Cadmium May Be Eel-Killer

Jessica Marshall - Discovery News, The European eel slithers its way more than 3,400 miles downstream to the ocean to spawn -- but cadmium accumulated upriver may make the voyage fruitless for the highly endangered species.

So finds a new study by researchers at the Centre National de la Recherche Scientifique in Arcachon, France who exposed eels to cadmium in the laboratory and found that although the eggs of cadmium-exposed eels initially matured faster in the ovaries, the eggs -- and the eels -- were more likely to die than unexposed eels.

"They show that a moderate cadmium concentration is already almost lethal to the eel," said Willem Dekker of Wageningen IMARES, a marine research institute in IJmuiden in the Netherlands, who was not involved in the new study.

The cadmium levels found in eel tissues in the experiment were less than those in eels captured from the Gironde estuary in southwest France. Even at these lower concentrations, four of eight cadmium-exposed eels died in the last three weeks of the experiment, while none of the unexposed eels died in that period.

European eel populations have declined sharply since the 1980s, with only 10 percent surviving each generation, and an overall dropoff of up to 99 per cent since the 1960s, Dekker said.

Cadmium exposure is certainly not the only cause of this decline, the researchers noted. Overfishing, contamination from other pollutants, and hydropower stations that act like "meat grinders" to migrating eels also pose threats, Dekker said.

The eels have a four-stage life cycle, which contributes to their susceptibility to cadmium, the researchers say. In the first stage, larvae drift toward the European coasts after hatching in the Sargasso Sea. The larvae morph into glass eels, a transparent form, which enter rivers and begin to move upstream.

The glass eels (a delicacy that currently fetches about $1,000 per kilogram) then develop into juvenile yellow eels. These can live for 10 to 20 years in rivers and streams, storing fat for their 3,400-mile migration back to the Sargasso Sea, during which they do not eat.

"They can spend more than 10 years in a river and they accumulate a lot of cadmium," said study author Patrice Gonzalez of the Centre National de la Recherche Scientifique.

As they begin the migration, the eels transform into their fully adult stage -- a silver form with a darkened back and a white belly. Once they reach the coast, their life is a mystery: No fully adult eels have ever been caught in the ocean, and their spawning has never been observed.

Their adult migration phase is where the cadmium does the most damage, the researchers found.

After exposing the eels to cadmium, the researchers simulated the six-month migration in a tank with a continuous current, and provided hormones to begin the egg maturation process, which the eels will otherwise not do in captivity. As the eels depleted their fat stores for energy, a big dose of stored cadmium was released also. This is the stage during which the eggs and eels died in the laboratory.

"We don't know exactly what the mechanism is," Gonzalez said. "But cadmium is probably an endocrine disruptor."

Although the researchers demonstrated the problem only in the laboratory, not in the environment, Dekker noted, "It's realistic to assume that it is a problem."

"The bottom line is, we have so many concerns with the eels," he said.

The good news is that the European Union last year implemented a recovery plan for European eels, Dekker said, with a target of returning the spawning stock to 40 percent of what it would be without humans' impact. "We hope to turn the tide in coming years."

SOURCE : Discovery News