Metal pollution is toxic for endangered eels

New research finds that cadmium acts as an endocrine disrupter in European eels during their 6000 kilometer migration.

One of the world's most bizarre creatures is vanishing. Freshwater eel populations began crashing worldwide in the 1980s. The decline has been rapid, and scientists think eels are probably succumbing to a variety of ills, including overfishing, habitat loss, pollution, and eel-chewing hydropower turbines.

The European eel (Anguilla anguilla) is a prime example. The number of its young has dropped by perhaps as much as 99% in 20 years, and the conservation group International Union for Conservation of Nature (IUCN) classifies the eel as critically endangered. Now, another potential threat to some European eel populations has emerged, according to research published in ES&T (DOI: 10.1021/es703127c). Researchers in France report that cadmium, a widespread metal contaminant in rivers and estuaries, interferes with the eel's complex reproductive cycle. Although cadmium is probably not the main cause of eel declines, scientists say that it could complicate efforts to save the species because toxic pollution takes years to clean up.

European eels have a notoriously complex life cycle that puts them at special risk from pollution. Larvae transform into transparent glass eels and then into yellow eels, which can live for up to 20 years in estuaries. Finally, the eels turn silver and head for the Sargasso Sea in the middle of the North Atlantic Ocean to spawn. They swim for up to 6 months straight—6000 kilometers—without eating. Before the journey, they store large amounts of fat, and with it, fat-soluble pollutants such as dioxins and PCBs, as well as cadmium.

The eels spawn just once and then die. The process is so cryptic that fully mature European eels have never been caught in the wild.

"We have shown that eels contaminated by cadmium are not always able to stock sufficient lipids to migrate," says study coauthor Magalie Baudrimont of the University of Bordeaux (France). The team captured female eels in the Loire River and exposed half of them to cadmium in the laboratory. The cadmium levels of eels in the experiment were environmentally relevant; for example, eels living in the cadmium-polluted Gironde estuary in southwestern France have been found with even higher concentrations of the metal in their tissues. Next, researchers injected the eels with hormones that stimulate reproductive maturation and placed them in swim tanks, where they swam constantly for several months to simulate natural migration.

Initially, female eels that had been exposed to cadmium showed faster follicle development in their ovaries compared with eels not exposed to cadmium. But as the fish swam during the simulated migration, the developing eggs of cadmium-exposed eels died (in a process called atresia), and many of the eels died as well.

Baudrimont explains that as the eels burned fat while swimming, the cadmium previously stored was released and accumulated in the ovaries. The cadmium may be directly toxic to reproductive tissues, or it may alter hormone levels. Either way, she says, the eels appear to spend part of their energy reserves detoxifying the cadmium and cannot develop fully mature gonads.

"That's an absolutely classic demonstration of an endocrine-disrupting compound," says fish physiologist Alan Kolok of the University of Nebraska Omaha. These compounds typically have different effects at high and low doses, he says. In this case, follicle stimulation occurs at low cadmium levels, whereas toxicity occurs at higher doses. Kolok has found reproductive effects of cadmium in fathead minnows as well but says this is the first study he has seen that demonstrates that internal stores of cadmium can be released during fish migrations at levels high enough to be toxic. Some migrating birds "keel over and die" after long migrations because of pesticides released during flight, he says, and other migrating fish, such as salmon, might be affected similarly.

Baudrimont says that cadmium is a contributor to but certainly not the sole cause of worldwide eel decline, and other experts agree. "You can't prove any of the single causes, and it would be very risky to focus on just one," says Willem Dekker, a senior fisheries scientist at the Institute for Marine Resources and Ecosystem Studies (The Netherlands) who studies the eel's decline. Dekker worked with the European Commission to develop an eel restoration plan, which was enacted as an EU-wide directive in 2007 that requires countries to develop management plans.

The eels won't spawn in captivity, making them all the more difficult to save. Guido van den Thillart, a fish physiologist at Leiden University (The Netherlands), is working on hormone-producing implants that would help eels mature in the laboratory. "If we can reproduce eels in a regular way, we can see what the effects of cadmium and other contaminants are in the total reproduction of the animal and their population," he says. —ERIKA ENGELHAUPT