“Fall migration” conjures up images of honking, south-bound geese and monarch butterflies on their way to Mexico. Each autumn we marvel at the long distance migrations made by many other birds and animals.
Some dragonflies make long distance movements, and salmon make well known spawning runs from the ocean to freshwater spawning grounds. But there are other migrations that go on largely unnoticed.
One of the most impressive unseen animal migrations is made each fall by American eels. Every autumn mature American eels migrate from freshwater river systems of eastern North, Central and South America to the Sargasso Sea in the North Atlantic Ocean.
It’s a long journey made by adults returning to the spawning grounds of their birth after spending 10 to 30 years in freshwater river systems.
Despite their wide geographic distribution, the migration to the Sargasso Sea means all American eels make up a single breeding population.
Eels are elongated, snake-like fish. They can absorb oxygen through their skin as well as across their gills. This enables them to move over land across wet grass or mud to bypass barriers as they move up and down waterways.
The eel’s life begins in the Sargasso Sea, a 700- by 2,000-mile patch of warm ocean in the North Atlantic defined by four distinct currents. Within these bounds, the Sargasso Sea is characterized by seaweed of the genus Sargassum, which floats en masse on the surface there.
This forest of seaweed also serves a protective refuge for other small marine animals. After hatching, some sea turtles, for example, travel to the Sargasso Sea and stay for years until they grow large enough to be protected by their sheer size.
When mature adult eels return to the Sargasso, females lay 20 to 30 million eggs, males fertilize the eggs, and then the adults die.
After hatching, billions of tiny transparent larva, called leptocephali, get swept away by the currents. Only chance determines which direction they are carried.
At the mercy of the currents, it takes the leptocephali about a year to reach the Atlantic coastal waters. At this point the leptocephali transform into glass eels, transparent juvenile eels that lack pigmentation.
Glass eels are strong swimmers and as the currents take them to coastal waters, they head for estuaries. In estuaries (the mouths of rivers), glass eels acquire pigmentation, are known as elvers, and begin swimming up rivers, especially during high tides.
Elvers grow to about five inches in length, and eventually transform into yellow eels, the fourth stage of life. The yellow eel stage lives for as many as 30 years as they work their way up unobstructed river systems.
Females can grow to four feet in length; males get about half that size. Some eels travel hundreds of miles upstream. Some eels choose to remain in estuaries, these mature much more rapidly than those that head upstream.
When memories of the Sargasso Sea call them home, eels turn black, their eyes enlarge, they get fat and their digestive system degenerates.
Now they are silver eels, and they somehow return to the Sargasso Sea.
Exactly how eels find their way back to the Sargasso Sea is unknown. They are most active and migrate at night, so perhaps they use celestial cues. Other possibilities include orienting to geomagnetic cues or using odor trails to find their way home.
Interestingly, European eels live a parallel life and make a mirror-image migration. They, too, spawn in the Sargasso Sea, but currents sweep them east to European estuaries and rivers.
The life history of both American and European eels is unusually complex. Biologists call such fish catadromous. Catadromous fish are born in the ocean, grow and mature in fresh water, then return to the ocean to spawn.
On the other hand, more familiar game fish such as salmon and striped bass are anadromous species. They hatch in freshwater, live most of their life in the ocean, and then return to freshwater to spawn.
Until science can explain how eels navigate, we are left to simply marvel at their migratory prowess. In any case, it makes bird migration seem like child’s play.