There are few animals that arouse a fascination similar to that of the eel (Anguilla anguilla). That fish with the body of a snake, covered in slippery slime and with surprising agility, almost impossible to grasp, which discreetly walks its reputation as a scavenger on dark seabeds. The one that served to pay taxes, fed the fauna and people of Europe and North Africa, and supported one of the few commercial freshwater fisheries in this part of the world. But the most intriguing thing has always been its origin. Where do they come from?
This question is the enigma of the eel, which has captivated naturalists throughout history. Aristotle, Pliny, Aldrovandi And till Sigmund Freud they racked their brains trying to explain the mere existence of animals that seemed not to reproduce. Each one contributed proposals to each one more imaginative, spontaneous generation included.
The key progress in solving the enigma is due to the effort and commitment of Johannes Schmidt, who embarked during the first years of the 20th century searching for the origin of European eels.
Shortly before, at the end of the 19th century, the Italian John Grassi had discovered that small, transparent, leaf-shaped marine fish known as Leptocephalus brevirostriswere actually juvenile forms of the eel, which today we call leptocephalic larvae.
When approaching the European coasts the leptocéfalas they become eels, the way they enter rivers and wetlands. Grassi’s find made it clear that the eels came from the sea. But the sea is very big.
At first, the Mediterranean was thought of as a breeding place for eels, but Schmidt captured leptocephalic larvae in the Atlantic and observed that they became rarer as they entered the Mediterranean. He also realized that the size of the larvae was variable and thought that the area of origin of the eels would be the one in which the smallest leptocephalians were found.
He embarked on the titanic quest of fishing for leptocephalians across the North Atlantic, noting the position and size of each one, always looking for the smallest ones, each time closer to an area east of Florida.
In 1923 he published his work and since then we say that eels reproduce in the Sargasso Sea. Although surprising, we have learned very little more about the eel’s breeding range and its journey there since Schmidt’s work.
No one has ever caught an adult eel (with reproductive organs) at sea, much less around the Sargasso Sea, where its fertilized eggs have not been detected either.
The great journey of the eel
When technological development allowed it, various teams installed transmitters on eels close to starting their journey, hoping that they would indicate their exact breeding area.
Thus we learned fascinating things about the journey of the eels. For example, that they do not eat during their entire marine journey of thousands of kilometers. Or that in their constant swimming they change depth between day and night, with differences of more than a thousand meters.
Animals tagged in Ireland and Scandinavia followed the intended route to the Sargasso. The same thing that those who left from the south of France did, crossing the Strait of Gibraltar. But the transmitters that marked all those routes hardly moved away from the European coasts.
Nearly a hundred years after Schmidt’s work was published, an international team has taken another historic step in solving the eel puzzle.
For the first time, it has been possible to follow the journey of the eel to its supposed breeding area, which has turned out to coincide with the proposal by Schmidt in 1923.
For this purpose, eels from the Azores Islands were marked, the territory closest to the Sargasso in the entire distribution area of the species. These eels save several thousand kilometers of travel compared to British, Danish or Italian eels, making it more likely to follow them to their final destination. So it was. Of the 26 tagged eels, 5 entered the Sargasso Sea and one arrived right at the breeding area marked by Schmidt.
However, the riddle of the eel is not solved. We have confirmed that the eels swim where we thought they would. But we still don’t know the exact breeding site, its depth, its separation from the breeding site of its sister species (the American eel, Anguilla rostrata), how they breed there, and what the eels look like when, after a very long journey, they dedicate the little energy they have left to reproduce, before dying.
Worst of all, we may run out of eels before we finish unraveling its enigma.
A critically endangered fish
The eel is immersed in a population collapse. Since 1980 its abundance has collapsed by more than 95% and today it is considered a critically endangered species, the maximum level of threat. Our grandparents wouldn’t have believed it.
In the Iberian Peninsula, the eel has lost 85% of the territory it occupied historically, due to the barrier effect of the reservoirs. Today it seems exotic to us that people fished for eels in Palencia, Soria or Albacete, but before the proliferation of reservoirs it was common.
When dams do allow passage of upstream eels the result can be even worse, as the journey downstream often involves traversing hydroelectric generating turbines, with little chance of survival.
Eel fishing is a centuries-old industry, but commercial exploitation is more recent: eel fishing in the Guadalquivir began in the 1970s. This may have led to severe overexploitation of the population.
The fight to save the eels
The International Council for the Exploration of the Sea (ICES) proposed on November 3 an absolute ban on eels in all habitats, all life stages and for any purpose from 2023.
It would be very important that regional, state and European institutions strictly implement these moratoriums.
The eagerness of the Asian markets for eels (following the collapse of the local species) has made the illegal trade in European and American eels look more like the drug trade that to a fishing activity.
The ease of transporting eels in plastic bags allows the development of this illegal activity. Genetic analyzes show that European eel meat, whose export is prohibited, is common in Asian stores. From the East, he often makes the trip back to Europe.
The international transfer of eels has also facilitated the spread of parasites that can make it difficult for the remaining animals to travel to Sargasso.
As if that were not enough, incipient biological invasions pose an additional threat to the eel. Catfish and blue crab are especially worrying.
Knowing the enigma of the eel and having just unveiled one of the longest-lived mysteries in natural history is a precious goal. But far more precious is the eel itself. To lose it would be to lose a unique animal, with a unique role in the ecosystems it occupies, plus thousands of years of fascination, mystery, sustenance, and human culture. It’s important that it doesn’t happen. You have to try.
This article was originally published on The Conversation.