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Twenty years after the Exxon Valdez oil spill,little remains in Prince William Sound toremind the eye of 1989’s striking images ofoiled birds and sea otters, or of armies ofworkers in protective gear toiling to cleanblackened beaches. Today, the waters of thesound are turquoise and the shorelines bristlewith life, almost none of it human.

But researchers are still studying thespill’s persistent aftereffects: Even as manyspecies have recovered, others continue tostruggle. Some may still come in contactwith the oil that lingers, tucked away belowthe rocky surfaces of the beaches.

Scientists—some of whom have studiedthe spill for the entire 2 decades and are nowlooking to retire—are taking stock of theirresults and working to determine how (andwhether) they might encourage furtherrecovery of the ecosystem. This month, theExxon Valdez Oil Spill Trustee Council,which oversees research and restoration,released its summary report. “It’s not justresearch for the sake of research,” saysCatherine Boerner, a restoration specialistwith the Trustee Council. The science nowlooks squarely at how to manage the injuredspecies, she says, such as by opening a fish-ery or culling predators.

Before the incident, researchers had lim-ited understanding of the long-term effectsof a big spill. The Valdez studies are thelargest, longest, and most expensive everdone. They suggest, for example, that oil

may persist much longer than expected,affecting intertidal organisms, and thatchronic exposure to low levels of oil caninflict subtle damage on wildlife. Many ofthe hundreds of scientif ic reports are“incredibly influential papers” that “will becited for a long time,” says marine chemistChristopher Reddy of Woods Hole Oceano-graphic Institution in Massachusetts, whodid not work on the spill.

Despite the mountain of studies, gov-ernment-funded and Exxon-funded scien-tists still clash over the spill’s long-termeffects and whether Exxon should pay thegovernment an additional $92 million foryet more research. Government scientistssay Exxon researchers don’t accept goodevidence, while Exxon scientists chargebias, too. Government-funded studies tendto be “bleak and negative,” says Alan Maki,an environmental scientist who oversawExxon’s research until he retired in late2007. “This spill has not behaved much dif-ferently than what you would expect fromstudies of other spills,” he says.

Some questions provoke less rancor butstill may never be answered, such as whythe Pacific herring populations crashed. Inpart, because of the complexity of theecosystem, “we’ll never know,” says StanleyRice, who manages research on the oil spillat the National Oceanic and AtmosphericAdministration’s (NOAA’s) Auke Bay Lab-oratories in Juneau.

Aftermath of a disaster

The spill occurred just after midnight on 24 March 1989, when the supertanker Exxon

Valdez ran aground on Bligh Reef in PrinceWilliam Sound. Forty million liters of crudeoil ended up in the sea and on the beaches,making it the largest spill in U.S. waters. Theimmediate impact was dramatic: About250,000 sea birds died, along with 22 killerwhales, 2800 sea otters, 300 harbor seals,and untold numbers of fish eggs.

Scientists rushed to study the ongoingeffects. Their efforts received a huge boost in 1991, when Exxon agreed to pay$900 million in a civil settlement with the U.S. and Alaskan governments to restore Prince William Sound. The TrusteeCouncil has dedicated some $180 million to research, with the rest used to preserveland and reimburse cleanup expenses. Theresearch efforts will continue indefinitelythanks to an endowment fund, currentlyabout $100 million.

The company now known as ExxonMobilhas sponsored its own research, and the sci-entists it funds have published or presentedmore than 400 peer-reviewed papers andtalks. Over the years, their conclusions haveoften clashed with those of the government-funded researchers.

For example, one of the largest efforts hasbeen to track the fate of the oil remaining inthe sound years after the spill. In 2001, ateam led by Jeffrey Short, a chemist then

Published by AAAS

with NOAA, randomly sampled 91 beachesin the oiled parts of the sound, digging 9000 pits. Short estimated that 55,000 litersof oil remained, spread across and underneath11 hectares of beaches.

David Page, an Exxon-funded chemist atBowdoin College in Brunswick, Maine,insisted, after conducting his own sampling,that the government estimate was too high.Even though he later came to accept Short’sresults, he and others still questionedwhether the lingering oil is affectingwildlife. They argue that other sources ofhydrocarbon pollution outweigh what littleoil remains from the 1989 spill. The remain-ing oil, says Page, is sequestered. “If it wereavailable to be harming wildlife, it wouldhave been long gone.”

Government researchers challenge thoseclaims. In 2005, Short’s team resampled 10of the beaches where oilremained in 2001. They reportedin 2007 in Environmental Science

& Technology that the oil wasdecaying at just 0% to 4% peryear. “It will persist for decadesup to a century,” says Short, whoretired from NOAA a few monthsago and now works for Oceana, amarine conservation group. Inanother study reported last year inMarine Environmental Research,Short’s team found that biologi-cally active contaminants in theregion were predominantly fromthe oil spill; he also thinks thatbiomarkers such as a particularliver enzyme reveal that organ-isms have been exposed to oil.

These conflicting claims have fiscal con-sequences: The 1991 settlement contained aso-called reopener clause allowing the gov-ernment to claim up to $100 million more ifby 2006 unanticipated damages from thespill appeared. That year, the governmentasked Exxon for $92 million to f ind andremove the remaining oil, arguing that it waspersisting longer than expected. Both sideshave agreed to postpone negotiating thereopener until the government finishes moreoil studies, perhaps 2 years from now, saysCraig O’Connor, a lawyer with NOAA.

A pod dwindles Scientists on both sides agree that manyspecies have recovered in Prince WilliamSound, including bald eagles, cormorants,salmon, and river otters. But the oil, govern-

ment scientists think, has had severe impactson at least two photogenic animals: killerwhales and sea otters.

The two pods of whales photographed andidentified in the oil slick in 1989 each lostabout 40% of their members around the timeof the spill, says Rice. “That is just totallyunprecedented,” he says. One pod is recover-ing slowly, but the other, originally com-prised of 22 whales, has lost all of its femalesof reproductive age and is down to seven oreight members. Eventually, Rice says,“they’re going to become extinct.”

Because the two unrelated pods declinedso suddenly and at the same time, researchersargued last year in Marine Ecology Progress

Series, the deaths were almost certainlycaused by the spill when the whales breathedoil fumes or ate contaminated prey. ButExxon scientists say the deaths can’t be con-clusively linked to oil.

Meanwhile, sea otters have reboundedin most of the sound, but populations

remain low in some heavily oiled areaswhere oil lingers in the intertidal zones.U.S. Geological Survey biologist JamesBodkin f itted 16 otters with time-depthrecorders and reported in February at ameeting of the Alaska Forum on the Envi-ronment that shallow intertidal diggingrepresented about 18% of female seaotters’ dives. “They’re going to get expo-sure to oil,” says Rice.

Most scientists do agree about the fateof at least one injured species: Pacific her-ring, whose populations are only 15% oftheir prespill numbers. In the late 1980sbefore the spill, the herring f ishery inPrince William Sound was worth $12 mil-lion and the population was at a recordhigh. The year after the spill, the populationseemed high again—estimated at 120,000

tons—and the fishery opened. But then in1993, the population crashed: Only 20,000tons of herring appeared.

Was this due to the spill? Many scientiststhink not. A poor bloom of plankton in 1992left the fish hungry and vulnerable to dis-ease, says fish pathologist Gary Marty ofthe British Columbia Ministry of Agricul-ture and Lands in Canada, who has beenstudying the herring since the spill. He andTerrance Quinn of the University of Alaska,Fairbanks, developed a model that he sayscan “describe every blip in the populationfor the past 15 years.”

But Richard Thorne, an acousticsresearcher at the Prince William Sound Science Center, says hydroacoustic monitor-ing results suggest that the spill—and thesubsequent 3 years of f ishing—caused the population to crumble. In 1993, hestarted conducting annual hydroacousticsurveys, which use sonar to count fish. Heand Gary Thomas, a fisheries scientist at the

University of Miami, noted thatthe acoustic results correlatewell with aerial surveys of herring spawn, which have beendone every year for more than 30 years, and suggest the declinebegan in 1989.

Unfortunately, most herringstudies stopped after 1990, soneither side in this debate hasdata about the critical precollapseyears. As a result, researchersmay never know for sure, saysGeorge Rose, a fisheries conser-vation expert at Memorial Uni-versity in St. John’s, Canada. “Ina way, it doesn’t matter,” saysNOAA’s Rice. “We need to knowwhy they don’t come back.”

That’s where $2 million of this year’sresearch has focused. With a better under-standing of factors such as disease, predators,and climate change, researchers hope to helpthe fish rebound. Ideas include establishing aherring hatchery or targeted fishing for someof the herring’s predators, like pollock. Thebest thing for the fish may simply be to pro-tect them from fishing and other causes ofmortality, Rose says: “In rebuilding naturalsystems, the main ingredient is patience andthe other one is protection.” Patience indeedis called for, many researchers agree. Twodecades may span most of a scientific career,but they hardly register in the transformationof an ecosystem.

–LILA GUTERMAN

Lila Guterman is a science writer in Washington, D.C.With reporting by Jacopo Pasotti.

www.sciencemag.org SCIENCE VOL 323 20 MARCH 2009 1559

NEWSFOCUS

Digging for oil. Oil rises from below the surface on

a Prince William Sound beach.

Swimming against the odds. One killer whale pod is slowly recovering from the

oil spill, but another is headed for extinction.

Published by AAAS