Peak migration activity usually occurs during September and October during the last quarter of the moon and is enhanced by dark, stormy nights and rising water levels. Yellow eels may also be found migrating seaward in the autumn but they are believed to be moving to overwintering sites within the river or estuary. A few eels have been captured offshore along the continental shelf but the migratory routes and behaviour of silver eels at sea are unknown. From the northern areas of their geographical distributions, silver eels may migrate sooner and in a more immature state than from southern areas, thus permitting them to reach the spawning grounds at similar times and reproductive condition. American eels spawn in the western part of the Sargasso Sea, with peak spawning occurring between January and March.
Larger females spawn more eggs than do smaller females.
One specimen of 724 mm in length and weighing 755 g was estimated to contain 2.6 million eggs. No mature adult has ever been caught in the Sargasso Sea and their spawning behavior is unknown. It is presumed that the adults die after spawning . A year or more may be taken to reach Canadian waters. On the way, the larvae undergo metamorphosis which involves a change to typical eel shape.