December 7, 2021

I, Science

The science magazine of Imperial College

Emma Brown looks at how new genetic research on the mysterious and fragile world of the American eel might help conservation projects
Eels migrate to the Sargasso Sea to breed

“Eels are sending us a really major signal that we’re harming our freshwater environment in a dramatic way,” said John Casselman, retired government scientist and Queen’s University biology professor. Casselman grew up on the shores of the St. Lawrence River and recalls seasons of eel abundance before dramatic declines began in the 1980s. Freshwater populations of the American eel are estimated to have dropped by 99% in some locations due to factors directly linked to humans. Eels have survived for 125 million years on Earth, but they may face their biggest threats yet from human activity.

Despite their cultural prevalence around the world – from the human food chain to legends of lake monsters – some of the eel’s life cycle remains a mystery to scientists. Researchers at the University of Laval, with findings published in Current Biology, have solved a piece of the puzzle by proving there are genetic differences between eels that mature in different environments.

American eels complete an incredible roundtrip journey in their life cycle. The larvae are believed to hatch in the Atlantic’s Sargasso Sea near the Bermuda Triangle – no one has
actually ever witnessed this – then float on ocean currents to be deposited along the east coast of North America. Some juveniles travel further upstream into freshwater systems, but as adults they all return thousands of miles to the Sargasso to spawn decades later.

eels 92431_web
An eel conundrum: How do the larger freshwater eels and smaller saltwater eels retain genetic differences (shown in this study) when they mate randomly in the Sargasso Sea?

“What we found is that genes affect whether an eel can survive freshwater or brackish environments,” said Scott Pavey, lead author of the study and former postdoctoral fellow at the University of Laval. The researchers screened the eel genome in 45,000 places and found 99 genes that differ between the two ecotypes. Their findings explain why some large-scale conservation projects were unsuccessful as the differences relate to characteristics like growth rate and heart development.

Commercial fisheries operators along the Eastern seaboard also witness the after-effects of experiments conducted further inland. “The [results] came as no surprise, as we already saw that the transplanted eels, though doing well in numbers in Lake Ontario, were maturing at far smaller sizes than the traditional female silver eels,” said Mitchell Feigenbaum, President of South Shore Trading in New Brunswick. Feigenbaum is referring to conservation projects like an Ontario Power Generation and Ministry of Natural Resources joint effort starting in 2006 that transplanted over 4 million juvenile eels to try to restock the Great Lakes system.

“Once they transplanted all these eels from the Maritimes into Ontario, they matured at a much smaller age and a lot of them out-migrated from the system,” said Matthew Windle, a research scientist at the St. Lawrence River Institute of Environmental Sciences (SLRIES) in Cornwall, Ontario. “That’s a concern because the Sargasso Sea is really far away so they likely need a large energy reserve to make that huge journey,” he said.

Eels have to negotiate dam turbines

Even before they reach the open ocean, eels must conquer hydroelectric dams’ turbines. “In the mid-1980s they actually jammed up the turbines so badly that some of them had to be pulled and cleaned. It was apparently a rotten, stinking mess,” said Casselman. Eel ladders have been installed at some dams in their path, but these only help young eels get upstream; adults travelling downstream face high mortality rates.

Fishing of juvenile eels may also place a strain on populations, although increasingly restrictive quotas are thought to somewhat mitigate the impact. The young eels are caught as they enter river systems, flown to growing facilities, raised to market size and sold for consumption in foods like sushi’s unagi. These eels, 70% of which are consumed in Japan, are almost exclusively American due to export bans in Europe and the near-extinction of Japanese eels. “It’s one of the most valuable fisheries on the planet per unit weight,” said Windle. According to the Maine Department of Marine Resources, the per-pound average for juvenile elvers increased from $874 in 2014 to $2,172 this year.

As if dam turbines and consistently high market prices weren’t enough to shake the foundations of the species, they must now also contend with changing ocean currents due to climate change and a deadly bladder worm thought to have hitched a ride on ships from Asia. The good news is, according to Feigenbaum, “Eels are coming to the east coast in sufficient numbers to repopulate previously-blocked estuaries.” The key is then getting them to their destinations safely, and doing so across many different regional and international jurisdictions. There are many ongoing collaborations focused on helping freshwater-based eels, including a citizen science data collection project between SLRIES and local divers and the Canadian Wildlife Federation’s Ottawa River eel project.

In light of the University of Laval’s findings, experiments like the stocking project of 2006 may well have been doomed from the beginning. However, Casselman believes that even if some trials are not entirely successful from a numbers perspective, there is value gained from keeping eels in “society’s consciousness”: “I think they should be stocked if for no more reason than so we have an association with them. If we lose that association then there’s a very serious problem of not caring about eels.”

Emma Brown is studying for an MSc in Science Communication

Images: Sargasso Sea, by U.S. Fish and Wildlife Service (Wikimedia Commons);   American eels (Anguilla rostrata) by Guy Verreault/Current Biology 2015 (Creaticve Commons); NB Power’s Grand Falls generating station floodgates are open during the Saint John River annual spring flood (New Brunswick Canada) (Wikimedia Commons)

Citation: Scott A. Pavey, S.A et al (2015) RAD Sequencing Highlights Polygenic Discrimination of Habitat Ecotypes in the Panmictic American Eel. Current Biology 25, 1666–1671. DOI: