Eel program tracks slippery prey

Anyone encountering "glass eels" in the shallow, muddy waters of a Virginia tidal creek might well assume that the creatures were born nearby, and perhaps just a few hours earlier. After all, the eels are tiny—easily held in the palm of your hand—and exhibit all the youthful wriggling of spring.

But it turns out that the eels that migrate up Virginia's rivers in March and April hatched almost a year earlier and a world away—in the inky blue waters of the Sargasso Sea south of Bermuda.

Members of the VIMS' American Eel Monitoring Survey team have been monitoring the spring migration of juvenile eels since 2000, partnering with fisheries teams in other East Coast states to provide the first consistent picture of the eels' coast-wide population. Understanding the dynamics of the species' population is key to its effective management and protection. 

Troy Tuckey, head of the eel monitoring team at VIMS, says a number of factors could be contributing to the eels' decline, including "dams, fishing pressure, and infection by a parasite." The decline is so severe that in 2006 the U.S. Fish and Wildlife Service considered the eel—once a staple of local cuisine—for protection under the Endangered Species Act.

But comprehensive data on the coast-wide population of American eels and their year-to-year recruitment success remains limited, largely due to the complexity of the species' life cycle. "Yellow" eels spend up to 25 years in inland waters before maturing into "silver" eels that migrate to spawn in the open ocean. The spawn—nearly transparent, ribbon-like larvae known as leptocephali—travel the Gulf Stream to the east coast of North America, transforming into "glass eels" and then "elvers" as they swim up coastal tributaries from Florida to Newfoundland. The trip from the open ocean to coastal waters takes from 9 to 12 months.

It is these "young-of-the-year" juveniles that are sampled by the VIMS team—at sites on the James, York, Rappahannock, and Potomac rivers—and by the other eel-monitoring teams in rivers along the Atlantic seaboard.

Only now, after nearly 10 years of sampling, are these teams closing in on the long-term and geographically spaced records needed to gauge the eels' coast-wide population.

"We're just getting to the point where we have enough data to understand recruitment variability," says Tuckey. Their data suggest that ups and downs in local recruitment might be due to eels jumping off the Gulf Stream at different points along the coast in different years. "There might be a ‘flip flop'," says Tuckey, "in which declines in our glass eel numbers in one year are offset by increases in the number of glass eels migrating into New England, and vice versa."