Mysterious Marathoners

The American eel arguably has one of the most fascinating and complex migratory life cycles of any animal in Missouri.

Life for an eel begins as an egg spawned in an area of the open ocean called the Sargasso Sea. This spawning area — 2 million square miles of warm water located within the North Atlantic Subtropical Gyre and bounded on all sides by the clockwise flow of major ocean currents — lies northeast of the Greater Antilles Archipelago.

From eggs, they grow into leptocephalus, or “slim head,” larvae. At this stage they are flat, transparent, and jellylike. As they passively drift on ocean currents, scientists think they feed on tiny particles also floating freely. After the larvae reach their maximum size — and as the ocean and gulf coast currents pull them toward the shallower but more nutrient-rich continental shelf — they transform into “glass eels.” Rounder and still transparent, glass eels gain the ability to swim. As they enter North and South America’s brackish estuaries and bays, they again transform into “elvers” and “yellow eels.”

Elvers are small, perhaps 4–10 inches in length typically. They can be found in estuaries, marine, or tidal rivers. Yellow eels are named for their yellow-green or olive-brown coloration. Some yellow eels stay in salt water; some will move to freshwater. Others will shift between the two environments. Yellow eels are sexually immature adults.

“They apparently separate themselves by male and female,” said MDC Science Unit Supervisor Dave Herzog. “Males tend to stay in the salt-brackish water. Smaller in size, they grow to sexual maturity a lot quicker. Females travel up the mainstem rivers.”

Missouri’s anglers only encounter female American eels.

“We have not verified if there are any males,” Herzog said.

Herzog posits this natural sorting may be an efficient way for the species to allocate valuable resources, since being a female and growing eggs requires more investment than being a male. By allowing female fish access to all the freshwater food and shelter resources, this gives the species a better chance of survival.

In their native freshwater range, American eels can be found as far north as Greenland and as far south as Venezuela. They inhabit waterways throughout most of the eastern United States and even portions of the Great Plains and Southwest.

A mature female yellow eel can grow to 5 feet. It can take between five and 20 years for a freshwater yellow eel to gain sexual maturity. Eels that remain in estuarine or marine waters mature earlier due to more food and a faster growth rate.

“The species is panmictic. This means the entire population of American eel is mixing genetically,” Herzog said.

Unlike other animal species that often adapt to small, precise locations and thus become genetically differentiated over time, American eels share a common gene pool.

In their final phase, they become silver eels. At this point, prompted by their proximity to salt water, the fish undergoes an amazing series of physiological changes that enable their ability to return to the Sargasso Sea. This allows them to transform from bottom dwellers to oceanic travelers. Fat reserves gained after years of freshwater feeding fuels their long journey. Their eyes double in size and become more sensitive to blue to enhance their ability to see. Their stomachs dissolve. Their gill and dorsal fin structures change.

As mature silver eels, they migrate thousands of miles back to the Sargasso Sea, making them challenging to study.

American eels are “facultatively catadromous,” meaning they must spawn in the ocean but feed and grow in brackish estuaries and freshwater to complete their life history. As opposed to “anadromous” fish, which are born in freshwater, migrate to the ocean to grow and feed, and return to freshwater to spawn. Salmon, for example, are anadromous.

Mysterious Breeding

Science has not yet completely nailed down how eels breed; the vast size of the American eel’s native range has defied scientists’ attempts to document spawning. Scientists can attach radio trackers to the fish, but with so many obstacles on the journey, the ability to follow a tracker is limited.

“We’ve never seen them reproduce. It’s never been photographed,” Herzog explained. “It’s a big ocean. They are traveling thousands of miles.”

But, based on insights into how other fish breed, it’s likely a female eel expels 400,000 to 2 million eggs into the ocean, which are subsequently externally fertilized by numerous males. Once they spawn, they’ve completed their entire life cycle, and it is assumed they die. An adult eel has never been known to return to the continent.

How they navigate to the Sargasso Sea is also still mysterious. Like us, they have five senses they use to explore their world. But like other long-distance migrators, they appear to possibly have one more: the ability to perceive magnetic fields.

“We don’t entirely understand how they navigate long distances. We think it’s magnetic, but we don’t exactly understand how the organism interprets a magnetic field because humans don’t navigate by magnetism,” said MDC Scientist Andrew Glen.

Eels are not the world’s longest migrators — that award goes to arctic terns. Terns travel 25,000 miles from their breeding grounds in the arctic down to Antarctica. But with lifetime journeys of 3,000-plus miles, it’s fair to say eels are long-distance migrators.

Males can reproduce at five years, but they may wait for 20 years for their female cohort to return, or they may breed with older females — another point for future study.

How do males even find the drifting eggs?

“That’s a great question,” Herzog said. “But one not answered yet. They just do. And they must. Because they persist. We speculate a lot because we just don’t know.”

The Fate of Missouri’s Eels

At this time, American eels are not considered endangered or threatened, according to an in-depth species status assessment by the U.S. Fish & Wildlife Service. They are listed on Missouri’s Species of Conservation Concern Checklist. North American populations have declined, said Herzog. The construction of dams to control flooding and interstates to promote travel has interrupted their ability to access former habitats.

“That’s when I think the decline of the American eel began,” he said.

When the tallest dam on the Mississippi River — Lock and Dam No. 19 near Keokuk, Iowa — was completed in 1913, anglers caught small, upstream-bound elvers by the bushel basket load, Herzog lamented.

“Any barrier can create an impediment to the American eel, in terms of its life history and completing its migration pattern,” he said.

This causes a problem for the species because the further a female eel travels away from the ocean, the larger she becomes. The largest eels are found in northern sections of the Mississippi River.

“Longer equals older equals larger equals more successful,” he said.

A fattier fish can carry more eggs. Females that fail to access the furthest sections of their former range may be less successful.

Despite these impediments, American eels are still considered a commercially viable fishery. Although eels aren’t considered a delicacy in the U.S., many sushi-loving Americans may be inadvertently eating them — better known as “unagi” — in restaurants when they order “caterpillar rolls.” Europeans consider smoked eels fine table fare. In addition to smoked, eels are known to be quite tasty fried or pickled. In the U.S., glass eels are caught and transported to aquaculture fisheries overseas in Japan to finish growing, Glen said.

The species’ future is almost as murky as the brackish water where they live. Scientists know eels are transferring nutrients — energy — from the Sargasso Sea to the lower Mississippi. If the transfer slows, that energy is lost. Sometimes a species’ connection to other lifeforms isn’t immediately known; the organism’s relevancy is only realized after it has vanished.

“Hindsight is 20/20,” said Herzog.

Herzog believes eels may have been a part of early Americans’ diets long ago, but humans’ use of the fish wasn’t widely recorded. Today, they are mostly by-catch to commercial fishermen and a curiosity to recreational anglers in Missouri.

He would like to see eels abundant on the landscape once again.

“We don’t want to lose or remove another connection people have to enjoying angling and the outdoors,” he said.

Conserving Eels

To that end, MDC conducted a management evaluation of the species to better understand how the species’ life history relates to their commercial harvest.

“What is the impact of harvesting mostly adult females?” Herzog asked. “We know if we sacrifice them before they arrive to the Sargasso Sea, we are pulling them out of reproduction and preventing millions of eggs from being spawned. How can we ensure that is sustainable?”

Answering questions like these are the work of MDC scientists. Herzog and his team study the fish’s movements, which is why they know American eels linger on rocky beds outside Cape Girardeau and a few other bedrock locales up and down Missouri’s mainstem rivers. By dissecting a fish’s otolith — inner ear bone — scientists can count annuli to reveal the animal’s age and can measure strontium and calcium levels, clues to the fish’s environmental travel history.

Longer term, scientists do not know how climate change will impact eels but studying them could help scientists predict their future.

Eels can tolerate excess carbon dioxide in the bloodstream, which may be why they survive in places of low oxygenation, like swamps and the Gulf of Mexico’s hypoxic zone.

“But we don’t know what their thermal tolerances are,” said Herzog, adding warmer temperatures may impact the insects and crayfish they eat. And droughts and frequent hurricanes may worsen females’ migration path to Missouri waters.

“It’s all very speculative, but the concerns are all very real,” Herzog said. “There’s so much to learn, really.” 

Experiencing Eels

Most people have not experienced seeing a live American eel in nature, but those who have often are anglers.

“We have over 1 million licensed anglers in this state, and so there is a lot of potential for them to come into contact with American eels,” said MDC Science Unit Supervisor Dave Herzog.

American eels (Anguilla rostrata) occur in a variety of stream types, but they are most abundant in ones with continuous flows and moderately clear water. Protected by logs, boulders, and other cover, they prefer to lurk in deep pools. They can live in almost any moderate to large lake or stream accessible from one of our state’s big rivers, but they also survive in places separated by surmountable land barriers.

Conservationists in some east coast communities have constructed “eel ladders” to improve population numbers.

“American eels are amazing climbers,” Herzog explained. “Although eels cannot walk across land like a mudskipper, or breathe air and survive out of water for extended periods of time like the northern snakehead, they can and do slither through wet grass and mud and can absorb oxygen through their skin and gills.”

Solitary and territorial, they prey on smaller fish, aquatic insects, and crayfish. In turn, they are the prey for wading herons, diving eagles, and darting bigger fish.

Quite muscular and slippery, eels are encased in a thick layer of mucus — an excellent barrier to viruses, bacteria, and predators.

“It takes two or three strong people to grasp an adult female eel firmly with a dry towel for examination,” said Andrew Glen, scientist with MDC.

But because of their snakelike shape, they are sometimes reviled by humans who don’t understand this harmless, ancient fish. Some eel species do generate electric shocks, but not American eels.

“It’s safe to handle them,” said Glen.