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Why do mature freshwater eels migrate from rivers to the ocean?

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American eel (Anguilla rostrata) — eel The American eel (*Anguilla rostrata*) is found in habitats along the coast of eastern North America.

Mature freshwater eels migrate from rivers to the ocean primarily to reach their spawning grounds, where they reproduce and subsequently die.12

These fish are classified as catadromous, meaning they spend the majority of their lives in freshwater environments before returning to the sea to breed. For species such as the European and North American eel, this journey involves traveling to specific oceanic areas, such as the Sargasso Sea.13

During the maturation process, these eels undergo significant physiological changes, such as the development of enlarged eyes, as they transition into their "silver eel" phase to prepare for the downstream migration to the sea. While the exact mechanisms guiding their return to these distant spawning sites are not entirely understood, contemporary research suggests that they utilize a combination of chemical, geomagnetic, and geographic cues to navigate.13