Duck Winter Survival Rates Stabilize More than a Decade After Exxon Valdez Oil Spill

Newswise — Although thousands of birds and mammals were killed immediately following the Exxon Valdez oil spill of 1989, the long-term effects of oil exposure on the region’s wildlife remain a concern.

In the article “Female Harlequin Duck Winter Survival 11 to 14 Years After the Exxon Valdez Oil Spill,” Daniel Esler and Samuel A. Iverson present results of their study of female harlequin duck survival rates during the winters of 2000–2001 to 2002–2003. The article is featured in the April 2010 issue of The Journal of Wildlife Management, published by The Wildlife Society.

Harlequin ducks were especially vulnerable to the Exxon Valdez spill because the oil invaded their habitat—the nearshore intertidal and subtidal areas—and remained in the sediment through at least 1998, according to soil samples. Research also showed that the ducks remained exposed to the oil and continued to suffer even a decade after the spill.

“This new understanding was in stark contrast to the conventional wisdom at the time of the spill that effects on wildlife populations should be short-lived and present only in the acute phase of catastrophic spills,” Esler and Iverson write.

Their study examined whether the spill continued to adversely affect the ducks 11 to 14 years later by tracking survival rates and a biomarker known as cytochrome P4501A (CYP1A) induction, which indicates exposure to oil. They followed 138 ducks at sites in Prince William Sound, Alaska, including some from areas where the beached oil was deposited and some from unoiled areas.

During those winters, they found that the ducks’ exposure to residual oil was ongoing but that winter survival rates did not differ between ducks living at areas oiled by the Exxon Valdez spill and ducks living at nearby unoiled areas—a change from a few years earlier.

“Our findings contrast with those from directly comparable studies from the mid-1990s in which survival rates were 5.7 percent lower in oiled areas compared to unoiled areas,” Esler and Iverson write. “Our results indicate that it took roughly a decade for survival of female harlequin ducks to recover following the Exxon Valdez spill, which is much longer than had been assumed that deleterious effects on wildlife populations would be expressed.”

Although ducks appear to remain exposed to oil in some areas, the researchers note it likely was not enough to affect winter survival rates. “With the volume of oil remaining in intertidal beaches declining over time … one would expect diminishment of both exposure and effects, though not necessarily on the same timescale. That appears to be the case with harlequin ducks in Prince William Sound.”

Esler and Iverson caution about “subtle or sublethal effects” in the long term for the ducks and suggest continued monitoring of their oil exposure levels. Because levels were measurably higher among exposed ducks for a decade after the oil spill, wildlife managers now must consider immediate and longer-term survival issues.

“This further highlights the importance of population-level effects during the chronic phase of exposure to residual oil, despite a common perception that most effects on wildlife populations occur during the acute phase, the weeks or months immediately following an oil spill,” they write.

Esler and Iverson suggest that these findings could be used with other demographic measures to “build population models that evaluate the chronology, mechanisms, and severity of negative effects of oil spills, as well as the process by which population recovery occurs.”

The full text of the article is available at http://www2.allenpress.com/pdf/wild-74-03-471-478.pdf

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About The Journal of Wildlife ManagementThe Journal of Wildlife Management, published since 1937, is one of the world’s leading scientific journals covering wildlife science, management, and conservation. It is published eight times per year by The Wildlife Society. To learn more about the society, please visit: http://joomla.wildlife.org/.