Spaghetti-like transparent glass eels are one stage in the American eel’s life cycle. They become pigmented after entering fresh water. Image: Peter Hodson
By Holly Drankhan
A mucous-covered female American eel reproduces only once in her lifetime, converting her entire 4- to 7-pound body into eggs.
But her tissues may also carry a lethal legacy from the Great Lakes water basin – fat-soluble organic pollutants.
Along with overfishing, parasites and dams, pollutants may be responsible for the drastic decline of the snake-like eel in Lake Ontario, according to a study that will be published this October in the journal Science of the Total Environment.
To spawn, American eels travel more than 1,000 miles from Lake Ontario and other North American lakes to a region of the Atlantic Ocean called the Sargasso Sea. The boundaries of the oval-shaped sea are defined by four changing ocean currents, but its western edge tends to run near the coast of North America from Maine to the Caribbean.
Large yellow eels reside in North American lakes until returning to the Sargasso Sea to spawn. Image: Peter Hodson
After hatching at sea, the eel’s offspring cross a number of shipping locks and dams to reenter freshwater.
Survival concerns prompt new research
The number of eels entering Lake Ontario across a hydroelectric dam on the St. Lawrence River at Cornwall, Ontario, dropped from 1.2 million in the early 1980s to below 10,000 for most of the 1990s and early 2000s, according to the Ontario Ministry of Natural Resources, or OMNR. Although this number increased to 40,000 in 2010, the Canadian agency classifies the eel as endangered.
“This species is 125 million years old,” said study author John Casselman, adjunct professor at Queen’s University in Kingston, Ontario. “It will survive, there is no doubt about that. But it is not surviving in our presence.”
Concerned for the eel’s survival, Casselman prompted the Ontario Ministry of Natural Resources to freeze and archive eels collected from eastern Lake Ontario. Researchers from Queen’s University analyzed the tissues of eels collected in 1988, 1998 and 2008.
The highest percentage of contaminants that they found are polychlorinated biphenyls (PCBs) and organochlorine pesticides, including DDT. Both groups of chemicals were banned from use in the United States and Canada in the 1970s because they killed birds and wildlife.
The ban on chemical use and discharge appears to have helped the eel, Casselman said.
Contaminant concentrations in eel tissues from the early 1980s were twice those known to kill lake trout embryos. Since then, the concentrations of organic pollutants in eel tissues decreased at an average annual rate of 9.1 percent.
“We have a glimpse that we have been doing a good thing,” Casselman said. “Let’s keep it up for the future.”
The decline in eels entering Lake Ontario began in 1984, some 20 years after organic pollution peaked in Lake Ontario sediments, according to the study.
A number of factors complicate the relationship between pollution and population, said Peter Hodson, study coauthor and professor at Queen’s University School of Environmental Studies.
One is that the eels live a long time. After hatching, transparent, leaf-like larvae float on ocean currents. When they reach cooler freshwater, they metamorphose into spaghetti-like glass eels and eventually yellow eels. The females reside in the smallest of the Great Lakes until they are 15 to 20 years old, when they return to the sea to spawn.
“That long, stretched-out lifestyle means that a fish exposed to the peak of contaminants in Lake Ontario in the 60s isn’t spawning until possibly the early or mid 80s, and we won’t see the offspring of that fish in Lake Ontario until late 80s, early 90s,” said Hodson.
Tracking progress
The fact that eels are cleaner is certainly good news. But overfishing, habitat loss, parasites and dams may continue to keep their numbers low, Hodson said.
Keeping track of progress is hard. The journey of the floating larvae is random. The offspring of an eel do not necessarily return to the same water body, making it impossible to track the contaminants from one area across generations, Casselman said.
Additionally, there is no long-term data set for eels entering Lake Ontario, Hodson said. This study used data from just one hydroelectric dam in Cornwall, Ontario. The dam has two eel ladders constructed in 1975 that allow researchers to count the eels crossing them.
“One of the problems with the management of eels throughout North America is that each state or province is managing the eel on their own, and each one has a different level of interest depending on the size of the fishery and their jurisdiction,” Hodson said.
They put in different levels of effort and may use different data collection and analysis methods, he said. “So it is a bit of a hodge-podge right now, and I think the different jurisdictions throughout North America really have to work together and do things in a consistent way.”
Although much about the American eel remains a mystery, scientists continue to search for answers to help its recovery in the Great Lakes basin and across the continent.