Wading into Buffalo Creek’s knee-deep water, a group of biologists were checking in to see how American eels were faring in their adapted central Pennsylvania home — by giving them an electric jolt.

The shock was delivered in the name of science: They wanted to find out whether the eels —apart from the sudden electric charge — were otherwise getting fat and happy in the creek.

With battery packs in their backpacks, two biologists placed a circular wand that sent a steady electric current into the creek’s clear water.

Sediment plumes puffed from the bottom as dazed eels emerged from the sediment or out from under rocks, only to be scooped up in a net by other biologists who placed the squirming snake-like fish into a bright orange 5-gallon pail.

“We’re not going five or 10 feet without catching an eel now,” said Joshua Newhard, a fisheries biologist with the U.S. Fish and Wildlife Service Maryland Fisheries Resource Office.

That wouldn’t have been the case just a few years ago. Eels had essentially vanished from the Susquehanna after the construction of large hydroelectric dams downstream in the early decades of the 20th century.

But four years ago, biologists began stocking this Susquehanna tributary in Union County with eels whose migration up the river was halted at Conowingo Dam, as well as with eels collected near Ocean City, MD.

Roughly 100,000 eels had been stocked in the creek prior to the mid-August electro-shocking survey, and another load of 25,000 was due the next day.

And, other than the annoying electric jolt, the eels seemed to be doing well in their new homes. Most of the eels were only 3–7 inches long when released. Many of those placed into the buckets were nearly a foot long.

A century or two ago, eels in this area reached lengths of two or three feet. Over time, the biologists want to see if these eels will get that large and — if so — how long it takes.

“We are putting these eels into a habitat where they haven’t been in a long, long time, so we would expect them to grow fast for a few years,” said Steve Minkkinen, who heads the USF&WS Maryland Fisheries Resource Office.

Although they look like snakes, eels are fish, and were once one of the most abundant species found in East Coast streams where, according to some estimates, they may have accounted for a quarter of all fish biomass.

But life for eels has proven perilous. After the adults breed in the Sargasso Sea in the mid-Atlantic, their tiny offspring make a long journey to the coast, where some remain in brackish coastal waters while others migrate to the farthest river headwaters. There they live most of their lives — sometimes decades long — before returning to the Sargasso to spawn, and die.

Overfishing, pollution and habitat alterations — especially the construction of dams — have taken a toll on eel numbers, and some scientists think changes in ocean circulation and other impacts from climate change may be affecting their migrations to the coast.

Whatever the reason, eel populations are at an all-time low along the East Coast, and they are candidates for listing under the Endangered Species Act.

Before stocking, there were virtually no eels on Buffalo Creek or Pine Creek — a West Branch Susquehanna tributary.

The stocking on the creeks, considered a pilot project, began with funds from a mitigation project along the Susquehanna near Sunbury. It may be a prelude to a far bigger effort. Funding to support a long-term program to collect eels below Conowingo Dam and truck them upstream may be part of the license agreement now being negotiated between state and federal officials and the dam’s owner, Exelon Corp.

Much of the interest in bringing back the eels is actually driven by mussels. Recent research has shown that eels are critical for the reproduction of the eastern elliptio, a common freshwater mussel. The larval stage of freshwater mussels needs to live for a time on a fish “host” before it drops off and begins to grow on its own.

For the eastern elliptio, eels are the main host, laboratory research has shown. Without eels, the Susquehanna elliptios, which are the most numerous mussels in the nearby Delaware River, are disappearing. Young mussels are largely absent from the river and there is little sign of reproduction.

Minkkinen reached into Buffalo Creek and pulled out of the water an eastern eliptio a bit more than 3 inches long. “That could be 50 to 100 years old,” Minkkinen said.

He noted that Deer Creek, just below Conowingo Dam, has plenty of eels, and lots of juvenile mussels. “We see lots and lots of little mussels there,” Minkkinen said. “Here, we just see big ones.”

The hope is that restoring eels will boost the mussel population and ultimately improve water quality. Freshwater mussels, like oysters in the Bay, are powerful water filterers.

There are early signs that the eel stocking might help. Eels captured in Buffalo Creek and brought to the service’s Annapolis lab this spring were found to be “infected” with mussel larvae, as the biologists had hoped.

What’s happening in the wild won’t be known for a while. Young mussels are about the size of a grain of sand, so biologists don’t expect to be able to find any in the creek for five to 10 years.

In the meantime, biologists want to know how eels will fare in the creek, which has changed a lot since eels were abundant. Surveys, like the electro-shocking work Minkkinen and his crew were conducting, are intended to provide clues about how well eels — and other fish — will fare if eels numbers continue to increase.

Eels, along with brook trout, were once top stream predators.

The “top predator” position is now open for competition in Buffalo Creek and many other streams.

Newhard pulled a temporarily stunned smallmouth bass out of the creek. “It’s going to get fat and happy on eels,” he predicted.

The creek is, in fact, filled with nonnative predators like smallmouth bass, brown trout and rainbow trout that were not here, at least not in significant numbers, when eels were plentiful, and they might take a bite out of eel numbers. “Everything loves to eat an eel — especially another eel,” Minkkinen said.

The biologists are conducting separate surveys, isolating 70-meter stretches of the stream at a time and collecting all of the trapped fish in that area to assess, in a fish-eat-fish environment, which species are winners and which are losers as eels take up residence.

“We want to see if the biomass of the fish changes as we restore eels to the river,” Minkkinen said. “We also want to see if the species are changing.”

For now, the eels appear to be doing well. After about an hour of wading and shocking roughly 200 yards of stream, four buckets were heavy with eels. A sedative was added to each bucket — otherwise handlers would quickly learn what “slippery as an eel” truly means.

As a group of Mennonites from an adjacent farm gathered to watch, the biologists began measuring each eel and injecting it with a chip — a passive integrated transponder, or PIT tag — which can be detected during future surveys with a wave of an electronic wand. (Eels initially released into the creek were too small to be tagged).

In all, 93 eels were captured — about one for every two linear yards of river surveyed. The catch included two eels that had been tagged the previous year.

One recaptured eel grew from 211 millimeters last year to 263 mm (about 10 inches) this year.

“That’s about 2 inches,” Newhard said. “That’s faster than most things are growing in this creek.”

The second recaptured eel had grown even more — nearly 3 inches — since last year. “It’s certainly faster than the ones left below the dam are growing,” Minkkinen added.

The measuring and tagging completed, the reviving eels were returned to the orange buckets and taken back to Buffalo Creek. For now, the creek, named for a species long vanished from Pennsylvania, appears ready to welcome back a different long-vanished species.