After swimming greater than 2000 kilometres – from a river estuary in western Victoria to the Coral Sea off northern Queensland – it’s doubtless the feminine eel identified solely as “179353” met her finish in the huge, heat stomach of a whale.
How do we all know? For three months earlier than this Australasian short-finned eel disappeared, Victorian scientists had been monitoring her lengthy journey up the east coast to search out the mysterious location the place these eels go to spawn.
Using 40 gram satellite tv for pc transmitters connected to their dorsal fins, scientists tracked 16 eels, together with 179353, to pinpoint their location, temperature and depth, and observe their route. It’s the first time, worldwide, that any eel migration has been tracked from freshwater rivers to their spawning location. But immediately, they misplaced observe of 179353.
Luckily, a beachcomber on Lizard Island, about 1620 kilometres north-west of Brisbane, discovered her satellite tv for pc tag with a damaged antenna and posted it again to the scientists, permitting them to entry further information from its reminiscence.
“It showed the eel  had probably been eaten by a whale or a shark,” says Wayne Koster, a senior scientist at the Arthur Rylah Institute, half of the Victorian Department of Environment, Water, Land and Planning.
“All of a sudden it shot up to 37 degrees and stayed there for a couple of days. That’s the internal temperature of a mammal like a whale”.
Eel populations have declined dramatically over the previous 50 years in lots of areas of the world and short-finned eels are actually thought-about “near threatened” by the International Union for the Conservation of Nature.
A crucial half of their life cycle is the mysterious migration from freshwater waterways to distant oceans for a single spawning (when eggs and sperm are launched into water). This spawning has by no means been recorded.
Mr Koster and a workforce of consultants have revealed new analysis on uncovering the migration pathway of the short-finned eel in the journal, Scientific Reports. This species is present in waterways from western Victoria to southern Queensland.
Mr Koster and his colleagues launched the 16 feminine eels (that are bigger than the males and higher suited to carrying tags) into the Fitzroy and Hopkins rivers, and tracked two of them about 2500 kilometres into heat tropical waters in the areas round New Caledonia. The different tags had been shed a lot earlier.
Once eels enter the ocean they don’t feed, counting on saved fats reserves to gasoline them for the four-month journey.
The monitoring confirmed the eels swim a kilometre underwater throughout the day and are available again in direction of the floor at evening, most likely to keep away from predators.
“One of the really interesting things was they don’t all go the same way,” says Koster. “About half of them headed directly east across Bass Strait and then north, but some went south around Tasmania. It’s a much longer journey but gets them into deep water off the continental shelf much quicker.”
Previously, the solely clues to the spawning location had been small quantities of eel larvae picked up by trawlers in the south-western Pacific Ocean.
These younger, known as glass eels, drift on ocean currents for as much as a 12 months earlier than swimming up rivers, the place they could dwell for many years, feeding on small fish and crustaceans like yabbies.
Like heaps of freshwater fish (sure, eels are fish) are diadromous – they migrate between recent and saltwater to take benefit of totally different sources at totally different life phases.
The short-finned eels are often olive brown, however after they migrate down the river to breed they endure an unimaginable metamorphosis into “silver eels”, with a pale stomach and darkish again that helps them camouflage. Their eyes additionally darken and enlarge to see in the ocean depths
Eels have a selected cultural significance to First Nations individuals.
Gunditjmara conventional elder Denis Rose mentioned Aboriginal individuals in south-western Victoria constructed and used subtle aquaculture methods all through the Budj Bim cultural panorama to take advantage of eel migrations at the least 7000 years in the past. Gunditjmara individuals name them kooyang.
“Kooyang are a very hardy fish, they could be smoked and traded over large distances,” says Rose. “They’re still relatively plentiful, but we recognise eel stocks around the world are under pressure from overfishing and pollution and we want to keep an eye on the health of our eels.”
The Budj Bim panorama was just lately inscribed on the World Heritage listing as one of the world’s most intensive and oldest aquaculture methods.
The eels’ lengthy migration journey exposes them to predators. Most of the satellite tv for pc tags got here off early, which signifies a excessive stage of predation, most likely by sharks or whales.
This highlights the must maximise the quantity of eels that start the lengthy journey, says Mr Koster. This contains eradicating bodily limitations, putting in fish ladders and guaranteeing ample river flows to cue the eels for migration.
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