Ever wondered how many eggs an American Eel lay each year?
I set out to find the facts and figures. Read my blog post about the full life of the American eel here.
American eels live an extreme example of a full circle life: they’re born in the Atlantic, live for five to twenty years in freshwater, and then venture back out to the ocean to breed and perish. During their entire lives, this is the only time American eels will spawn.
Reproduction and laying eggs of eels has long been a mystery.
No one has ever witnessed American eels spawning, and no one knows what happens to the eels afterward.
But, in recent years, researchers have been starting to put together a picture of what happens in the sea.
Want to know how much an American Eel weigh?
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Table of Contents
So, How Many Eggs Can an American Eel Lay in a Year?
Some estimates state that female eels release 4 million eggs once they return to the Sargasso Sea, while others put the number around 20 to 30 million fertilized eggs.
The older and longer the female, the more eggs she can carry.
The largest and most fertile of females reaching the higher egg estimates, typically are reported to come out of inland Canada.
A 2018 article in the journal Marine and Coastal Fisheries took a stab at estimating how many eggs American eels carried, also known as their fecundity, from four different studies.
These sampled female American eels were found to be carrying between 0.5 to 22 million eggs.
Generally, short females early in their migratory route have fewer eggs.
Things that can lead to female eels carrying more eggs include:
- The eel’s body length.
- How far along in her migratory route she is.
Once eggs are laid, it’s hard to know what exactly happens next.
Much of what happens to eels in the open ocean is still largely a mystery.
We know that the eggs hatch and the larvae find their way to freshwater, and it’s believed that, after their long journey, the adults pass away.
How do American Eels Reproduce?
When an American eel reaches reproductive maturity, their body undergoes dramatic physical changes to transform it from bottom dwellers in rivers to ocean fish.
American eels will stop eating entirely and rely on fat reserves to make the long journey.
When ready to spawn, the physical changes eels undergo are:
- Degeneration of their gut
- Their eyes growing in size
- More blood vessels in their swim bladders
- An increase in fat reserves
Male American eels reach sexual maturity sooner in life, usually when around five to ten years old, and begin their migration to the Sargasso Sea.
Female eels live for longer before reaching maturity, the largest going for over twenty years before beginning their migration.
American eels can travel thousands of kilometers in this migration to their breeding grounds.
One eel was tracked by researchers all the way from Nova Scotia to the Sargasso Sea, traveling roughly 2,400 km in 45 days.
Once American eels reach the Sargasso Sea, however, the rest is still a mystery.
No researcher has ever observed American eels spawning in the wild.
It’s currently believed that, after spawning, American eels die.
Based on where larvae are found on the eastern seaboard, researchers have built models to try and simulate how the larva make it to shore.
Their models suggest that at least a little bit of swimming is involved, or else we wouldn’t see such a wide spread of larvae across so much of eastern North America.
Despite the lack of attention American eels may get, they live a life fit for a fable.
They live their whole lives before their bodies begin to change for the migration to spawn. And after traveling thousands of kilometers to the same sea that they were born in, they breed and vanish.
American eels (Anguilla rostrata) are a catadromous fish, meaning they spend most of their lives in freshwater before migrating to the sea to breed. Reproduction of American eels in the wild, however, has proven to be an elusive area.
It has long been believed that American eels spawn in the Sargasso Sea, a region of the North Atlantic Ocean bounded by the Gulf Stream, North Atlantic Current, North Atlantic Equatorial Current, and Canary Current.
Nice to know: How Many Volts Does an Electric Eel Have?
The Sargasso Sea was proposed as the single spawning area of American eels as far back as 1904, but it took until 2015 for researchers to finally track an adult eel along its migratory route.
By tagging eels with pop-up satellite archival tags, or PSATs, researchers from the Université Laval in Québec and Dalhousie University in Nova Scotia tracked migrating American eels.
While it has now been confirmed that an adult American eel did travel to the Sargasso Sea, there still hasn’t been any observed egg laying in the wild. Whatever effect environmental changes may have on their spawning and reproduction is unclear.
Hatched larvae drift and swim to the continental shelf, where they then leave the ocean and transition to living in brackish water and freshwater.
American eels land in most corners of the Atlantic coastline, where their adaptability helps it survive in so many diverse habitats.
American eels go through a cycle of transformations. In freshwater, American eels metamorphose through different life stages where they exhibit different physical characteristics.
The life stages of American eels:
- Leptocephali (larva)
- Glass Eels
- Yellow eels
- Silver eels
Males tend to mature and return to the Sargasso Sea within five to ten years, while females can live in freshwater and brackish water for twenty years before migrating to the sea.
One mature female eel recorded in Nova Scotia lived to age 43.
Eels kept in aquariums will typically live around ten years.
Interestingly, eels are sexually undifferentiated as larvae and glass eels. It isn’t until they’re already in freshwater that they begin to differentiate into males in females.
Often, eels that grow quickly as glass eels tend to become males while eels that grow slowly at the start of their lives develop as females.
The environment a glass eel experiences seems to largely influence the sex it’ll develop into.
When in aquaculture, glass eels usually differentiate into males. It’s suspected that this is because of stress and cortisol levels.
Females tend to be the ones to venture further inland, swimming upstream, and can grow to five feet in length. Males usually stay in estuarine waters and can reach three feet.
American eels range as far north as Greenland and have been reported as far south as Venezuela.
While their movements aren’t as well recorded outside of Canada and the U.S., American eels are believed to also currently range through eastern Mexico and the Caribbean.
Do all American eels eventually migrate back to the Sargasso Sea?
Some researchers categorize American eels as “facultative catadromous,” meaning that their migration pattern may not be as obligatory as once believed.
But to the best of current knowledge, American eels do make the trip back to the sea when it’s time to spawn.
How do American eels know when it’s time to spawn?
When an American eel is ready to spawn, its body begins to physically change for the journey. Little is known, however, on what triggers those changes.
How many eggs actually get fertilized?
American eel eggs are fertilized externally (after being laid) and are believed to hatch within a week or two. But what happens during spawning in the Sargasso Sea is still largely a mystery.
Read also: This Is The Life Cycle of The American Eel.