Eels are fish known for their serpentine bodies and unique life cycles. Unlike most fish that spawn in freshwater and live in the ocean, eels are catadromous, meaning they reproduce in the sea and spend a significant portion of their lives in freshwater or brackish environments. Their journey from tiny ocean larvae to mature adults involves several distinct stages, each with unique characteristics and challenges. This complex transformation makes understanding the early life stages of eels important.
The Ocean Journey of Larval Eels
The life of an eel begins in the open ocean, specifically for American and European eels, in the Sargasso Sea in the western North Atlantic. After hatching from tiny eggs, eel larvae emerge as leptocephali. These larvae are transparent, flattened, and leaf-shaped, appearing almost invisible in the water column. American eel leptocephali grow up to 85 millimeters in length.
Leptocephali drift with ocean currents, such as the Gulf Stream, on journeys that can span thousands of miles and last for months, or up to two years for European eels. During this drift, they undergo metamorphosis, transforming into “glass eels” as they approach coastal waters and estuaries.
The glass eel stage is characterized by their transparency and a more cylindrical, eel-like shape. They are still small, around 2 to 3 inches long and resembling spaghetti in thickness. This oceanic migration occurs before their transition to inland waters.
Arriving in Freshwater: The Elver Stage
Upon reaching coastal areas, glass eels begin their migration into estuaries and then into freshwater rivers and streams, marking the beginning of the “elver” stage. This transition involves pigmentation of their bodies, as they lose transparency and develop a darker, brownish-green coloration. Elvers are small, worm-like young eels, around 8 centimeters in length, seeking habitats further upstream.
Their upstream journey shows their resilience, as elvers overcome natural and man-made obstacles. They can wriggle over damp areas, climb wet spillways, and ascend vertical surfaces like waterfalls and dams, sometimes by piling up their bodies by the tens of thousands.
These mass migrations, often called “elver runs,” are visible at instream obstacles during late winter to early summer, depending on the region. They are influenced by water temperature and rainfall. This ability allows them to colonize a wide range of freshwater habitats.
Growing Up: The Yellow Eel Phase
Once elvers have migrated upstream and settled into freshwater or brackish habitats, they enter the longest developmental period of their lives, known as the “yellow eel” stage. This phase can last from 5 to 20 years, depending on factors like sex, resources, and temperature. Eels develop a yellowish-brown coloration, giving them their name. They grow large during this time, with some European eels reaching up to one meter in length and weighing as much as 6 kilograms.
Yellow eels are primarily carnivorous, feeding on insects, small fish, and crustaceans. They prefer habitats with muddy bottoms, submerged vegetation, or crevices where they can hide and ambush prey. While many yellow eels inhabit freshwater rivers and lakes, some remain in estuaries or coastal marine waters. This prolonged growth stage allows them to accumulate energy reserves for their eventual return to the ocean to reproduce.
Protecting Young Eels
Young eels, in their various life stages, face threats that have led to population declines globally. Habitat loss and fragmentation are concerns, mainly due to dams and other barriers that impede their migrations between oceanic spawning grounds and freshwater growing habitats. Hydropower turbines can also harm adult eels during their seaward migration. Pollution from agriculture and urban areas degrades their aquatic environments, impacting their health and survival.
Overfishing, particularly of glass eels for aquaculture, poses a threat, with illegal trade being a lucrative black market activity. Climate change, affecting water temperatures and acidity, also influences eel migration patterns. Conservation efforts address these challenges. Building “eel passes” or “ladders” helps young eels bypass dams and other obstructions, reconnecting fragmented river systems. Habitat restoration projects, alongside international regulations and monitoring programs, are being implemented to protect eel populations and ensure their survival.