ByShi En Kim
Published February 9, 2024
The narrow strip of murky water rippling along the concrete-lined banks of Japan’s Dotonbori River in Osaka may be the last place anyone would think to find wildlife, let alone an endangered species.
Japan’s third most populous metropolitan area, Osaka, is famous for its round-the-clock entertainment, endless shopping districts, and seas of crowds. At the heart of its bustling downtown is the Dotonbori River, where millions of tourists flock annually to bask under the iconic billboards, their multicolored glare spilling into the river. The water itself is not particularly attractive, with an expert once advising that swimming in it would be like “jumping into the toilet bowl.”
But the inhospitable depths of the Dotonbori hide a familiar creature: The river is home to Japanese eels, exactly the kind you would be served if you ordered unagi at a Japanese restaurant. Last year, the Research Institute of Environment, Agriculture and Fisheries, in the Osaka Prefecture, and the Mainichi Broadcasting System Television jointly announced the capture of 11 individuals in Dotonbori’s waters. It was the first record of live Anguilla japonica there.
The eels in these boat-laden waters are a shining example of wildlife’s resilience in urban centers, an encouraging sign for the species and a reminder that cities can be biodiversity refuges deserving of protection.
Yoshihiko Yamamoto, the institute’s lead researcher, says that he hopes that his team’s find will spark public interest in everyday conservation. “I’d like people to learn more about the connection between our daily lives and the places we live in,” he says. “If people think that eels and other aquatic creatures may be living in their local rivers, they will become interested in biodiversity and conservation.”
Riverine urbanites
Despite their name, Japanese eels live not only in and around the islands of Japan but also coastal China, the Korean peninsula, and the northern Philippines. After spawning in the ocean, the eels move inland, spending most of their lives in rivers and estuaries, like salmon but in reverse. Japanese eels can tolerate a wide spectrum of conditions, from brackish waters at river mouths to freshwater further upstream.
Despite their adaptability, Japanese eels—in Dotonbori and elsewhere—don’t have it easy. Their numbers have been declining since the 1970s, after two decades of economic prosperity in Japan led to a boom in riverine construction projects. The government built dams and transformed meandering rivers into ramrod-straight channels to reduce flooding. Habitat destruction, pollution, and artificial lighting have further contributed to their dwindling count.
The ubiquity of unagi in restaurants and supermarkets may give a false sense of security regarding the eels’ abundance in the wild. Nearly all of the unagi supply on the Japanese market is from farmed stock—captured from the wild as juvenile glass eels and raised in captivity until they’ve matured into yellow eels. Under government-regulated catch limits, Japanese fishermen catch as much as 80 percent of wild glass eels that enter the rivers.
While the population size is unknown, the fishing haul volume is telling: The global catch of Japanese eels in all life stages has dropped from a high of 3,600 tons in 1969 to 120 tons in 2019. In 2014, the International Union for Conservation of Nature listed A. japonica as an endangered species. Despite the numbers, Japanese eels are not rare—yet. They have a widespread footprint, as determined from the DNA particulates they shed into the environment.
Genetic research conducted in 2021 found that eels likely reside in most rivers that snake inland from the Pacific coast and further north, in the tidewaters cradling the Sea of Japan. These genetic clues had also been detected near estuaries surrounding the Osaka Bay, hinting at the presence of live eels in the river, but it wasn’t until Yamamoto and the Mainichi Broadcasting System Television’s excursion that their presence was proven.
Pit stop or prison?
Japanese eels are shy. These nocturnal fish hide between rocks or burrow in the mud during the day, making them hard to catch. Not that you should try—eel fishing is often illegal in urban rivers, including Dotonbori.
One clear November evening in 2022, with an autumn chill lingering in the air, a research vessel bobbed on the surface of the Dotonbori. On board and in front of rolling cameras, Yamamoto’s team—which also included local entertainer Ken Kojima—hoisted up 10-foot-long lines with trailing arms of bait-tipped hooks. Yamamoto and Mainichi producer Takeshi Ozaki had spent countless hours wrangling for government permits to fish in Dotonbori for an educational variety TV program, and their efforts were finally about to be rewarded. Dangling at the ends of the lines were a handful of Dotonbori’s best-kept serpentine secret: yellow-flank eels, about two feet long and a half-pound heavy. Three live eels are now on public display at the Osaka Biodiversity Research Center.
Some rivers host eels that escape from captivity or are placed there by fish farmers. Yamamoto’s tissue analysis of several of the Dotonbori dwellers revealed that they likely migrated of their own accord from the ocean.
The lifecycle of Japanese eels is one of intermittent transformation. Younglings hatch in the ocean, then drift to the coast as willowy, transparent glass eels. As they settle in estuaries or migrate upstream, the juveniles fill out into black-backed yellow eels and live as homebodies, generally keeping within half a mile of their hideout and on the same side of the river. After five to 10 years—or, for one Methuselah, 22 years—the eels undergo a final metamorphosis into chonky silver eels and make one last journey to their original birthing grounds near the Mariana Islands to spawn.
The Dotonbori’s eels may have naturally migrated from the sea, but their presence still raises questions. The Dotonbori, arguably Japan’s most famous urban river, is far from an ideal choice for a home. “It feels less like a river and more like a drainage channel,” says Leanne Faulks, a freshwater biologist at Nagano University who wasn’t involved in the Dotonbori survey. “It is hard to imagine that you would find some hiding spot there.”
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The water quality in the Dotonbori has improved over the years, but the river is still fairly polluted, and it’s unclear how that may affect their survival and breeding. “We need to study the growth and sexual maturation of eels in urban rivers and also whether they will potentially contribute to spawning in the ocean,” says Jun Aoyama, an aquatic researcher at the University of Tokyo who wasn’t involved in the discovery.
Another important question: “Are they there by choice?” Faulks asks. Are the eels long-term inhabitants of the river or simply passing through on their migration? Or, given their long lifespans, they might be stuck, fenced in by human-made structures that were erected after their arrival and before they were ready for their pilgrimage back to the Pacific.
Yamamoto calls for fish ladders—sloping structures that allow fish to hop over an obstacle—to be placed along the Dotonbori and other urban rivers to ensure eels don’t get trapped when they’re ready to return to the ocean.
He and other researchers also are dropping ishikura, rock-filled cages, into concrete-smoothened rivers as artificial homes for the eels to nest in, helping ensure the eels—
however long they’re in the river—have a safe shelter for suited for their reclusive lifestyles.
A beloved delicacy, threatened
There’s another possible reason for A. japonica’s steep decline: overfishing. Tackling the demand side of the equation could be the most challenging measure of all, given Japan’s culinary fixation on eels.
Japan alone accounts for over 70 percent of the global consumption of freshwater eels. Like most Japanese people, Yamamoto and Ozaki admit that they too love eels as menu items as much as they do as research subjects.
So far, artificial breeding hasn’t been able to keep up with demand in Japan, estimated to be at least 40 tons, or 200 million glass eels, a year. With the country’s capacity to produce fewer than a thousand captive-bred eels annually, the hunting pressure on wild populations remains high. Illegal imports and poaching of Japanese eels further threaten their numbers in the wild.
Perhaps Japan’s eel craze is most apparent on Doyo no Ushi no Hi, a day in midsummer that’s one of the hottest of the year. To get through the heat, Japanese people gorge on nutritious unagi. The centuries-old tradition has turned into a celebratory fest, with roadside stalls and supermarkets nationwide selling kabayaki grilled eel.
The culinary love for eels is a hard habit to kick. The past years have seen calls to substitute Japanese eels with other eel species or even vegetables as ingredients, or do away with eels altogether. These alternatives haven’t caught on yet, but soaring Japanese eel prices have curbed consumer appetite for unagi somewhat. Researchers fear that the country will also start importing other species of eels en masse from Southeast Asia, where regulations are much looser.
Faulks herself avoids eating eels, though she recognizes that not everyone is prepared to make the same choice. For now, eel stocks remain stable, she says, thanks to the government’s annual cap for wild-caught glass eels at 21.7 tons. She’s currently trying to estimate the population of A. japonica using genetic techniques so that catch limits may be informed by current, concrete data.
The confirmation of eels in the Dotonbori is a promising sign that even heavily industrialized rivers can serve as habitat for wildlife. But there’s a difference between dodging extinction and thriving, and this discovery underscores the need to respect—and restore—urban waterways to support declining species. The Japanese eel may have been a mainstay of the country’s cultural past, but its present status is precarious at best, its future uncertain, its future uncertain.
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