Miguel Clavero Pineda, Doñana Biological Station (EBD-CSIC)
There are few animals that are as admired as the eel (acne eel). This snake-bodied fish, covered in slippery mucus and with amazing agility that is almost impossible to grab, stalks the dark seabed unnoticed in pursuit of its reputation as a scavenger. 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 that part of the world. But the most intriguing has always been its origin. Where are they from?
This question is the riddle of the eel, which has captivated natural scientists throughout history. Aristotle, Pliny, Aldrovandi and even Sigmund Freud racked their brains trying to explain the very existence of animals that did not seem to reproduce. Everyone made suggestions to everyone more creative, including the spontaneous generation.
A key advance in solving the puzzle is due to the efforts and commitment of Johannes Schmidt, who in the early years of the 20th century embarked on a search for the origin of European eels.
Shortly before this, at the end of the 19th century, the Italian Giovanni Grassi discovered that some small, transparent, leaf-shaped marine fish known as Leptocephalus brevirostriswere in fact juvenile forms of eel, which today we call leptocephalic larvae.
When approaching European shores, leptocephali turn into eels as soon as they enter rivers and swamps. 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 to be a breeding ground for eels, but Schmidt caught leptocephalic larvae in the Atlantic and noticed that as they penetrated into the Mediterranean, they became less and less common. He also realized that the size of the larvae was variable and thought that the area of origin of the eels would be where the smallest leptocephali were found.
He undertook a titanic search for leptocephalians across the North Atlantic, noting the position and size of each one, always looking for the smallest, each time approaching the area east of Florida.
In 1923 he published his work, and since then we have been saying that eels breed in the Sargasso Sea. Surprisingly, we have learned very little about the eel’s breeding range and journey there since Schmidt’s work.
No one has yet caught an adult eel (with reproductive organs) in the sea, especially in the area of the Sargasso Sea, where its fertilized eggs have not been found either.
The Great Journey of the Eel
When technological developments allowed, various teams placed transmitters on the eels shortly before they began their journey, hoping they would pinpoint their exact breeding area.
Thus, we have learned amazing things about the journey of eels. For example, that they do not eat throughout their entire sea journey of thousands of kilometers. Or that in their constant swimming they change depth between day and night, with a difference of more than a thousand meters.
Animals tagged in Ireland and Scandinavia followed their intended route to Sargasso. The same thing was done by those who left the south of France, crossing the Strait of Gibraltar. But the transmitters, which marked all these routes, almost did not move away from the European coast.
Nearly a hundred years after Schmidt’s work was published, an international team has taken another historic step in solving the eel mystery.
For the first time, it was possible to trace the path of an eel to its supposed breeding site, which, as it turned out, coincided with Schmidt’s proposal in 1923.
For this, eels from the Azores were noted, the territory closest to Sargassum in the entire range of the species. These eels save several thousand miles of travel compared to British, Danish or Italian eels, making them 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 directly at the breeding area noted by Schmidt.
However, the riddle of the eel has not been solved. We have confirmed that the eels swim where we thought they would. But we still do not know the exact breeding site, its depth, its separation from the place where its sister species breed (American eel, eel rostrata), how they reproduce there, and what eels look like when, after a very long journey, they spend what little they have left on breeding before they die.
Worst of all, we might run out of eels before we finish solving his riddle.
Endangered fish
The eel plunges into population collapse. Since 1980, its numbers have collapsed by more than 95% and today it is considered a species that is on the verge of extinction, the maximum level of threat. Our grandparents wouldn’t believe it.
In the Iberian Peninsula, the eel has lost 85% of its historical territory due to the barrier action of water bodies. Today it seems exotic to us that eels were caught in Palencia, Soria or Albacete, but before the spread of reservoirs, this was common.
When dams allow eels to pass upstream, the outcome can be even worse, as downstream travel often involves crossing hydroelectric plants 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, which 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) has proposed on November 3 to introduce an absolute ban on the use of eels in all habitats, at all stages of life and for any purpose from 2023.
It would be very important that regional, state and European institutions strictly observe these moratoriums.
The drive of Asian markets for eels (after the extinction of native species) has made the illegal trade in European and American eels more like a drug trade than a fishery.
The ease of transporting eels in plastic bags allows this illegal activity to develop. Genetic analyzes show that European eel meat, which is banned for export, is common in Asian stores. From the East, he frequently makes the trip back to Europe.
The international transport of eels has also contributed to the spread of parasites that can make it difficult for the remaining animals to travel to Sargasso.
As if that weren’t enough, nascent biological infestations pose an additional threat to the eel. Of particular concern are catfish and blue crabs.
To learn the riddle of the eel and just solve one of the longest-lived mysteries in natural history is a precious goal. But the eel itself is much more valuable. To lose it would be to lose a unique animal that plays a unique role in the ecosystems it occupies, as well as thousands of years of charm, mystery, livelihood and human culture. It is important that this does not happen. You must try.
Miguel Clavero Pineda, CSIC Senior Fellow, Doñana Biological Station (EBD-CSIC)
This article was originally published on The Conversation. Read the original.
Source: RPP
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