There are plenty of mysteries in the animal world, and some have persisted over the years.
Take, for example, the European eel (Anguilla anguilla). It is one of 19 known species of freshwater eel that has an incredibly widespread territory, yet surprisingly little is known about its reproductive cycle.
Bits and pieces of the puzzle have come together over time, but researchers are finding that the need to know more about this animal’s complex life cycle is more immediate.
Listed as Critically Endangered on the IUCN Red List since 2018, the European eel has seen drastic reductions in its population at all life stages.
To this day, scientists do not have every piece of the puzzle, yet the interest in this species continues.
Conservation is important not only environmentally but also commercially, as plenty of people rely on these eels as a source of income.
Understanding how eels reproduce is a key part of conservation efforts.
Researchers continue to discover information as to the migration routes, reproductive cycle, and exact breeding ground that can help formulate plans to protect this unique species.
In this article
Past Information on European Eels
It has been 100 years since Johannes Schmidt’s discovery in 1923 redirected the academic world’s thoughts on the European eel’s life cycle.
In his research, he found small, larval European eels concentrated in one general location in the Atlantic Ocean, a good distance from the continental waters where they live most of their lives.
For the longest time, however, no one could confirm how the eels traveled to this location nor how they reproduced.
Recent studies, discussed further below, provide new evidence of how these eels migrate to these spawning grounds.
European Eel Life Stages
What is known is that European eels spend the majority of their lives in Europe’s freshwater rivers and their lives can be divided into distinct stages.
The larvae of European eels are leaf-shaped and once hatched, they drift along with the Gulf Stream, eventually making their way back to the freshwaters of Europe and Northern Africa.
The shape of these larvae, also called leptocephalus, is a perfect adaptation for long-distance migration.
According to the United States Geological Survey (USGS), “the larvae drift in the plankton for an average of 7-11 months; however, this stage may last as long as three years.”
By the time they reach the brackish water at the outlets of their future continental homes, the eels are around 2.8 inches (7 cm) long and metamorphose into long, thin, and completely transparent forms.
This excellent camouflage adaptation earned them the name “glass eels.”
At this point, they will begin to gain pigmentation and head upriver.
Once in freshwater, they will continue to grow. In this so-called “yellow eel” stage, they exhibit yellowish pigment.
Males tend to remain in this stage for five to 12 years, while females can stay longer, anywhere from nine to 20 years.
After this time, they will transition to the silver eel stage and begin the reproductive cycle.
When they reach the age of between five to 15 years of age, they enter the “silver eel” life stage, where they reach sexual maturity and are ready to reproduce.
At this time, this genetically similar population from a variety of locations embarks on an exhausting migration of between 3,100 and 6,210 miles (5,000 to 10,000 kilometers) from their home waters and across the Atlantic, finally stopping in the Sargasso Sea.
European Eel Breeding Ground
The National Oceanic and Atmospheric Administration (NOAA) states that the Sargasso Sea is a body of water without land boundaries.
Located entirely in the Atlantic Ocean, its boundaries are ocean currents collectively called the Northern Atlantic Subtropical Gyre.
This system of rotating ocean currents includes the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south.
The Sargasso Sea makes excellent spawning grounds for the European eel.
The Sargassum seaweed, for which the area is named, is a prominent mat of floating algae that harbors much sea life.
Here, the eels come together in a massive group.
External fertilization of the females’ released eggs is then believed to occur, with the leptocephalus larvae hatching sometime later.
It is believed that female European eels can carry up to three million eggs per kilogram of body weight.
Researchers also suppose that this species dies shortly after reproduction.
The future of this species cannot be guaranteed without dedicated conservation efforts.
These eels have a massive natural range that extends throughout Europe and Northern Africa as well as the critical portion of the Atlantic Ocean as its breeding ground.
Because of this habitat range that includes both freshwater and marine environments, the eel is subject to human impacts from many angles.
Agriculture, transportation and development, resource mining, and modification of the natural environment are top on the list. Disease, pollution, and climate change are additional stressors that can impact eel reproduction.
Several groups and government entities have undertaken projects aimed at protecting and recovering eel populations. The following are two such organizations.
Zoological Society of London
Hailing from the United Kingdom, the Zoological Society of London (ZSL), continues its studies on the number of juvenile eels entering the Thames River Basin.
Data collected since 2005 indicate that the “annual recruitment of the European eel across its range has reduced by 90-95% since the 1980s.”
The primary impediment to eel migration, which is key to their reproductive cycle, is the immense number of barriers along the Thames.
ZSL has determined there are more than 2,000 such barriers, known as weirs, that divide the river into impassable fragments.
As part of its management plan, ZSL is currently mapping these barriers and either lobbying for removal or the installation of pass-throughs to support eel migration.
The impact of these efforts is yet to be determined, but the initiative for conservation is present and ongoing.
Sustainable Eel Group
The Sustainable Eel Group (SEG), a conservation group based in Belgium, estimates that over one million migration barriers exist along the eels’ European migration routes, resulting in significant declines in the eel population.
This decline has been measured since the 1980s, with the number of glass eels arriving from the ocean estimated to be lower than 10 percent of the population 40 years ago.
The illegal trade of glass eels is especially concerning.
SEG estimates that more than 300 million glass eels were captured and illegally exported to locations in Asia. Since 2017, efforts have been underway to monitor this trade.
After the European Union Eel Regulation was implemented in 2007, the eel population has again begun to rise, indicating the success of regulating the “protection and sustainable use” of this species.
Placing the eel on the IUCN Red List has also been beneficial.
As part of its conservation plan, SEG seeks to remove or bypass the numerous barriers that prevent eel migration to allow access to vital territory for this species.
Recently completed Tracking Study Data continue to be gathered to understand the reproductive process of Anguilla anguilla in efforts to protect this species.
A recent study completed by Wright RM et al. strove to “gather direct evidence on the final stage of the migration of European eels to the Sargasso Sea.”
The research team used pop-up archival satellite tags (PSATs) to tag a total of 26 silver eels in the waters of the Azorean Islands in late 2018 and 2019.
These eels were tracked from 40 to 366 days as they made their way to the breeding grounds of the Sargasso Sea.
While underway, the team measured the eels’ travel speeds at between 1.8 to 7.4 miles (2.9 to 11.9 km) each day.
Of the 26 female silver eels tracked, six made it to the Sargasso Sea, while others provided data along the migration route.
The team noted that “tag pop-up locations confirm[ed] the south-westerly trajectory of [the eels’] migration in the direction of the Sargasso Sea,… covering straight-line distances of 272 to 2275 km [169 to 1,414 miles].”
The study was not able to determine the method the eels use to migrate along this route (i.e., whether the eels used detection of magnetic fields, currents, or other methods of navigation); however, it did provide “the first direct evidence of migrating adult European eels reaching the presumed breeding place in the Sargasso Sea.”
With conservation of the species in mind, understanding the reproductive process for European eels is critical.
As more research is conducted and information discovered, protective measures can be put in place to allow this species’ population to grow to healthy, sustainable levels.
We know that the eels’ life cycle and reproductive process spans freshwater rivers and marine environments.
We also know that these processes are not quick, that eels spend almost a year floating in sea currents as larvae and up to 20 years in fragmented, often polluted fresh water before transitioning back to the sea to reproduce.
Conservation efforts currently underway are seeing progress. Eel populations in some areas are trending slightly upward with the strategic implementation of protective measures.
With continued efforts to protect the migratory and breeding environments, the European eel could bounce back.