Marine mammals are so polluted that they are poisoning endangered condors, according to a new study. California condors (Gymnogyps californianus) that fed on marine mammal carcasses had high blood levels of several toxic chemicals, including broken-down forms of the banned pesticide DDT. The findings could explain why condors living on the coast in Big Sur have thinner eggshells and lower breeding success than their inland cousins.
“It is showing pretty solidly that if you’re a condor from Big Sur, you’re spending time on the coast, you’re eating marine mammals, and it’s going to be detrimental to your reproduction because marine mammals have higher levels of contaminants,” said Carolyn Kurle, an ecologist at the University of California, San Diego and one of the authors of the study published last month in Environmental Science and Technology.
California condors are immense vultures that feed on carrion, including the washed-up carcasses of whales, seals and sea lions. Most of these marine mammals are top predators, and their bodies accumulate mercury and long-lasting organic pollutants such as polychlorinated biphenyls (PCBs), flame retardants (polybrominated diphenyl ethers, or PBDEs), and chlorinated pesticides including DDT. Chemicals in these categories have been shown to disrupt hormones, increase cancer risk and damage the nervous system.
The U.S. banned production of PCBs and DDT in the 70s, but in the mid-20th century, chemical companies pumped thousands of tons of the toxins into the ocean near California’s Channel Islands. The Channel Islands are still heavily contaminated, and they also happen to be the breeding site for more than 95 percent of California sea lions (Zalophus californianus), the marine mammal most often eaten by condors. DDT and its breakdown product DDE are known to cause eggshell thinning in birds, and past research has found that coastal condors have thinner eggshells than condors from inland populations. The coastal birds’ eggs are also 20-40 percent less likely to hatch.
For the new study, the researchers aimed to pin down the connection between condor eggs and sea lions, as well to investigate the birds’ exposure to other pollutants. Using stable isotope analysis, they showed that coastal condors got 8-52 percent of their food from marine mammals. Compared with inland condors, these sea lion-fed birds had 12-100 times more mercury, PCBs, PBDEs and chlorinated pesticides in their blood. The researchers estimate that 40 percent of coastal condors have DDE levels high enough to cause eggshell thinning.
“We don’t know what those [other] pollutants are doing to condors, but we can assume that they’re not doing anything good, because they’re known carcinogens and endocrine disruptors and neurotoxins,” said Kurle.
Pollution from marine mammals isn’t the only threat facing condors. The most immediate threat is lead poisoning, which condors get from eating animals shot with lead bullets. Lead kills condors quickly, while the chemicals in marine mammals are likely to cause long-term harm to populations, says Kurle.
“We were really worried about lead poisoning, which is still a huge problem inland, but now we’re also worried about contaminants from the marine system,” Kurle said. “There doesn’t appear to be any wild food source that’s 100 percent safe for these birds.”
|Nala Rogers is a science writer at The Wildlife Society. Contact her at email@example.com with any questions or comments about her article.