In part one of this two-part series on eels, I noted that despite our centuries-long relationship with them, some parts of these creatures’ lives remain a mystery.
What’s clear is their impact on our lives: When the Green Line was extended past Boylston Street station after 1913, workers discovered underneath the station prehistoric tidal eel and alewife fish weirs with 65,000 stakes that were estimated to have been put in place 2,000 years earlier. (You can read more about this fish weir at Arlington Street Station itself.) They were important for sustaining the 102 Mayflower colonists and the ship’s roughly 30-person crew. Only about half of them survived that first winter on board the ship.
In March, they left the ship and came into contact with Tisquantum, a member of the Patuxet band of the Wampanoag. One of the first things he did was to teach the passengers and sailors how to capture eels. By late 1621, leader Edward Winslow wrote, “In September we can take a hogshead [a 63-gallon barrel] of eels in a night, with small labor, and can dig them out of their beds all the winter.”
They were an important food source for many in the Boston area into the early 1900s.
Sadly, today their population is at historic low levels due to overfishing and habitat loss. Still, eels are common all along the coast of Massachusetts, as well as in ponds and rivers that reach the ocean.
Young eels’ flat bodies metamorphose into rodlike glass, something like glass noodles. What were semi-hard structures in their spine and head now turn into bone. Their digestive tract rearranges, and their scent organs enlarge. Once they swim into freshwater rivers, the two- to four-inch eels acquire a grayish pigment, and are called elvers. Weighing about as much as a drop of water, elvers migrate, sometimes hundreds of miles, up our rivers against the current.
In the Mystic River, for example, they travel upriver to the Mystic Lakes Dam, where an eelway was installed in 2012. The elvers swim up a textured ramp into a box. People from the Mystic River Watershed Association monitor the box, count the thousands of eels that show up every year, and transfer them to the upper Mystic Lake. There, the eels develop into yellow eels, which live in the lake for 5 to 25 years, dining at night on insects, worms, small fish, clams and crabs. They are difficult to spot because they hide beneath rocks and in sediment during the day.
Eventually, female eels can grow to be four feet long and weigh 16 pounds. Males are smaller, usually less than 16 inches. Males reach maturity in about 5 years, but the females take 20 or more years.
In the fall, when it is time to become reproductive adults, the eels stop eating. Their bodies rely on stored fat to produce the energy they need to swim back to their birthplace. They wait for a dark, moonless night, when there are fewer predators, to swim over the top of the Mystic Lakes Dam or through downstream passage gates and migrate the 1,000 miles back to the Sargasso Sea. During this time, they complete another metamorphosis called silvering, transforming into sexually mature fish with reproductive organs. These gray-backed, silver-bellied fish have just one mission — to return to their spawning grounds to reproduce.
These silver eels are prepared for life in the ocean: Their eyes grow larger to see in the ocean depths, and their digestive tract degenerates to make way for developing sex organs. Their hearts and lungs enlarge to support their long-distance travels. Females shed calcium and phosphorous from their ribs to provide nutrients for egg production. By the time these females reach their spawning grounds at some unknown location in the Sargasso Sea, their health is rapidly deteriorating. In late winter, the females manage to release as many as 22 million eggs deep underwater. The males expel clouds of sperm to fertilize the eggs. Once fertilized, a small percentage of eggs float to the surface where they hatch into larvae. The adult eels die, paving the way for the new generation to venture out unencumbered by the old brood.
Eel Facts
If you’ve heard the phrase “slippery as an eel,” it’s because eels are slippery. Although they have small scales embedded in their skin, their bodies are covered in a mucus layer, which makes them difficult to hold onto.
American eels breathe through gills like other fish, but they can also absorb oxygen through their skin for short periods. This allows eels to travel over wet grass or mud when looking for a suitable habitat.
Glass eels are illegal to harvest, except in South Carolina and Maine. (In Maine, a pound of glass eels can sell for more than $2,000, although in 2025 they were worth $895 per pound.) Indeed, the glass eel trade is so profitable that glass eels are said to be more valuable than their weight in heroin.
The South American electric eel is not a true eel at all. It is more closely related to catfish. It can generate up to 600 volts of electricity, five times more than you would get if you stuck your finger in a light socket. These fish use weak electric signals to navigate and stronger ones to stun prey.
People have never successfully bred eels in captivity.
Although eels are not pretty, they connect ocean ecosystems with freshwater rivers and lakes, essentially fertilizing inland waterways with ocean-derived nutrients. Eels are food for herons, gulls, cormorants, eagles, otters, raccoons, snakes and many other predators. In addition, they are the primary species that help mussels disperse. Adult mussels spread their larvae into water, and as adult eels swim by, the larvae attach themselves to the gills, where they are protected. The mussels then hitchhike upstream with the eels for a few weeks while they metamorphose into juveniles. Then the mussels drop off and settle into the sediment to grow into adults.
A stronger
Please consider making a financial contribution to maintain, expand and improve Cambridge Day.
We are now a 501(c)3 nonprofit and all donations are tax deductible.
Please consider a recurring contribution.