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Mar. 10, 2015 - Updated 04:16 UTC

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Putting Stamina Back in the Eel

Sep. 15, 2016

Dining on grilled eels is a mouth-watering tradition in Japan that's believed to provide stamina. But wild Japanese eels are disappearing from the menu.

The species is endangered, and conservation methods don't seem to be working.

Researchers in Kagoshima Prefecture are trying to figure out what the problem is, by seeing whether farmed and wild eels can co-exist.

"We’re researching how farmed eels interact with wild eels in rivers. These ponds simulate that kind of environment," explains one of the researchers.

They release equal numbers of wild and farmed eels into the ponds to see what happens.

Seventeen months later, the farmed eels are undernourished, and the wild ones are healthy and fat. After 20 months, 90% of the wild eels are still alive, but almost half of the farmed ones have died.

To find out why their survival rates are so different, the researchers put a wild and a farmed eel together in a water tank. The tank contains just one tube as shelter.

The 2 eels compete for territory. It's the wild eel that is the aggressor. About 9 times out of 10, the wild eel ends up occupying the tube.

"It was surprising to see eels being this aggressive. I think farmed eels have little chance of surviving in an environment where they’re in competition with wild eels," says Chuo University Associate Professor, Kenzo Kaifu.

The findings shed light on why releasing farmed eels in rivers where wild eels live has not been successful. The university researchers are now looking for scenarios in which the eels will not be in competition with each other.

They noticed that in rivers with weirs or other artificial barriers that are too high for wild eels to get over, they cannot swim far upstream. Farmed eels might have a greater chance of survival if they are released upstream.

There are also rivers where eels don't live. The team is now examining if they might be suitable for farmed eels.

"We’re starting to develop more effective and efficient methods. We hope our work will make a difference in helping the eel population to recover," says Kaifu.

Though eels spend most of their time in rivers, they travel to the deep seas to breed, which makes it difficult to track their behavior.

Scientists are carrying out studies in the South Pacific, where eels are abundant. A third of all eel species can be found in the region.

Markets in the region offer various kinds of fresh fish, but eels are rare. That's because the fish appears as a god in South Pacific legends -- as can be seen in the Fiji Museum, where a ritual club shaped like an eel is displayed.

Gods aren't meant to be eaten, and as a result, large numbers of wild eels can be found in rivers in the South Pacific.

A team of researchers from Japan and other places are studying their habitat in American Samoa. Katsumi Tsukamoto, a professor at Nihon University, is a leading expert on this fish. He thinks the Pacific is an ideal place to do research.

Many things are unknown about the ecology of the dwindling Japanese eel population, such as how they reach maturity. Studying South Pacific eels will give the researchers insights into their Japanese cousins.

"Tropical eels are genealogically close to ancestral species. So studying tropical eels can provide clues in researching Japanese and other eels in the temperate zone, including how they have evolved," says Tsukamoto.

About 30 researchers from 9 countries and territories spent 3 months on a Japanese research vessel. They sailed from New Caledonia to Tahiti to study eels in the South Pacific.

Their main objective is to find eel larvae. Known as leptocephali, these larvae range in length from a few millimeters to up to one centimeter. Team members hope that by carefully studying them, they can locate the eels' spawning grounds and migration routes.

In early August, leptocephali were found off the coast of Fiji. DNA analysis revealed they were a species found in Australia. The ones that are a centimeter long are believed to be 2 to 3 weeks old.

"The decline of the Japanese eel has made South Pacific eels a target for human consumption. We have to figure out soon how large the stocks are and the eels' habits. Then we can achieve another purpose of our survey -- to come up with guidelines for managing the eel population," says Tsukamoto.

The South Pacific is still a relatively safe habitat for wild eels. Researchers hope the knowledge gained here could be applied to Japan's dwindling eel population, and help lead to the successful management of precious eel resources.