eels have been identified, including three longfins. The biggest of the haul weighed 2.3kg and was 85cm long.

The tub of eels is then taken to an estuary on the Manukau Harbour, where there’s a good mixture of fresh and salt water, to help the eels adjust.

Dr Jellyman says people are increasingly recognising the importance of helping eels migrate, with most species under some kind of threat.

“Anything we can do to enhance the migration of eels to sea is worthwhile.”

The Department of Conservation deems shortfin eels, which are also found in Australia and some Pacific Islands, as a non-threatened species.

However, Aotearoa’s endemic population of longfin eels is declining, with human activities such as pollution, vegetation loss near habitats, overfishing and dam construction all having an impact.

Hone Pene​ of Ngāti Haua published a 2004 thesis about the depletion of tuna (eels) in the Waikato due to commercial fishing and pollution.

He says tuna have long been a well-respected taonga (treasure) and a special delicacy for iwi and tangata whenua.

Legend holds that Kupe, credited by Māori as the first person to discover Aotearoa, once saw “beautiful tuna” swimming in the river “and the rest was history”.

During Pene’s childhood, catching tuna was one of the first jobs for whānau at the marae or tangihanga, so that manuhiri (guests) could enjoy “the best of our delicacy kai”.

“If you’ve got eels/tuna on the table, it’s like ‘wow’.”

Pene said that in Morrinsville, where he grew up, iwi would “take a step back” after the first big post-summer rain, to sustain the population during the migration.

“If we don’t let them go, it’s game over. It’s not just about our generation, it’s about our tamariki and mokopuna to come.”

These days, up to 137 tonnes of native eels – including longfins – are caught by commercial fisheries and sent overseas as food.

As Oord and Stockle release the Lower Nihotupu Dam migratory eels into the estuary, there’s a real sense of accomplishment.

The pair have caught and released a total of 44 migratory eels from West Auckland’s dams so far this season, which is in its final days.

If those eels decide they don’t want to migrate, they can live happily in the estuary or find their way back to the dam. Any migratory eels in the dam that were missed will have another chance next season.

And once the offspring of this season’s migratory eels find their way back to New Zealand, Oord and Stockle will be there to help them back into the dam.

Stockle says they both love working with the eels.

“It’s definitely one of the best parts of the job.”

Story Source: https://www.stuff.co.nz/environment/128896617/aucklands-trapped-dam-eels-get-a-human-helping-hand-for-epic-pacific-migration

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