Last week I was crossing the River Thames on the way to work in London, and I happened to see a cormorant emerge from the water with a thrashing eel in its mouth. The bird juggled the fish, skillfully managing to position it so it could swallow the wriggling animal headfirst.
While it was a disaster from the eel’s point of view — after all, they can live for up to 20 years before breeding — it was for me quite an encouraging sight. The number of European eels has crashed in recent years, so to see one in the Thames is a good sign. Even if it was being eaten.
Eels — whether European or Japanese — are elusive, mysterious creatures. We have eaten them for millenniums, but we still don’t understand the basic details of their reproductive behavior. They spend most of their lives in freshwater rivers, but then migrate in vast numbers, with Japanese eels swimming thousands of kilometers to spawn at an unknown location near Guam and the Mariana Islands. European eels have a similarly mysterious spawning ground somewhere in the Sargasso Sea.
That ignorance is now more pressing than ever, as both Japanese and European eels are officially classed as being in danger of extinction.
To foodies, unagi (Japanese eels) need no introduction. A famous, cherished national dish, some 100,000 tons of the animal are eaten in Japan each year. (This accounts for around 70 percent of the worldwide consumption.) The “u” of unagi is often written in hiragana on shop signs in the shape of an eel.
When I worked at the National Institute for Environmental Studies in Tsukuba, Ibaraki Prefecture, one of my colleagues was famous for his love of eel. He couldn’t wait for Doyo no Ushi no Hi (Midsummer Day of the Ox), the traditional day when eel is consumed, supposedly to counteract the summer heat.
But consumption of eel is now year-round, and as a result, supplies are becoming scarce. By some counts, numbers have crashed by around 95 percent from the 1960s.
In response, the International Union for Conservation of Nature has now put Japanese eels on its Red List of Threatened Species. People may be becoming aware that the amount of eel being consumed is unsustainable, and even the slow-moving Fisheries Agency might act to protect the animal.
What people don’t seem to appreciate, however, is just how mysterious the eel is.
“Although our ecological research is often covered by the media and we have public lectures and symposiums, many Japanese people don’t know about much of the mystery of eels,” says eel biologist Mari Kuroki, of the Department of Aquatic Bioscience at the University of Tokyo.
Kuroki has written a book for children, “Unagi no U-chan: Daiboken” (“Adventures of U-chan the Eel”), so that they will at least learn of the dangers faced by eels.
Foodies may know that the best eels are held to come from Lake Hamana in Shizuoka Prefecture, and some people think this is also where they reproduce. Not so.
Eels live for some years in freshwater lakes and brackish estuaries before they mature and become “silver eels.” Then, no longer feeding, the eels start heading out to the ocean. All over East Asia eels leave their rivers and start swimming.
They swim for weeks, covering thousands of kilometers. How do they know where to go? How do the baby eels, when they are hatched in the depths of the ocean, get back to freshwater? We don’t know.
“The spawning behavior and the spawning migration route still remains a mystery,” says Kuroki.
Some facts are known. The baby eels feed on something called marine snow — a shower of dead plants and animals that slowly and continuously drifts down through the ocean. The peculiar nature of this food makes raising baby eels a tricky business.
The route of the journey back is unclear. Some part of the route is determined by ocean currents, and some by active swimming.
“Overfishing is apparently the main threat,” says Kuroki. Overfishing affects the numbers of young eels that can establish themselves in rivers, because commercial fisheries remove young eels to raise them in farms.
But fishing also affects the mature animals — these are caught by fishermen (in the olden days by men using cormorants to catch the eels for them) as the eels migrate back out to sea.
“Also, the degradation of the river environment is a serious factor,” she adds.
Kuroki and her colleagues are tagging eels with radio transmitters in order to better understand the route taken by these unusual animals. One thing recently discovered about European eels is that they can be eaten by whales.
To some extent, eels are at the mercy of ocean currents. And these are known to be changing as the ocean gets warmer as a result of climate change.
“The return route to the ocean and the mechanism of adult eel migration is still not well validated,” Kuroki says. “If they use ocean currents, a slight global climate change could have a big influence.”
One of the things I love about Japan is how a respect for nature is deeply woven into society, culture and legend. Eels are a prime example. They appear in many rakugo comic stories, and lend themselves to various idioms.
And the famous legend of the country being balanced on the back of a catfish, whose wiggling is supposed to account for earthquakes? In some regions the foundation animal is said to be an eel.
So in that sense the animal is the literal foundation of the entire country. The species must be preserved!
Rowan Hooper (@rowhoop on Twitter) is the news editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human).”