Article: Lazarus, R. S., Rattner, B. A., McGowan, P. C., Hale, R. C., Schultz, S. L., Karouna-Renier, N. K., & Ottinger, M. A. (2015). Decadal re-evaluation of contaminant exposure and productivity of ospreys (Pandion haliaetus) nesting in Chesapeake Bay Regions of Concern. Environmental Pollution, 205, 278-290. DOI: 10.1016/j.envpol.2015.05.026
A Silent Spring in Chesapeake Bay
Did you know that Chesapeake Bay is known as the “osprey garden of the world?” In fact, Chesapeake Bay is the largest nesting area of ospreys in the world! Even though ospreys are, well, birds, they are situated high up in the estuarine food web, making them an important aquatic predator (Fig. 1). Ospreys can actually dive into water to catch a live fish for dinner, which has earned them the nickname “fish hawk”! Because ospreys are so ecologically important, they have been well studied in Chesapeake Bay for decades.
Unfortunately, in 1970, osprey nesting pairs were at an all-time low. This decline was a largely the result of habitat degradation and pesticide pollution. If you are familiar with Rachel Carson’s ground-breaking book, Silent Spring, you may recall that the pesticide DDT was linked to the thinning of bird egg shells, including ospreys (Fig. 2). Osprey diets are rich in fish and shellfish, which bioaccumulate organic pollutants and transfer them up the food web in a process known as biomagnification.
Luckily, DDT was banned in 1972 and by the 1995-1996 season, observed osprey nesting pairs had doubled to 3500. While the concentration of DDT declined in osprey egg shells, other contaminants, such as PCBs remained high in certain regions and new threats from PBDEs were being detected.
In May 2009, an Executive Order was signed to help monitor the recovery and health of critical Chesapeake organisms, especially in U.S. EPA designed Regions of Concerns (ROC). This study specifically looks at the recovery and status of the osprey.
The health and recovery of Chesapeake Bay osprey populations were determined by observing reproductive success and measuring organic pollutant levels in eggs and blood samples from March to July in 2011 and 2012. The study sites selected focused around Regions of Concern in Baltimore Harbor (MD), Anacostia (DC), and Elizabeth River (VA). Regions of Concern are areas with significant urban and industrial inputs with a legacy of organic chemical usage. The nearly pristine Poplar Island was used a reference site since low levels of pesticides have traditionally been measured animal life.
Lazarus et al. (2016) measured the reproductive success of osprey pairs (Figure 3) by observing how many eggs per nest hatched and survived to become fledglings (baby osprey older than 45 days). Osprey eggs were also measured for thickness. Additionally, both egg and blood samples from nestlings (ospreys 40-45 days old) were analyzed for 11 PBDEs, 5 alternative brominated flame retardants, 129 PCBs, 44 organochlorine pesticides, and methyloxytriclosan.
Ultimately, organic pollutants concentrations from this 2011-2012 study were compared to a similar analysis from 2000-2001 in order to assess if osprey populations are recovering in Regions of Concern.
What did they find?
25 of the 44 pesticides measured were detected in the osprey eggs. The greatest concentrations were found in Baltimore Harbor and Anacostia and were 2 to 11.5 times greater when compared to the “clean” reference site at Poplar Island. But, only 1 egg in Baltimore Harbor had a concerning concentration of p,p-DDE, a breakdown product of DDT. Additionally, concentrations of PCBs were up to 5 times greater in Baltimore Harbor and Anacostia than the background site and methyloxytriclosan was found in all samples from the Anacostia Region of Concern.
Good news, there were no significant differences in egg shell thickness between all sites. On average, there were 3 eggs laid per nest and 94.2% of hatchlings would survive to be fledglings (making 1 to 1.4 fledglings per nest). For comparison, in Anacostia in 1970, the average was 0.55 fledglings per nest! This suggests things are looking up!
A decade later: how are the osprey?
DDT and other organic pollutants have been attributed to osprey population declines throughout the United States. Luckily, those dangerously low Chesapeake Bay osprey populations in the 1970’s doubled by 1995 nearly 25 years after DDT was banned.
Osprey survival, chemically speaking, is better today than it was forty years ago. The number of fledglings per nest has doubled and eggs do not appear to be thinning. In fact, the average egg shell thickness measured in this study is very close to pre-DDT era thicknesses! Moreover, in 2011-2012, more nests were observed in the Regions of Concern, increasing 4-fold in some areas. While nest increases could be a result of improvements to food availability or even more favorable weather, declines in pollutants are likely to be a contribution.
While our chemical legacy was frequently detected in osprey eggs and fledglings, the overall consensus was that things are getting better. Pesticides, such as DDT, continue to decline and were detected at concentrations below the threshold which would cause egg shell thinning. PBDEs also declined, but were highest by waste water treatment plants, suggesting that sewage is a large source of flame retardants. While PCB concentrations remained high, there were no large-scale effects observed on osprey productivity.
Overall, osprey populations are rebounding and organic pollutants concentrations in Regions of Concern are getting closer to clean-region concentrations. Ospreys continue to be resilient and ever so awesome (Figure 4).
I received a Ph.D. in oceanography in 2014 from the Graduate School of Oceanography (URI) and am finishing up a post-doc at the University of Maryland Center for Environmental Science (Horn Point Laboratory). I am now the Research Coordinator for the Delaware National Estuarine Research Reserve.
Carbon is my favorite element and my past times include cooking new vegetarian foods, running, and dressing up my cat!