Recently I had the opportunity to meet Jake, a star sea lion at the San Diego Zoo’s Wegeforth Bowl. When a sea lion like Jake leans in for kiss, what is the first thing you think of? Perhaps you wonder if he brushed his teeth or if there is a towel near by to wipe away the slobber? In any case, I bet you wouldn’t wonder, as I did, about how many environmental contaminants are present in his blubber. Sure, this may sound a bit grim and out of place, but I can assure you that my reasons for thinking this are part of a larger, intriguing story, in which I am completely immersed as I pursue my master’s degree through the University of Missouri. This story involves a pesticide that was banned 40 years ago, the Los Angeles County sewer system, and San Diego Zoo Global’s continued commitment to the recovery of the California condor.
No need to worry about Jake and other sea lions living in zoos, as there are relatively low levels of contaminants present in their tissues. Those living off of the California coast, however, conceal some of the highest detectable levels in any marine mammal species of DDE, the breakdown product of the pesticide DDT. How did levels of DDE come to be so high in sea lions? The answer is shockingly simple. From the 1940s to 1970s, the world’s largest manufacturer of DDT dumped hundreds of pounds of it down the drain daily, where it traveled through the Los Angeles sewer pipes offshore to the Palos Verdes shelf. This dumping occurred over 30 years ago; however, these chemicals are particularly resistant to degradation and still exist in shelf sediments. As tiny critters feed on the sediments, they accumulate DDE. From the smallest invertebrates to fish and larger animals, more and more DDE creeps its way up the food chain to a top predator, the sea lion.
As sea lions migrate up the coast, they take DDE with them. Some do not survive the annual journey and therefore become part of a larger food web that includes the California condor. With over 50 reintroduced condors circling the coastal cliffs, it is a real possibility that a beached sea lion would become a welcomed scavenging opportunity. While this find is an amazing feast for a condor, should we be concerned about the DDE it conceals?
To answer this question, we can imagine traveling the California coast on a birding tour in the mid-1900s. We would start along northern Channel Islands where the peregrine falcons are being reintroduced after a population crash. Next, at Santa Barbara Island, we could see a shrinking cormorant population that hasn’t produced a chick in years. Finally, on Santa Catalina Island we would look for a bald eagle, but without luck as they have disappeared. Each of these populations has one thing in common; birds were exposed to high levels of DDE, and they produced notably thin eggshells.
Reintroduced condors currently feed higher up the food chain than the above-mentioned coastal birds, and as you may have already guessed, are producing thin-shelled eggs. Our colleagues at Ventana Wildlife Society have found that eggshells of coastal birds are 34 percent thinner than those of inland birds. This has contributed to a 50 percent decrease in successful hatching of coastal chicks. This discovery has led managers to successfully take in eggs from the wild, incubate them in captivity, and allow the wild parents to incubate and fledge captive-laid eggs.
Multiple lines of evidence suggest a link between thin eggshells and DDE exposure in coastal condors, but how can we possibly be sure without exposing condors to harmful chemicals? DDE is an endocrine disrupting compound (EDC). It can disrupt normal endocrine (hormone) pathways and functions, like reproduction, by activating hormone receptors, producing an unnatural hormone-like signal, ultimately resulting in weak eggshells. Luckily for the condors, our lab is developing a reliable assay within a test tube to determine the sensitivity of condor receptors to DDE and other EDCs. Determining which EDCs most strongly interact with condor receptors gives us a clue as to which will be most disruptive to condor reproduction. Connecting this knowledge with EDC assessments of habitats will assist conservation managers in locating release sites for California condors. Our hope is to safely release these magnificent birds into a historic habitat that provides a rich, contaminant-free diet, allowing the hatching of many wild condors in the future.
Rachel Gerrard is a research technician for the San Diego Zoo Institute for Conservation Research.