European eel

Photograph by Joel Sartore, National Geographic Photo Ark
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A European eel at the Environmental Education Center of the Ribeiras de Gaia in Arcozelo, Portugal.

Photograph by Joel Sartore, National Geographic Photo Ark

What is a European eel?

European eels are fish with one pair of small pectoral fins and long, snake-like bodies.

As catadromous fish, European eels spend most of their adult lives in freshwater rivers, streams, and estuaries before returning to the open ocean to spawn and lay eggs. As young larvae, baby eels drift around the sea for between seven months and three years.

Life cycle

European eels go through several easily recognizable life stages on their journey to adulthood. As larvae, eels are known as leptocephalus because of their thin heads and wider bodies that give them a leaf-like shape, but as they age, the animals become longer and thinner. As their bodies become larger and more translucent, the fish become known as “glass eels.”

Next, as the eels enter freshwater systems, they transform again, gaining pigmentation and becoming known as “elvers.” For the next six to 20 years, the eels put on weight and length while also developing lemon-colored undersides. This is what’s known as the “yellow eel” stage. Yellow eels travel continuously upstream toward cooler, less salty water until they become reproductively mature.

Finally, the fish transform again into “silver eels” with a metallic sheen and large eyes, at which point they migrate thousands of miles back out to the Sargasso Sea, where the animals spawn, lay eggs, and die.

Much of the European eel’s life cycle while out at sea remains a mystery.

Habitat and diet

This species is nocturnal and secretive, preferring to burrow into mud and under stones during the day. At night, European eels emerge to feed on a variety of food sources, depending on their life stage. Their diet can include everything from other fish to mollusks and crustaceans, to even insects, worms, and carrion.

European eels are strong swimmers, but they have impressive climbing skills, too, allowing them to navigate obstacles such as dams in their upstream journeys. There are even reports of European eels leaving the water altogether and entering fields, where the species feasts on slugs and worms.

Threats to survival

By some estimates, European eel numbers have declined to less than one percent of historic levels. As such, the International Union for Conservation of Nature lists the species as critically endangered.

Interestingly, scientists have yet to pinpoint one major threat that can be attributed to the European eel’s decline. There is, however, evidence to suggest that many different factors may have each taken a toll. Barriers to migratory routes, such as dams and hydroelectric turbines, are one likely cause. One study found that Europe had a total of 24,350 hydropower plants in 2012, with that number projected to keep going up.

Studies have also pointed toward climate change, diseases and parasites, habitat loss, pollutants, and predation as threat factors. Exploitation of the eels for food is another concern, as the species was once a staple in local communities across the continent. The species has also been exported to Asia to supplement markets there.

Conservation

In 2007, European Union member states instituted environmental management programs designed to make sure at least 40 percent of silver eel stocks were able to make it back to the sea when it came time to mate. These programs include efforts to reduce commercial and recreational fisheries, improve habitats, and even change the timing of hydroelectric turbine schedules so that they don’t run when the eels are most active.

In 2009, the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) added European eels to its Appendix II list of species in which trade is monitored and regulated.

While increases in European eel numbers from 2011 to 2013 are encouraging, the IUCN recommends continued monitoring and conservation efforts to in order to make sure the species continues to recover.