Elvers

Elvers are pictured in a transport tank, waiting to be released. Source: Mike Martinek / Normandeau Associates Inc., from: https://www.dep.pa.gov/OurCommonWealth/Pages/Article.aspx?post=70.

Editor’s Note: This is a column on area watersheds by Blyden Potts and guest columnists to spread awareness of the area’s tributaries and the efforts of area volunteers to keep them clean.

 

The Susquehanna River Basin Commission (SRBC) released dozens of eels into the Conodoguinet this week, at Ridley Park in Camp Hill. The juvenile, 1-3 year old, American Eels raised by students at East Pennsboro High School, as part of SRBC’s “Eels in the Classroom” program. You might say the eels graduated.

“Eels in the Classroom” is a very small-scale experimental SRBC project to raise eels in classroom settings, to teach students about their ecology, and aid in the restoration of eel populations. I assume the program is modeled on the well-known “Trout in the Classroom” program offered by Trout Unlimited, which has students learn about the ecology of trout while raising them.

Eels are elongated, snake-like fish. Most species live their entire lives in marine environments, but those known as freshwater eels are catadromous: They live most of their life in freshwater, but migrate to the ocean to spawn, the opposite of anadromous fish, like salmon, that live in the sea but migrate up rivers to spawn in freshwater. The American Eel is the freshwater eel species native to the eastern United States.

The life cycle of freshwater eels is even stranger than their serpentine shape. They spawn and lay eggs somewhere in the Sargasso Sea. Science still is not quite sure just where. The eggs hatch into larval eels, called leptocephali. They bear little resemblance to adult eels. Each leptocephalus – “thin head” in Greek – is transparent, and only a few centimeters long. They drift with the Gulf Stream for months.

At about 6 centimeters, they undergo a metamorphosis into “glass eels,” becoming elongated, and begin their trek into coastal estuaries. As they travel through brackish water, they transform from marine to freshwater organisms, gain skin pigmentation, and become juvenile eels, known as elvers, still only 5 or 6 inches long, though already a year or two old. They head upstream, where they will mature into adult “yellow eels,” which can grow as large as 4 feet long and weigh more than 15 pounds. Late in life, they transform again, into “silver eels,” as they return to the sea to spawn and die.

Eels are nocturnal predators, but what they eat changes as they grow. Glass eels are so small they feed on detritus particles. Elvers mainly eat insect larvae. Small adult eels prey on diverse macroinvertebrates and small fish. Large adults have a diverse diet, heavy on crayfish and fish.

American Eels were once a very common fish in eastern U.S. rivers, but populations were wiped out where dams were constructed that blocked upstream migration. As dams have been removed, and fish ladders or bypasses added to remaining dams, eels are returning to parts of their native range where they have long been absent. To aid eel restoration, elvers are collected at dams, like those on the lower Susquehanna, then driven upstream and released back into the river.

There is a link between the eels and a species of freshwater mussel called the Eastern Elliptio. Mussels are filter feeders. Each one filter cleans up to 10 gallons of stream or river water each day. Collectively, across all the watersheds that feed the Chesapeake Bay they are filtering. They can live for decades, but their populations have declined historically. The larval form of the mussel is parasitic, and lives in fish gills. Their preferred host is the American Eel. So the absence of eels adversely impacts mussel populations.

Restoring eel populations should increase future Elliptio numbers, increasing total filtration, and yielding cleaner and clearer streams and rivers.

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