Adult American eels live at the bottom of the Hudson River and its tributaries year-round, but those of breeding age will leave to spawn in the Sargasso Sea, hundreds of miles off-shore in the mid-Atlantic. Males mature at a smaller size; any eel over 2 feet is likely female.

The Hudson River is home to a significant population of American eels (Anguilla rostrata), a fascinating migratory species that plays a vital role in the river's ecosystem. These eels are "catadromous," meaning they are born in the Sargasso Sea in the Atlantic Ocean and migrate as tiny, transparent "glass eels" into freshwater estuaries like the Hudson every spring. Once in the river, they grow into "yellow eels" and can live there for 10 to 30 years before returning to the ocean to spawn and die. The Hudson River Eel Project, a long-running citizen science initiative, involves hundreds of volunteers who help count and migrate these eels past dams to reach better habitats upstream. While you might not see them frequently because they are bottom-dwellers and often nocturnal, they are abundant throughout the river from New York Harbor all the way up to Troy. They are known for their thick slime coat, which makes them incredibly difficult to catch by hand. Despite their snake-like appearance, they are true fish and remain a key indicator of the Hudson's biological health and the connectivity of its many tributaries.