The American Eel’s Spawning Run

Water levels in our local ponds, creeks and swamps have been high this fall and were topped up even more by Sunday’s rain, boding well for the tail end of the spawning run of one of our most unusual wildlife species: the American eel (anguilla rostrata). Studies have shown that rainfall is one of several factors that stimulate their downstream migratory spawning movements, which peak during the months of September through November.

As a general rule, males reach maturity and are ready to spawn earlier than females, with the youngest mature males being 5 years old and their maximum length measuring 1.5 feet. Mature females can be up to 25 years old — the maximum recorded age is 43 years — and 5 feet long. Any American eel measuring greater than 2 feet in length would be a female.

This is a one-way, 1,000-mile trip for the mature eels in our area. Once they reach their spawning grounds in a stretch of the western Atlantic Ocean known as the Sargasso Sea, named for the floating brown seaweed found in the area that provides a moving habitat and cover for many marine invertebrates and small fishes, they mate, the females lay eggs, and both adult males and females perish.

Considered one of the most prolific fishes, female American eels may have as many as 20 million eggs. No one has ever witnessed spawning or even fertilized eel eggs. Nor has anyone ever found adult eels, dead or alive, in the Sargasso Sea.

One recent study using satellite tags tracked a single mature female 1,440 miles from its release site on the Nova Scotia coast to the northern edge of the Sargasso Sea, a journey that spanned 45 days, for an average of 32 miles per day. Tagging studies also revealed that migrating eels suffer high predation rates from other fish on their migration routes.

The belief that eels spawn in the Sargasso Sea is based on finding larval eels in that area of the ocean. The spawning grounds are estimated to be in an area south of Bermuda and east of the Bahamas, and spawning is estimated to take place sometime in February.

Another fascinating aspect of the eel’s fall spawning run involves their metamorphosis from what is known as their sexually immature “yellow phase” to their mature, breeding “silver phase.” These profound changes include enlargement of the eyes and pectoral fins, thickening of the skin, gonad development, changes in the metabolism of skeletal muscle, degeneration of the digestive tract and cessation of eating, and, of course, changes in color.

The American eel population once comprised over 25 percent of the total number of fish found in coastal streams along the Atlantic Seaboard. Recent declines in their population due to dams, hydropower turbines, harvesting and other causes prompted a review of their status by the U.S. Fish and Wildlife Service in 2015 for listing as endangered — and that review concluded that listing was not warranted. Still, the International Union for the Conservation of Nature states that the American eel is at very high risk of extinction in the wild.

There are still several important gaps in our knowledge of this important and interesting species, including many details of the silver eel migration and the specific location of the spawning area. Future improvements in satellite tags may uncover these mysteries.