The Book of Eels, Patrik Svensson [bill gates best books .TXT] 📗

things, like many others, destined never to be learnt before the world comes to its end. Or perhaps—but here I speculate, here my own curiosity leads me by the nose—the world is so arranged that when all things are learnt, when curiosity is exhausted (so, long live curiosity), that is when the world shall have come to its end. But even if we learn how, and what, and where, and when, will we ever know why? Why, why?”

IN SPITE OF ALL OBSERVATIONS AND ATTEMPTS TO UNDERSTAND (until the end of time), there is thus still a lacuna in the story of the eel. We know silver eels leave in the autumn, when the eel darkness descends, usually between October and December. The tiny willow leaves, the leptocephalus larvae, appear in the Sargasso Sea in the spring; the smallest specimens usually between February and May. Which should mean breeding happens around this time. Which in turn gives us a time frame for the eel’s journey. It has at most six months to get there.

Yet even so, it’s something of a mystery why the eel sets its course for the Sargasso Sea and nowhere else. Lots of animals migrate for breeding purposes, but few undertake a journey as long and difficult as the eel, and few are as stubbornly fixated on one single place thousands of miles away, and few do it just once before dying.

There are theories claiming only the Sargasso Sea has the right temperature and salinity for the eels’ propagation. It’s also a fact that eels have been around so long the continents have moved; the first eels likely had a much shorter distance to travel. But as the landmasses of our planet have changed, drifting apart inch by inch over the years, the eel has refused to adapt. It still needs to return to its birthplace, to the exact location it once came from.

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