The mystery of the European eel begins with basic curiosity: ever since the Stone Age, fishers have caught eels in European rivers, lakes and coastal waters, but no one had ever seen eels spawn or found fertilised eggs. At some point in their lives, adult eels simply vanish from European freshwater streams and lakes, spawn somewhere else and never return. Where do they go? No one knew. In 1904, this inspired Schmidt to launch a research project that would continue for almost two decades until he found an answer – or at least a partial one. Some mystery still remains, as we shall see later.

Schmidt’s research was inspired by his mentor, the oceanographer Carl Georg Johannes Petersen (1860–1928). In 1896, Petersen had proven that the sexually mature silver eel was simply a more mature version of the freshwater-dwelling yellow eel and not a distinct species, as had been assumed. During the early 20th century, the two men’s research followed separate paths. From 1908 to 1910, Schmidt embarked on a major project with two expeditions to the Mediterranean to test whether that might be the eel’s spawning grounds, as hypothesised by the Italian zoologist Giovanni Battista Grassi (1854–1925).

The method was brilliant in its simplicity: following the eel larvae – tiny, transparent leaf-like creatures – back to the place where they were the smallest would lead the researchers to their place of origin, which might be in the Mediterranean. However, Schmidt’s findings soon showed that the larvae grew bigger, not smaller, as the expedition progressed. Thus, their findings did not lend support to Grassi’s hypothesis.

After his years in the Mediterranean, Schmidt turned his gaze westwards. As head of the Carlsberg Research Laboratory’s physiological department, he organised several major expeditions during the 1920s. On the ship Dana (later called Dana I) in 1920, and from 1921 on the custom-built oceanography vessel Dana II, he and a team of researchers collected thousands of samples on long voyages across the North Atlantic. They measured every single larva and determined its age, and gradually a clear picture emerged: the farther west they sailed, the smaller the larvae were.

When Dana II returned from the 1922 expedition, Schmidt was finally able to identify a specific area: the Sargasso Sea, east of Florida and south of Bermuda. Here, in open water with strong currents and lots of free-floating sargassum seaweed, he found the smallest eel larvae, some just a few millimetres long. Thus, a key part of the mystery was solved.

Schmidt’s discovery was a milestone in zoological research. He had proven that the European eel lives for decades in freshwater but then travels several thousand kilometres across the Atlantic Ocean to spawn in the Sargasso Sea. From here, the larvae return to Europe, carried by the Gulf Stream – a life cycle that is both impressive and extremely vulnerable to modern-day threats, such as overfishing and climate change.

Despite this crucial finding, the eel’s life cycle is still only partially understood. To this day, no one has observed an adult eel spawn in nature or found a fertilised egg. Schmidt located the spawning grounds, but we still do not know how the spawning takes place or what happens to the eels afterwards. The search for answers continues, both out of scientific curiosity and because insights into the eel’s biology are key to the survival of the species and to future aquaculture.

The success of Dana II was followed up by the Carlsberg Foundation’s Oceanographical Expedition Round the World 1928–1930, led by Schmidt. At the time, this was the largest single grant in the foundation’s history, and the extensive collections formed the basis for decades of research and also served as a model for later Danish circumnavigations of the globe. Schmidt was mainly known for his expedition research and his study of the life cycle of the eel, but he also worked on issues of direct relevance to Carlsberg. One of them was the study of the hereditary traits of hops, such as bitterness and aromatics, with the goal of improving the plant and thus the quality of the beer.

The chapter is written by Kristian Sjøgren.