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Effort to help eels to pay environmental dividends

Effort to help eels to pay environmental dividends

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by KARL BLANKENSHIP, Bay Journal News Service

A decade ago, Steve Minkkinen and a team of biologists pulled into a boat ramp along a tributary to the West Branch of the Susquehanna River in Pennsylvania.

Their pickup was hauling a blue tank filled with hundreds of squirming eels, ranging in size from large earthworms to small snakes.
To some, they had about as much appeal. One woman watched as the creatures poured from the tank into the creek.

“Well,” she told Minkkinen, who heads the Maryland fisheries office of the U.S. Fish and Wildlife Service, “I’m never swimming in this river again.”
She was only getting a glimpse of what was to come.


Efforts to bring the slithery fish back to the East Coast’s largest river have accelerated. This year alone, more than 620,000 eels were returned to habitats they had dominated until the last century.

Although eels were once an abundant food for American Indians and early settlers, the river was devoid of them just two decades ago – the result of massive dams built in the early 1900s, which blocked their migrations.
Eels fell off the radar of the public and resource managers alike.
Restoration attention (and funding) turned to getting the higher profile American shad back upstream.

Those efforts absorbed tens of millions of dollars but have largely failed to date: Since 2008, shad restoration efforts have succeeded in moving fewer than 2,400 fish beyond the four dams on the lower Susquehanna. During that same period, biologists trucked 800 times as many eels upstream – more than 2 million – at a fraction of the cost.

Started on a whim with a shoestring budget, the American eel restoration effort is transforming the ecology of the river. Surveys show they are not only surviving, but growing fast and spreading throughout the Susquehanna basin, where they once accounted for a quarter of all fish biomass.

Biologists hope the returning eels will prey on, and slow the spread of, rusty crayfish, a troublesome nonnative invader. They also predict that the eels will boost populations of water-filtering mussels, which may eventually help improve water quality in the river.

That’s not all. Biologists now hope the surprising Susquehanna success will have consequences that reach far beyond the river or the Chesapeake Bay.

Eel populations have plummeted throughout their North American range in recent decades and are considered “depleted” today. But the Chesapeake region retains the greatest abundance of eels along the East Coast, and 40 percent of the habitat there is found in the Susquehanna basin.

Biologists hope that returning that vast area to productive eel habitat could help bolster eel numbers from South America to the Arctic Circle.
Eels have been surprising people for nearly as long as humans have existed. They live in a greater variety of habitats than any other fish in North America, from deep ocean waters to tiny headwater streams. They even crawl into ponds with no connection to any creek.

Especially perplexing for centuries was the question of where eels come from, as most have no sex organs, and no one knew where they spawned. Aristotle thought they were spontaneously produced in mud. Some thought they came from earthworms.

Scientists now know – at least they’re pretty confident – that American eels come from the Sargasso Sea, a large expanse of the Atlantic Ocean off Bermuda bordered by strong ocean currents that is known for massive beds of seagrass. No one has actually seen a spawning eel, nor even a dead post-spawn eel. But plenty of eel larvae are found in that area.

The larvae float with ocean currents for about a year until transforming into small, transparent “glass” eels that are capable of swimming, allowing them to break free of currents and head toward the coast.

They gain green-brown pigmentation, becoming “elvers” as they move into brackish coastal habitats, like the Chesapeake Bay, and upstream into rivers. At around 4 inches, they transform into larger yellow eels, an appearance they will retain for years or decades — not that most people see them, as they also become nocturnal and live under rocks and roots or in the mud.

They remain sexless until they are nearly ready to transform into their final stage: the silver eels, which are two to three feet long. These mature eels then make an enormous migration back to the Sargasso Sea to spawn. They have thicker skins and larger eyes to help survive the journey.

This is the opposite of anadromous species, like shads, salmons, river herring, striped bass and sturgeon. Those fish briefly visit freshwater rivers to spawn but live most of their lives in the ocean.

“Eels do everything backward,” Minkkinen said. Eels are the only “catadromous” fish in North America, breeding in the ocean but living most of its life in brackish or fresh water.

That unusual lifecycle may have contributed to the demise of eels, and is the reason the Susquehanna might be a key to their comeback.

An eel’s sex is not determined until later in life, and research suggests that those in dense populations tend to be mostly males. Those that reach sparsely populated headwaters are almost exclusively females. By congregating eels downstream, dams may be restricting the production of females needed to help the coastwide stock reproduce. Some crawl over, or around, smaller structures, but each can reduce the number of eels that get by. Large dams — like the 94-foot-high Conowingo — can totally shut down their passage.

If the river were to be fully reopened, Minkkinen estimates that the Susquehanna alone could eventually support 11 million (mostly female) eels. Some think that number is low.

That’s important because, unlike anadromous fish that return to their native rivers to spawn, the entire eel population breeds as a group in the Sargasso Sea. Their offspring are flung across the coast by ocean currents, rather than returning to a specific river.

Therefore, a rejuvenated Susquehanna population, biologists hope, could help rebuild eel numbers all along the coast, which is near its all-time low. But it’s hard to say for sure because of the eel’s unique life cycle, and they are poorly studied compared with anadromous species.

“It’s really hard to fit eels’ life history into a quantitative model,” said Kristen Anstead, a stock assessment scientist with the Atlantic States Marine Fisheries Commission. “Eels just become – highly scientific term – weird all the time.”

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