More than anything, it’s still a mystery how the eel gets there. What route does it take? How does it find its way and how does it get there on time? How can an eel make it almost five thousand miles from the rivers and waterways of Europe across a deep ocean to the other side of the Atlantic in just a few months?
In 2016, a European research team published a report on the most extensive study ever of the European eel’s journey toward the Sargasso Sea. Over five years, a total of seven hundred silver eels had been tagged with electronic transmitters and released from different locations in Sweden, France, Germany, and Ireland.
As the eels turned west and the transmitters eventually fell off and floated to the surface, loaded with information, the researchers could form a picture of what their journey actually looks like.
At least that was the idea, but as is so often the case where eels are concerned, things didn’t turn out as planned. Of the seven hundred transmitters, only two hundred and six yielded any information at all. And of those two hundred and six eels, only eighty-seven got far enough into the sea for their information to reveal anything useful about what their journey had been like.
But data from eighty-seven silver eels’ journeys toward the Sargasso Sea is still far more than we had before, and the results revealed a lot about what a complex and difficult process this yearly migration really is. The first finding was that the eels swam both day and night and seemed to employ a deliberate strategy to avoid danger. During the day, they moved through the darker and much colder water at a depth of about three thousand feet. At night, under the cover of darkness, they rose up toward the warmer water nearer the surface. Even so, a large proportion of the eels disappeared during the earliest stages of the journey, falling prey to sharks and other predators.
What the researchers could also see was that not all eels are in a hurry. In theory, the journey to the Sargasso Sea is plausible. Experiments have shown that an eel swimming at normal speed moves slightly farther than half its length every second, and a silver eel on its way to the Sargasso Sea, which no longer hunts or eats or lets any of life’s distractions slow it down, can swim without stopping for at least six months using nothing but its fat reserves as fuel. If you draw a line on a map, from any given place in Europe to the Sargasso Sea, and calculate how fast it would need to swim in order to arrive by May at the latest, the eel’s journey is certainly possible. Very long and difficult, but possible.
Among the eels in the study there were, however, many that didn’t seem to realize what was actually required of them, or how little time they had. A few impressive individuals did cover an average of thirty-one miles a day, but others managed only two.
The eels also chose wildly disparate routes. Clearly, many roads lead to the Sargasso Sea. The majority of the eels released on the Swedish west coast, for example, chose a northerly route, up through the Norwegian Sea and then west across the northeast Atlantic. They all chose roughly the same path, apart from a single eel, which after reaching the Atlantic suddenly veered east and disappeared without a trace outside Trondheim, Norway.
The eels released in the Celtic Sea south of Ireland and in the French Bay of Biscay, on the other hand, headed south before turning west. One of them meandered about west of Morocco for more than nine months before making it all the way to the Azores.
The eels released off the German Baltic coast took different routes. Some followed the Swedish eels, setting their sights on the Norwegian Sea. Others headed south through the English Channel. But none of them reached the Atlantic.
The eels released from the French Mediterranean coast swam, predictably, west toward Gibraltar, but only three of them managed to get through the straits and into the Atlantic.
At first, the results looked random, to say the least. The eels’ movements traced strange patterns on the map, as though someone had tried to draw a maze blindfolded, or as though nothing was predetermined and every journey was the first. But at least one thing was made unambiguously clear: the majority of eels never make it to their spawning grounds. The long journey back to their birthplace remains for most of them a thwarted aspiration.
That may seem like a bleak outcome, both for the eels and for the scientific study. Not one of the seven hundred silver eels released could be tracked all the way back to the Sargasso Sea. It’s impossible to say if any of them reached it. Sooner or later, they disappeared into the depths, leaving the realm of human knowledge while their electronic transmitters floated up to the surface.
Nevertheless, the research team managed to draw some new and fairly remarkable conclusions from their observations. Their initial finding was that the eels’ migration is likely more complex than previously thought, but that it could be explained—at least in part. Because from the observations that at first seemed random and unpredictable, a pattern eventually appeared. Firstly, it was clear that the eel rarely takes the shortest route from its starting point to its goal. Its journey isn’t like the journeys of birds or airplanes. Nevertheless, all of Europe’s eels seem to rendezvous somewhere around the Azores, about halfway through their journey, and continue west toward the Sargasso Sea from there in much closer formation. If the journey starts in uncertainty and slight confusion, it becomes more deliberate as it progresses.
The researchers also discovered something else that complicates our understanding of the eels’ migration. When old specimens of leptocephalus larva caught in the Sargasso Sea were reexamined and compared for size and growth rate, they showed that the eel’s spawning season probably starts earlier than previously thought, possibly as early as December. That would mean breeding commences around the same time the last silver eels set off from the coasts of Europe, which only serves to make the question of how they get there on time even more vexing.
But the explanation, the researchers claimed, must, of course, be that all eels don’t make it across the Atlantic in time for the next breeding season. For some, the long journey back to the Sargasso Sea can take much longer. Perhaps eels simply adjust their speed and route according to their abilities. While some swim as fast as they can in order to reach the Sargasso Sea in early spring, some take a considerably more leisurely approach and wait for the next breeding season instead. While an eel setting off from Ireland, for instance, can travel west in an almost straight line and get there by spring, an eel coming from the Baltic Sea might aim to arrive in December, more than a year after it first set off. That would not only explain the differences in the behavior observed but also lend some kind of logic and relevance to what at first seemed random. Maybe eels are, quite simply, individuals, who not only have different abilities but also different means and methods of reaching their goal. Maybe they’re all aiming for the same destination, but no two journeys back to the origin are exactly the same.
AND THUS, ONE QUESTION REMAINS, AND IT IS ONE THAT APPLIES TO both eels and humans: How do they know which route will take them back to where they came from? How do they find their way back home?
That the eel has special abilities that make it skilled at navigating great distances has long been known. It’s well established, for example, that it has a phenomenal sense of smell. According to the German eel expert Friedrich-Wilhelm Tesch, who wrote the standard reference work The Eel in the 1970s, the eel’s olfactory sensitivity is on par with a dog’s. Put one drop of rosewater in Lake Constance, Tesch claimed, and an eel can smell it. It’s likely that eels use smell in some way during their journey across the Atlantic, either to locate the Sargasso Sea itself or at least one another. It’s also likely that the eel is sensitive to changes in temperature and salinity and that these might offer clues as to which way to go. Some scientists believe the eel’s well-developed magnetic sense constitutes its primary navigational tool. Much like bees and migrating birds, it can feel the earth’s magnetic field and is thus guided toward a certain destination.
We know what that destination is. And somehow, the eels know it, too. They know where they’re going, even if th
e routes they choose can be both meandering and unpredictable. But how they know is one of the mysteries still surrounding the eel question, one of the enigmas even scientists cherish.
Rachel Carson, for her part, described the eel’s inherited knowledge about its origin as something more than an instinct. In Under the Sea-Wind, she writes about how the fully grown and sexually mature eels one autumn suddenly feel a “vague longing for a warm, dark place,” and how these eels, who have lived their long lives “beyond all reminders of the sea,” in lakes and rivers, now set off into the unfamiliar open ocean, finding there something familiar, something they recognize, a sense of belonging “in the large and strange rhythms of a great water which each had known in the beginning of life.”
Do they remember where they came from and where they’re going now? Do they remember their very first journey across the Atlantic as tiny, transparent willow leaves? No, perhaps not in a human, conscious sense, not according to our definition of memory. But when the European research team who followed the more or less successful attempts of seven hundred eels to reach the Sargasso Sea tried to explain how the eels find their way back to their birthplace, they still described the experience as a kind of memory. It seemed, they wrote, as though “eels follow olfactory cues originating in the spawning area or that eels navigate using oceanic cues imprinted or learned during the leptocephalus phase.”
Because what their study revealed more than anything was that the farther the eels got, the more they seemed to end up following a predetermined route. Simply put, they seemed to follow the Gulf Stream and the North Atlantic Drift, but in the opposite direction. As though a memory, a map, had been ingrained in them when they made the journey from the Sargasso Sea to Europe as tiny, transparent willow leaves, and as though that memory had survived in the eels, remaining constant through all their metamorphoses, for ten, twenty, thirty, or fifty years, until one day it was time to make that same journey in reverse, straight toward the mighty ocean current that had once carried them helplessly to Europe.
AND SO THE SILVER EEL FINALLY COMES HOME TO ITS BIRTHPLACE, its Sargasso Sea, and at the same time, it disappears out of sight and our realm of knowledge. No one has ever seen an eel in the Sargasso Sea.
Some have tried, however. After Johannes Schmidt’s years-long expeditions in the early twentieth century, it would be a while before anyone set off for the Sargasso Sea to look for the eel again, possibly because Schmidt’s work was so persuasive, but perhaps even more likely because it was so discouraging. But the past few decades have seen an increase in research traffic to the Sargasso Sea, expeditions manned by some of the most prominent eel experts in the world. They’ve gone to seek deeper knowledge of the eel’s migrations and reproduction, to test existing theories by verifying or disproving them, but also to find what no one has yet been able to: a living eel in the Sargasso Sea.
The German marine biologist Friedrich-Wilhelm Tesch went on a major expedition with two German ships in 1979, the eventual result of which was the much-cited article “The Sargasso Sea Expedition, 1979.” The expedition took place in the spring and roved across large parts of the eel’s supposed spawning area. Tesch was able to employ his nets and trawls in the exact location where breeding was thought to occur; like Schmidt, he caught large numbers of tiny leptocephalus larvae, but other than that, he found no sign of the presence of eels. For example, seven thousand fish eggs were collected, but closer examination revealed that not a single one came from an eel. It goes without saying that researchers didn’t see any mature breeding eels either.
The American marine biologist James McCleave, who for more than thirty years has been one of the world’s leading eel experts, went on his very first marine expedition together with none other than Friedrich-Wilhelm Tesch in 1974 and undertook his first journey to the Sargasso Sea in 1981. Since then, he and his team have returned seven more times, using a range of sophisticated methods to try to catch at least a glimpse of an eel. McCleave has posited a theory according to which areas where different bodies of water of different temperatures meet—so-called front regions—provide eels with exactly the right conditions for procreation. It is in such locations that he has caught the smallest specimens of leptocephalus larvae, and it is also where he has most zealously looked for adult eels. James McCleave has sailed back and forth across these regions, with ships equipped with advanced acoustic instruments designed to pick up echoes from breeding eels in the deep. And he has, in fact, recorded echoes very likely produced by living, breeding eels; each time he has tried to catch them, however, his nets have come up empty.
During one expedition, together with a fellow marine biologist, Gail Wippelhauser, McCleave employed almost malicious cunning to lure the shy eels out of the depths. Their team had caught a hundred fully grown female American eels and injected them with hormones to induce sexual maturity. The plan was to bring these females on their expedition and place them in cages fastened to floating buoys in the middle of a front region in the Sargasso Sea. The females were intended as bait, to attract males who had swum there to spawn, and thus force them out of hiding.
But the eels were reluctant participants. The scientists kept the mature females in a laboratory and were about to drive them down to the docks in Miami ahead of departure, but before the ship had even cast off, the majority of the eels had died. By the time the expedition arrived in the Sargasso Sea, only five of the one hundred female eels were still alive.
Regardless, the five surviving eels were placed in cages and tied to the buoys, and McCleave and Wippelhauser took turns monitoring the movements of the buoys around the clock with the help of radar. But inexplicably, they managed to lose them. Eels and cages and buoys disappeared without a trace and were never seen again.
During another expedition, which Gail Wippelhauser undertook without James McCleave, the acoustic instruments picked up echoes from what was believed to be a large group of breeding eels; the researchers threw at it everything they had, lowering no fewer than six nets into the water. And yet there was no sign of any eels.
Another strange detail is, of course, that it’s not only living eels that have proved elusive in the Sargasso Sea. No one has ever spotted a dead one either, whether in the form of a corpse or as the victim of a larger predator. Swordfish and sharks have been caught with silver eels in their stomachs, but never anywhere close to the Sargasso Sea. A sperm whale was once caught off the Azores with an eel in its stomach that was on its way to spawn, but the Azores are pretty far from the Sargasso Sea. Once eels reach their breeding ground, they universally manage to avoid human detection in both life and death.
It should be said that there is no consensus on how significant it would really be to find a mature eel in the Sargasso Sea. Some scientists feel it’s beside the point, since we already know that’s where the eels are going. Others claim our knowledge of the eel’s life cycle can’t be considered complete until someone has observed an eel at its spawning ground. To these scientists, the elusive eel is something of a scientific holy grail.
In the past few decades, some researchers, such as James McCleave, have started asking another difficult question: If we can’t track all silver eels back to their birthplace, and in fact not even a single one, can we really be completely certain the eel breeds only in the Sargasso Sea? Granted, it took Johannes Schmidt almost twenty years to find the smallest of the tiny willow leaves there, but he had searched only a fraction of the world’s oceans. Schmidt himself wrote in 1922 that until all the seas have been trawled for eel larvae, it would be impossible to say for certain where the eel breeds, or at least where all eels breed. And virtually all eel expeditions since, including James McCleave’s, have focused on the already familiar region of the Sargasso Sea. Perhaps some eels go elsewhere entirely? It’s unlikely, but how can we know for certain?
Moreover, the Sargasso Sea is very large. Is it one big breeding ground, or are there several separate breeding grounds within its borders? Do the American and Europ
ean eels breed in exactly the same area, or do they prefer different locations? Some scientists, Friedrich-Wilhelm Tesch among them, have claimed that the American eel breeds in the western part of the Sargasso Sea while the European one stays farther east, but that the areas are partially overlapping. Others argue the collected leptocephalus larvae do not support such conclusions. All we know for sure is that when the tiny, transparent willow leaves leave the Sargasso Sea, European and American ones are intermingled, drifting helplessly along in the mighty ocean currents, while their parents appear to remain, die, and decompose.
HENCE, TO THIS DAY, THE WORLD’S LEADING ZOOLOGISTS AND MARINE biologists, the people who are most intimately familiar with the eel, are forced to qualify their reports and results with reservations. “We believe,” they’re obliged to say. “The data indicate . . .”; “It can be assumed that . . .” By patiently rejecting less likely scenarios, they are slowly moving toward a probability that in turn closes in on truth.
It can, for example, be assumed that what’s true of one of our eel’s closest cousins, the Japanese eel, is also true of the European eel. And when it comes to the Japanese eel, some of the classic aspects of the eel question are, in fact, slightly less enigmatic.
The Japanese eel, Anguilla japonica, looks essentially like its European counterpart. Its life cycle is also very similar. It hatches in the sea and drifts toward the coast as a willow leaf. It turns into a glass eel and wanders up waterways in Japan, China, Korea, and Taiwan. It becomes a yellow eel and lives out its life in fresh water before many years later turning into a silver eel and wandering back out into the sea to spawn and die. It’s a very popular fish for cooking, particularly in Japan, and it has long played an important role in East Asian culture and mythology, among other things as a symbol of fertility.
The Book of Eels Page 14