For biologist Jim Gelsleichter, a good day at work is one when he hooks enough bull sharks that his arms are covered in "shark burns" - rashes caused by rubbing elbows, so to speak, with the feisty rough-skinned fish. Fortunately, he and his team are catching juveniles, typically just a few weeks old, and around two feet in length. While one researcher holds down the squirming baby, another can safely collect 5mL of blood and insert a small nylon dart tag in the base of the dorsal fin. To each tag Gelsleichter affixes a pair of dime-sized silicone-rubber discs that absorb the chemicals in the coastal waterways of southwest Florida, where the sharks are swimming.
Gelsleichter, based at the Mote Marine Laboratory in Sarasota, isn't testing the sharks for chemicals typically associated with aquatic pollution, such as mercury and PCBs. He's looking for traces of common pharmaceuticals that millions of people take every day, including contraceptives, antidepressants, anti-inflammatories, and lipid-lowering drugs. Excreted from our bodies, small amounts of medications make their way through wastewater treatment plants and into the effluent pumped into rivers and streams.
In 1999 and 2000, the US Geological Survey (USGS) sampled 139 streams in 30 states for organic wastewater contaminants, including hormones and pharmaceuticals. Eighty percent contained traces of the chemicals. In most cases the levels were low, frequently less than 1 ppb (part per billion). But is that high enough to have an effect on aquatic organisms? "There are thousands of drugs going into wastewater treatment plants, and thousands of drugs coming out [in the effluent]," says Peter Fong, a biologist at Gettysburg College in Pennsylvania. "We know a lot more about what is out there than we know about how it affects aquatic organisms."
Fong found that fluoxetine (Prozac), a selective serotonin reuptake inhibitor (SSRI), began inducing premature spawning in male zebra mussels at levels of 20 ng/ml of water. Toxicologist Marsha Black, at the University of Georgia, found that fluoxetine exposure at the level of 6 ppb delayed development in mosquito fish. USGS scientists recently reported that male fathead minnows exposed to wastewater effluent from a major metropolitan sewage treatment plant began producing vitellogenin, a female egg protein.
Another popular villain is ethanyl estradiol, the estrogen mimic commonly found in contraceptive patches and pills, and the "poster child for the pharmaceutical problem," according to Gelsleichter. Karen Kidd, of the University of New Brunswick, recently reported that chronic low-level exposure to the chemical so damaged reproductive development in fathead minnows that the population in an experimental lake was nearly extinguished by the end of her seven-year study.
As far as Gelsleichter is aware, his study is the first to look at pharmaceuticals in sharks. Bull sharks are unique among their toothy clan in their willingness to leave the salty sea for brackish rivers and estuaries, which are more vulnerable to pollution from coastal runoff and wastewater discharge. Florida's Caloosahatchee River is a nursery area for juvenile bull sharks, Gelsleichter says, and is also home to six wastewater treatment plants.
So far, Gelsleichter and his coinvestigator, Nancy Szabo, of the University of Florida's Analytical Toxicology Core Laboratory, have detected no ethanyl estradiol in the samples since they began testing sharks in 2006. They did find trace amounts of six SSRIs: Of ten sharks sampled, one tested positive for all six, and nine tested positive for Zoloft. Gelsleichter stresses that the concentrations they discovered were very low, about 0.4 ng/ml of blood. (For comparison, a person taking Zoloft might expect blood levels of 30-200 ng/ml, he says.)
This summer, Gelsleichter and Szabo expanded the study to look for some additional chemicals, including those found in cholesterol-lowering medications such as Lipitor. "Cholesterol is the precursor for all the sex hormones," Szabo points out, and "cholesterol-lowering drugs could potentially be devastating for wildlife."
Perched at the top of the food chain, bull sharks keep smaller fish numbers in check; what happens to them could ripple through the entire ecosystem, Gelsleichter says. He is encouraged by the apparent lack of evidence that chemical pollution has affected the bull shark population. For now, he's still in the process of identifying candidate "problem drugs" that warrant closer study. "It's somewhat of a fishing expedition," he says, "if you'll excuse the pun."