The world's largest breeding population of ospreys is coping well with the long-lasting residues of toxic chemicals that were banned decades ago but remain in the Chesapeake Bay food chain at varying levels, such as the pesticide DDT and insulating chemicals known as PCBs. The resilient fish hawks are also showing few effects from two other groups of chemicals that have become widespread in the estuary—flame retardant PBDEs and pharmaceuticals intended for human use. Those are key findings of a three-year study led by US Geological Survey scientists, which follows up on a wide-ranging USGS survey conducted in 2001 of persistent chemical pollutants in the fish and fish hawks of the Chesapeake Bay, the United States' biggest estuary.
The researchers tested fish, osprey eggs and the blood plasma of osprey chicks in the Chesapeake Bay's tidal waters. In the ospreys' eggs they found high levels of PCBs at some locations. They also found residues of DDT and a related compound, p,p'-DDE, but at levels much lower than the ones that caused osprey and bald eagle population declines in the late 20th century. Both PCBs and DDT were banned in the 1970s. Further, the researchers found that young ospreys are being exposed to PBDEs, which are considered potentially toxic to wildlife. Yet these residues had no discernible effect on the big raptors' success in the Chesapeake region, where as many as 10,000 breeding pairs are expected to nest this season.
"Osprey populations are thriving almost everywhere in the Chesapeake," said Rebecca Lazarus, a researcher at the USGS' Patuxent Wildlife Research Center and the lead author of a report on the study's latest findings, published April 1 in Environmental Toxicology and Chemistry. "We found them nesting in some of the most highly contaminated areas in the Bay and we did not find any relationship between contaminants and their nests' productivity."
The scientists found one cautionary sign: the osprey nestlings' blood carried low levels of a biological marker for genetic damage. Levels of the marker were highest in one of the bay's most polluted areas, near Baltimore's Back River wastewater treatment plant, and osprey nests near that plant did poorly at raising chicks to adulthood. Baywide, the damage is not enough to affect the birds' overall ability to reproduce, but it may be having subtle, undetected effects, and warrants more research, Lazarus said.
Ospreys have just returned from winter homes in South America to Chesapeake Bay, the estuary one writer called "the osprey garden of the world." The bay's shallow waters and abundant fish attract roughly one-quarter of the Lower 48 States’ ospreys. The fish hawks usually return to the nests they used the year before. In March the males in each of the Bay's breeding pairs began gathering sticks to mend their nests. By mid-April most females will be brooding two or three eggs.
These charismatic fish hawks are one of the world's most widely distributed birds, found on every continent but Antarctica, and one of its most distinctive, with golden eyes, six-foot wingspans, and barbed talons adapted to hold wet, wriggling fish. Their global range, all-fish diet, and their role as a top predator make them ideal subjects for studies of water pollutants' paths through the aquatic food chain. The USGS research is the one of the world's most comprehensive studies of ospreys' exposure to toxic chemicals; a similar study on Pacific Northwest ospreys was published in 2008.
In the 1960s and 1970s scientists found the pesticide DDT was biomagnifying, becoming concentrated in ospreys and other fish-eating birds and causing females to lay eggs so fragile that they cracked under the parents' weight. The bay's osprey population fell to fewer than 1,500 pairs before DDT was banned in the U.S. in 1972. In 1979 Congress also banned PCBs, which can cause reproductive failure in animals. PBDEs, which were introduced as replacements for PCBs, are being phased out because of concerns about potential toxicity.
The EPA classifies more than 70 percent of Chesapeake Bay tidal waters as impaired by toxic chemicals. To track these toxics and their effects on bay ospreys, Lazarus and her colleagues collected fish, osprey eggs, and blood samples from 48 osprey chicks along Chesapeake Bay tributaries in Pennsylvania, Maryland and Virginia. Working during the spring and summer nesting season from 2011 through 2013, they included several sites the EPA considers pollution "regions of concern" – Baltimore's Harbor and Patapsco River; Washington, DC's Anacostia and Potomac rivers; and the Elizabeth River at Hampton Roads, Virginia.
In the first set of study findings, published in 2015 in the journal Environmental Pollution, the researchers found that in these heavily industrial, urban regions of concern, levels of the DDT breakdown byproduct were 80% lower than in the 2001 study, but PCB levels barely declined at all. Osprey eggs from developed areas had PCB levels three to four times higher than at nests on an island in the open bay.
"In fact the levels of PCBs have not changed significantly in the past 35 years, which tells you how persistent these chemicals are," said USGS ecotoxicologist Barnett Rattner, an expert on toxics in bay ospreys who led the 2000-2001 study and worked with Lazarus on the latest research. "Yet the birds are doing well. They're exposed to these toxic chemicals, which are biomagnified up the food chain, yet fortunately we do not see any really serious effects in ospreys."
In the next phase of the work, the researchers reported finding numerous human medications in Chesapeake Bay water samples, but only one in osprey chicks. Pharmaceutical compounds pass through humans' waste into wastewater treatment plants and septic systems, which discharge them into waterways. The scientists looked for 23 pharmaceutical compounds and an artificial sweetener and found 18 of them in bay waters and seven in fish. The drug diltiazem, used to treat hypertension in people, was found in all 48 chicks' blood samples, though at levels below those known to cause adverse effects in wildlife.
"Some of these chemicals are in the wastewater stream, but they do not seem to be biomagnifying in ospreys," Rattner said. Those results were published in 2015 in the journal Integrated Environmental Assessment and Management.
For more information on USGS science being used to help restore the Chesapeake Bay, visit http://chesapeake.usgs.gov/