Yes, many eel species live in freshwater for most of their lives. The true freshwater eels, all belonging to a single family, spend years growing in rivers, streams, lakes, and ponds before migrating to the ocean to spawn. Their lifecycle is one of the most remarkable in the animal kingdom, involving thousands of miles of travel and a complete overhaul of their body chemistry along the way.
How Freshwater Eels Split Their Lives
Freshwater eels are catadromous, meaning they grow up in freshwater but breed in the sea. This is the reverse of salmon, which do it the other way around. The American eel, the only freshwater eel species in North America, hatches in the Sargasso Sea (a region of the western Atlantic Ocean), drifts as a tiny transparent larva on ocean currents toward the coast, then swims upstream into rivers and lakes where it may live for 10 to 25 years before returning to the ocean to spawn and die.
European eels follow the same basic pattern, also spawning in the Sargasso Sea. Tracking studies have recorded individual European eels migrating more than 5,000 kilometers from western Europe toward the Azores region, with one eel traveling nearly 7,000 kilometers over 10 months. These journeys can take up to 280 days.
Not every individual actually enters freshwater, though. Studies using chemical signatures in ear bones have revealed that a significant portion of eels spend their entire growing phase in brackish or coastal marine waters, never moving into rivers at all. Scientists now describe eels as “facultatively catadromous,” meaning freshwater life is common but not required.
Five Stages of Development
A freshwater eel passes through five distinct life stages. It begins as a leptocephalus, a flat, transparent larva that drifts on ocean currents from the spawning grounds toward the coast. Once it nears shore, it transforms into a glass eel, a small, still-transparent juvenile that leaves the ocean current and begins moving into coastal waters.
As it pushes upstream, it becomes an elver, gaining pigment and growing stronger. It then settles into its main growth phase as a yellow eel, named for the brownish-yellow coloring along its body. Yellow eels are the stage most people encounter in rivers, lakes, and streams. They feed, grow, and remain in freshwater for years.
When the time comes to reproduce, the eel transforms into a silver eel. Its belly turns silvery white, its eyes enlarge dramatically, its pectoral fins lengthen, its skin thickens, and the muscles used for breathing grow significantly larger, likely to help the eel swim in deeper, less oxygen-rich ocean waters. The skull itself widens to accommodate the bigger eyes, which help the eel navigate the dark deep ocean. Silver eels stop eating entirely and begin their one-way migration back to the sea, where they spawn and die.
Where Freshwater Eels Live
In freshwater, eels occupy a wide range of habitats: rivers, streams, ponds, lakes, and estuaries. Their preferences shift as they grow. Young eels tend to favor shallow, fast-moving water like riffles and runs with gravel bottoms. In one study on a subtropical river, small eels were rarely found in water deeper than about 15 centimeters and preferred current speeds above 20 centimeters per second.
Larger eels are far less picky. They show up in habitats of any depth and flow speed, from muddy pools to boulder-strewn stretches. Both small and large eels strongly prefer natural, vegetated riverbanks and avoid areas lined with concrete or sand. Vegetation that extends into the water provides cover, and eels are most dense in these areas. This means habitat degradation, including river channelization and the removal of bank vegetation, directly reduces eel populations.
What They Eat
Freshwater eels are opportunistic bottom feeders that hunt primarily at night. Their diet shifts depending on their size and the time of day, but mayfly nymphs are a major food source across all size classes. Eels forage along the river bottom after dark, and their nighttime diet closely mirrors whatever invertebrates are available in the sediment. This nocturnal, bottom-feeding habit makes them hard to spot, which is one reason many people don’t realize eels are living in their local rivers.
How Eels Handle Salt and Freshwater
Moving between the ocean and a river would kill most fish. The salt concentration difference is enormous, and an animal’s cells need a stable internal environment to function. Eels manage this by completely reversing their internal water-management system. In the ocean, an eel’s body works to get rid of excess salt and hold onto water. In freshwater, it does the exact opposite: absorbing salts and flushing out excess water.
The gills are the main site where this exchange happens. Specialized cells in the gills use a suite of molecular pumps and channels to move sodium and chloride in or out, depending on the environment. Hormones coordinate the switch. One hormone acts as the “freshwater hormone,” boosting salt absorption in the gills and increasing water excretion through the kidneys. A different stress hormone helps the eel’s gut and esophagus adjust to saltwater conditions during the return migration. This flexibility is what makes the eel’s dual lifestyle possible.
Not All “Eels” Are True Eels
Several freshwater animals called “eels” are not closely related to true eels at all. The most famous example is the electric eel of South America, which is actually a type of knifefish more closely related to catfish. Electric eels live in the rivers and shallow pools of northern South America east of the Andes, hiding in woody debris and aquatic vegetation along river margins. They are nocturnal and secretive, occupying very specific habitats. For 250 years scientists thought there was only one species, but recent work has revealed multiple distinct species separated by geography, with lowland populations differing from those on the elevated shield regions of Guiana and Brazil.
Other freshwater creatures commonly called eels include moray eels (which are actually marine), swamp eels, spiny eels, and rubber eels. None of these belong to the true eel family. When biologists talk about “freshwater eels,” they mean the 16 or so species in the genus Anguilla, all of which share the same catadromous lifecycle.
Conservation Concerns
Freshwater eel populations are in serious trouble worldwide. The European eel is listed as critically endangered on the IUCN Red List, with significant declines in the number of young eels reaching European river systems over the past two decades. Dams block upstream migration, turbines kill eels heading downstream, and habitat loss reduces the quality of the rivers they depend on. Overfishing, pollution, parasites, and changing ocean conditions add further pressure. The American eel has also experienced steep declines across much of its range, prompting ongoing conservation reviews. Because eels need both healthy rivers and open ocean passages to complete their lifecycle, protecting them requires managing threats across thousands of miles of habitat.