The epic odyssey of Australasian eels to spawning grounds spans all the way from south-eastern Australia to the far reaches of the tropical Coral Sea near New Caledonia, according to a study published in Scientific Reports.
That’s a whopping 2500km journey through treacherous ocean waters.
Short-finned eels (Anguilla australis) are ecologically, economically, and culturally important creatures in Australia. They are top-order predators in freshwater streams and are valuable to Gippsland commercial fisheries. Beyond this, they have strong cultural significance to First Nations peoples. For example, the Gunditjmara people built complex aquaculture systems throughout the Budj Bim cultural landscape in south-west Victoria at least 7000 years ago – landscape that is now on the World Heritage list.
Sadly, eels have been in decline because of habitat loss and barriers to movement, and little was known about their oceanic odyssey to spawning grounds because of their complex life cycle.
Now, a team of researchers led by Dr Wayne Koster of the Arthur Rylah Institute for Environmental Research, in collaboration with Gunditjmara traditional owners, have tracked short-finned eels from Victorian estuaries right up to the Coral Sea using satellite archival tagging.
“Information of migration routes is really important to understand the potential impacts we might have on the marine environment,” says Koster. “That includes the impacts of human activities in the marine environment, like offshore development, deep-sea drilling, seismic testing, the other management applications.”
This is the first-ever example of eels being tracked long distance to a suspected spawning ground.
The complex life cycle of eels
“Their whole life cycle is pretty complex,” says Koster. “The eels we find in local rivers and streams were born thousands of kilometres away in the ocean.
“The larvae [baby eels] float on ocean currents for months and months, and eventually they make their way into freshwater. They will possibly stay there for 10, 20 or 30 years until they mature and begin a journey back down the river and into the ocean.”
After the mature eels return to spawning grounds – once again, travelling thousands of kilometres – they spawn just one time.
“After that they die” says Koster. “That’s the end of the journey for those eels.”
This makes conservation tricky because many eels die before they get back to the spawning grounds. This is why it’s so important to learn about what hazards the travelling eels face.
How to tag an eel
To understand the eel odyssey, the researchers tagged individual eels to keep track of them via satellite. But eel tagging is no easy task.
“It’s a lot of hard work with a lot of hard nights spent trapping,” says Koster. “The tags that we use are fairly big and so we put them on the biggest, strongest eels possible.”
The team used special cone-shaped nets to catch the eels, but for every 50 or 100 eels caught, often only one is big enough to actually tag.
“So we’ve done weeks and weeks of nights out trying to catch them,” remarks Koster.
But thankfully, the effort yields results.
Dangers of the eel odyssey
As it turns out, there were many dangerous things eels face on their journey.
The researchers found that the eels moved up and down in the waters depending on the time of day. When the sun was out, they dived deep and travelled at up to 900 metres below the surface, but sat at a shallow and warm depth of 100–300m during the night.
The changing depths may have helped the eels thermoregulate and avoid predators – the main obstacle that stops them reaching their tropical destination.
“There’s a lot of predation of the migrating eels in the marine environment,” says Koster. “Lots of the tagged eels were eaten by predators, like white sharks and whales.
“And that’s really important to know in terms of fisheries management.
“Management rules tend to be based around ensuring enough eels escape out of freshwater to breed, but it doesn’t factor in the possibility that a lot of them might not actually make it [to spawning grounds] because they’re getting eaten along the way.”
Fascinatingly, the eels don’t always follow the same route. Some of them choose the shortest distance, heading directly east across Bass Strait and then making their way up to tropical waters.
Others take a leisurely detour around Tasmania.
“It’s a longer journey that way, but it gets them into deep water much quicker and that might have the benefit of helping them to avoid predators,” explains Koster.
“But we don’t really know the mechanisms underlying that variation in the pathways yet.”
Deborah Devis is a science journalist at Cosmos. She has a Bachelor of Liberal Arts and Science (Honours) in biology and philosophy from the University of Sydney, and a PhD in plant molecular genetics from the University of Adelaide.
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