Efforts to restock eels in the Susquehanna River got a big boost this summer when biologists captured a surprising 135,478 elvers at Conowingo Dam and released them far upstream.

That was a big increase over the 84,961 last year, which had been the most captured since 2008, when eel-stocking efforts began.

The success was a surprise because biologists thought the huge amount of sediment that passed through the dam during Tropical Storm Lee in September 2011 might affect this year's migration.

"We had not predicted a great year," said Steve Minkkinen, who heads the U.S. Fish and Wildlife Service Maryland Fishery Resources Office.

In addition to the small, greenish elvers caught at the dam, biologists stocked about 30,000 glass eels — translucent eels recently arrived from the Sargasso Sea— which were captured near Ocean City, MD.

Minkkinen's office has been working in partnership with the U.S. Geological Survey Northern Appalachian Laboratory the last three years to stock eels in two Susquehanna tributaries — Buffalo and Pine creeks — and monitor their distribution and growth. Prior to that, some eels were released into Conestoga Creek in Lancaster County to test the feasibility of catching eels at the dam and trucking them upstream for release.

American eels spawn in the Sargasso Sea in the mid-Atlantic, and then migrate to coastal waters and up rivers where they live most of their lives until they return to the Sargasso to reproduce.

The Conowingo Dam, located just 10 miles upstream from the Susquehanna River's confluence with the Chesapeake, along with other dams on the lower Susquehanna, have blocked eel migration for nearly a century, causing them to vanish from the Bay's largest tributary.

The fish's disappearance may have taken a toll on some native mussels — and the river's health. Recent research has shown that eels are critical for the reproduction of the eastern elliptio, a common freshwater mussel. The larval stage of the mussel 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, laboratory research has shown that eels are the main host species. Without eels, populations of the elliptios, which are the most numerous mussels in the nearby Delaware River, are disappearing. Young mussels are almost totally absent in the river, and surveys in Buffalo and Pine creeks — located in Central and Northern Pennsylvania respectively — have found almost no evidence of mussel reproduction, although old mussels were present.

"We are hoping that the eels are going to help jump-start reproduction in those mussel populations by providing a host for them to complete their life cycle," Minkkinen said.

There is evidence that is happening. Biologists this summer recaptured several eels released in previous years. All had mussel larvae, known as glochidia, on them. "That gives me a lot of hope," Minkkinen said.

In addition, recaptured eels appeared to be growing rapidly, and dispersing through the Buffalo and Pine creek watersheds. Some of the recaptured eels, which were about 6 inches when released, are now way more than a foot long.

Eels in upstream habitats can grow to 2–3 feet long before they migrate back to the Sargasso after about two decades.

Getting eels upstream may be important not only for the mussels, but for the health of the river. The large eastern elliptio population on the adjacent Delaware River is estimated to be capable of filtering between 2 billion and 6 billion gallons of water a day, removing 78 tons of sediment.

That filtering capacity is largely gone from the Susquehanna, where mussel populations are depleted.

Eel releases in Pine and Buffalo creeks were slated to end this year as stocking goals for both were met and exceeded. In the last few years, 88,028 eels were stocked in Buffalo Creek and 122,047 in Pine Creek. The stocking was funded as part of a mitigation requirement related to a streambank project near Sunbury, PA.

In 2015 and 2020, biologists plan to resurvey mussel populations in the creek for signs of reproduction. Because young mussels are so small, and grow slowly, biologists think it will take that long for the mussels to get large enough to find.

Meanwhile, so many eels were caught at Conowingo this year that biologists also stocked 20,228 in Broad Creek in Maryland, which is upstream of the dam. Another 35,401 were released above the York Haven Dam, the fourth hydroelectric dam on the lower Susquehanna. Minkkinen said biologists may try stocking those two areas again next year.

The American eel population has plummeted along the East Coast in recent years, and the U.S. Fish and Wildlife Service is reviewing whether they should be protected under the Endangered Species Act.

Some biologists believe that helping eels reach upstream habitats may provide an important boost to their populations. The sex of eels is not set until they are several years old, and eels that migrate upstream disproportionately become females. Producing more females could boost reproduction in the Sargasso.