Dispersal Pathways of American Eel Larvae from the Sargasso Sea

At the end of their life cycle, American eel migrate to the Sargasso Sea from freshwater habitats along the east coast of North America to spawn planktonic eggs. The eggs develop into larvae that then have to reach freshwater habitats along the American coast within their first year of life. The exact spawning location in the Sargasso Sea and details of the larval journey are poorly understood. Here, a coupled biological-physical model was used to study how potential behavioral adaptations influence the ability of eel larvae to reach near-coastal waters. Several swimming behaviors are investigated: passive drift, random-walk swimming, and directional navigation with and without a preferred direction. Directional swimming with a randomly chosen direction improved larval success rates by >2 orders of magnitude compared to passive drift, and swimming primarily to the northwest further doubled these rates. Success rates also substantially increased for larvae with swimming abilities even slightly above an estimated average. Directional swimming resulted in a reasonable distribution of larvae along the shelf break, whereas other scenarios left gaps where no larvae reached the shelf, including near the Gulf of Maine where juvenile eels are abundant. Directional swimming yielded transit times of ~1 year, in agreement with observations. The model supported the southwestern Sargasso Sea as the probable spawning area.