Exxon Valdez - Legacy of a Spill

Thursday
May 13, 1999

Legacy of a Spill: Stories | Illustrations | Photos


Sound battles back, but threats linger

By DOUG O'HARRA
Daily News reporter



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A decade after the tanker Exxon Valdez ripped open on Bligh Reef and dumped at least 11 million gallons of North Slope crude, most of the oil has disappeared - dissolved and diluted by the relentless weather, collected and hauled or washed from blackened shores by workers.

Yet even as much of the spill zone blooms with health, toxic pockets of oil may still threaten marine life. Though Exxon calls these remnants insignificant, one federal scientist likened them to "an abandoned field of land mines" that could maim aquatic life well in the future.

Ovewr the years since the March 24, 1989, spill, Prince William Sound has become a demonstration of the resilience of nature and the persistence of North Slope crude.

Based on scores of studies, the state-federal Exxon Valdez Oil Spill Trustee Council lists only bald eagles and river otters as recovered among the 28 damaged species and resources. Among the 13 still healing are animals with signs of ongoing exposure to hydrocarbons. Pink salmon eggs may still be dying in streams that remain oiled. Along certain bays, sea otters and other species have yet to rebound.

Most alarming of all, a multiyear investigation by federal biologists found evidence that fish can be harmed by crude oil in much smaller doses than previously suspected.

Federal scientists at the National Marine Fisheries Service lab in Juneau have found that certain long-lived components of weathered oil - polynuclear aromatic hydrocarbons, or PAHs - are 100 times more toxic than previously believed, capable of killing or genetically damaging young salmon and herring at concentrations of less than one part per billion.

The investigation results were dramatic and startling - eggs that died, deformed spines and jaws among those that hatched, and significantly fewer returns from the sea, according to biologist Ron Heintz of Auke Bay Laboratory. "The lesson is, oil is way more toxic than we thought."

"What we found were effects that could show up throughout the whole life of the fish," added federal chemist Jeff Short. "Fish that had reared as eggs and alevins (newly hatched salmon) in oil-contaminated gravel had poorer marine survival. They were still impaired after they swam away."

Exxon and the scientists whose research it sponsors flatly reject the new findings, as well as the trustees' conclusion that significant healing remains.

"We see the Sound as essentially recovered," said Frank Sprow, corporate vice president for environment and safety and a chemical engineer. "There is a growing consensus in the scientific community that the Sound has returned to a thriving, robust and productive state."

It is faulty sampling - and not exposure to oil - that explains why researchers have found disproportionate numbers of dead salmon eggs in streams crossing oiled beaches, according to Ernest Brannon, director of the Aquaculture Research Institute at the University of Idaho in Moscow, Idaho. And the findings about oil's extreme toxicity contradict decades of previous work, said chemist David Page, a professor at Bowdoin College in Brunswick, Maine.

"This notion of minute quantities of petroleum causing this plutonium-like environmental damage can't be true," Page said. "I think there's this fixation on a very minute amount of very degraded oil that's left at very few places in the Sound that's doing very little damage."

The stakes couldn't be higher, with implications that reach far beyond any disputes over the Sound's recovery.

Because the PAH findings undercut broad assumptions about the danger from spilled oil, they suggest that national water quality standards for hydrocarbons may be inadequate, according to Stanley Rice, senior scientist for oil-spill studies at Auke Bay Lab.

Lethal concentrations of PAHs for fish eggs could be present in harbors, populated bays and chronic spill sites throughout the country. Even Alaska's standard for total hydrocarbons in water - the strictest in the nation at 15 parts per billion - may be 100 times too lax to create an adequate safety margin, according to the researchers.

"This may change how the public deals with oil," Rice said.

In Alaska, the discovery could help explain why certain populations of animals in the Sound have yet to recover despite the disappearance of nearly all the oil.

The discovery also raises questions about how to handle the next big spill. With crude so toxic for so long, perhaps people must do whatever it takes to remove it from the environment, Rice said, even if that means using controversial, aggressive techniques such as dispersants and hot-water washing that may cause additional short-term damage.

Putting these developments into context for the Prince William Sound ecosystem is complicated. On one hand, ongoing effects almost certainly involve relatively small numbers of fish exposed to the few remaining beaches with significant oiling, according to several state and federal biologists.

"The oil that's out there doesn't have a population effect on the herring and salmon that's out there," said Bruce Wright, National Marine Fisheries Service's liaison to the Trustee Council. "It's an isolated effect."

On the other hand, the remaining oil appears to be remarkably persistent, stubbornly resistant to further cleanup and weathering. If exposure still harms certain populations of pink salmon, what about other animals?

The Sound's herring population crashed in 1993 and 1994, the 4- and 5-year-old survivors of the spill year showing up with viral and fungal diseases that can be triggered by exposure to oil in the early stages of life.

Some beds of blue mussels are still absorbing hydrocarbons, concentrating them in their tissue, and passing them on to animals that eat them. Sea otters and several other species have not recovered in certain oiled bays, their blood containing physiological markers that appear only in animals that have recently metabolized hydrocarbons.

"You can certainly have a pretty credible hypothesis that all those things can be connected," said Robert Spies, chief scientist and consultant for the Trustee Council. "What has not been done is (to) go out and actually measure those concentrations of those compounds in the environment and close the loop logically."

"You have all these pieces of evidence on a tapestry that's really quite complicated," added marine ecologist Gail Irvine, with the Biological Resources Division of the U.S. Geological Survey. "When new information comes out, like the pink salmon and herring studies, that shows that toxicity can occur at a much lower level, then people have to revamp what they think about other species."

Just how far into the future will the spill extend its reach?

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'We see a system that is functioning, that is largely healthy, that is certainly recovering from the spill.'

The research on pink salmon covers only a tiny portion of a vast $225 million, decade-long scientific inquiry into the life and natural processes of southern Alaska's spill zone. Funded mostly by the 1991 court settlement with Exxon, the effort has amounted to what Exxon Valdez Oil Spill Trustee Council science coordinator Stan Senner called "a moon-shot, crash course on Prince William Sound and the Gulf of Alaska."

Researchers from government, academia and Exxon initially documented an unprecedented "body count" - at least 30,000 carcasses of 90 kinds of birds, 1,011 sea otters, 300 harbor seals, 151 bald eagles, as well as hundreds more individuals from dozens of species. They observed disruptions in the Sound's killer whale pods, including the loss of 13 individuals from the 36-member AB family. They tracked the oil as it fouled beaches from the heart of the Sound, on out past the Kenai Peninsula's outer coast, and as far as remote stretches of the Alaska Peninsula.

They also documented the oil's gradual and widespread disappearance through natural forces and human effort.

After the $1 billion court settlement in 1991, the focus shifted to measuring recovery or failure among 30 critical species or resources and then toward understanding the region's habitat as a whole.

In many instances, scientists had little understanding of the factors driving the complex Gulf of Alaska-Prince William Sound ecosystem, already undergoing dramatic changes before the oil spilled. Possibly triggered by an increase in ocean temperatures (and a shift from an environment dominated by crustaceans to one dominated by bottom fish), some species were crashing and some were at record levels. At the same time, humans had become an unprecedented factor through dramatic growth in commercial fishing and salmon hatcheries.

The oil - and the subsequent cleanup - hit this dynamic system with a tremendous jolt. The immediate exposure killed an estimated hundreds of thousands of animals and damaged habitats along 1,300 miles of shoreline. Gauging recovery from that time on would hinge on the ability to distinguish between trends independent of the spill and lingering damage from the oil.

As research got under way, scientists would track hydrocarbons into water, sediments and the food chain, an effort involving scores of projects and thousands of measurements. They would survey animals in the field, in some cases performing the first reliable population estimates.

Multimillion-dollar investigations were launched into the nature of the Sound's ecosystem, focusing on the forces that influence pink salmon and herring, as well as the relationships between seabirds and the fish on which they forage. When populations of sea otters, river otters, Harlequin ducks and pigeon guillemots failed to recover in formerly oiled areas, a multiyear study began to find out why.

Summarizing current understanding can be daunting. This week, a symposium sponsored by the Trustee Council will present or post 138 scientific papers among 13 categories by more than 200 investigators. Many of their findings later will be challenged, retested or revised.

But several key scientists agree that life in the spill zone has rebounded dramatically over the past decade. Most of the oil is gone, and many formerly damaged species are thought to be approaching good shape.

But not all habitats. And not all species.

"We see a system that is functioning, that is largely healthy, that is certainly recovering from the spill," said Senner, the trustee council science coordinator. "But full recovery has not been achieved."

"There is still some oil out there," added the National Marine Fishery Service's Wright. "I'm sure it's less than 1 percent of the original beach coverage, but it's not hard to find. ... Just this last year, I've heard (scientists on contract with) Exxon say you can't find Exxon Valdez oil out there. Well that's crap. Of course you can."

Still, Wright and other scientists caution, the continuing problems with certain species and habitats must be seen in the context of a functioning ecosystem.

"It's still growing an incredible amount of food, as it was before the oil spill," said Short, who has studied oil's effects on marine life in the Sound for the past 20 years. "The oil spill did not at any time threaten the integrity of that basic food chain. ... But that doesn't mean there aren't problems."

One major scientific issue has been the difficulty of distinguishing changes caused by oil from natural changes that might have occurred anyway, especially among species already in decline like harbor seals or forage fish. The Sound's overall pink salmon harvest plummeted in 1992 and 1993, then partly rebounded in 1994. Triggered by the spill? Biologists doubt it, saying only a fraction of the Sound's pinks came into contact with oil. Food supply, population dynamics and ocean temperatures almost certainly played the dominant role.

Given such a network of influences, identifying which forces might help recovery becomes even harder. Defining recovery takes on philosophical elements.

The Trustee restoration plan states that "full ecological recovery" means healthy populations of plants and animals can be found at levels that would have been there if the spill had not happened. Because gauging that is so difficult, the trustees sometimes use pre-spill numbers as a standard, Senner explained.

"The real benchmark here is not "Are things where they used to be?' but "Are things where they would have been without an oil spill?' " Senner said.

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'There's nothing else in life or in nature that is going to return to a condition of what it was in the past.'

Exxon maintains that the Trustee Council doesn't fully acknowledged the sheer complexity of the Sound's system and the natural tendency to change on its own. As a result, Sprow said, the council's notion of recovery is "flawed" and simplistic.

"I just tend to throw up my hands at (the goal of) returning to 1989 conditions," he said. "There's nothing else in life or in nature that is going to return to a condition of what it was in the past."

Exxon's portrays its definition of recovery as broader, with little emphasis or reference to pre-spill conditions.

"We would see recovery taking place when a healthy biological community has been restored, one that is characteristic of what one would expect in the area," Sprow said.

By that definition, species like pink salmon ought to be considered recovered, Sprow argued. "There have been (several) record harvests of pink salmon since the spill."

Likewise, the concern over scattered remnants of oil is misguided. "There certainly are some scattered locations where there is some residual oil," he said. But that oil is inert and harmless, he added.

"The signal from the spill is basically undetectable," added Page, the Bowdoin College chemist and key investigator in several Exxon-sponsored studies.

State, federal and Trustee-sponsored scientists disagree with Exxon's positions over these issues and others. Bitter arguments have erupted in academic journals over the methods used to estimate total bird mortality as well as the sources of background hydrocarbons in the Sound's sediments.

This last issue becomes especially important when scientists try to track down the origin of current oil exposure in animal populations. Is the Exxon Valdez spill the Sound's only important source of oil pollution - or do others exist?

For years, some Exxon-sponsored researchers have asserted that boats and human activities have introduced significant amounts of hydrocarbons into the environment. Several studies also argue that hydrocarbons from well-known oil seeps along the Gulf of Alaska have been showing up in the Sound's deep sediments for thousands of years, suggesting that its marine life has evolved with hydrocarbons present and can therefore cope with them.

But several federal scientists dismiss these findings as incorrect and self-serving. Years of experiments using mussels - each of which filters tons of seawater every month and accumulates trace pollutants in its tissues - show the Sound's waters have been virtually free of hydrocarbons except for the first years after the spill or near heavily oiled sites, according to Short and other NOAA scientists. And in a study released this winter, NOAA and USGS scientists say that the source of native hydrocarbons in the Sound's deep sediments is almost certainly biologically inert coal dust from the Bering River coal fields and not seeps from Katalla and Cape Yakataga along the Gulf Coast.

Page maintains that Short's conclusions are wrong - that the seeps produce enough oil and that they match the background hydrocarbons. But the whole argument is "a very academic and nit-picky thing," he added.

"The important thing all along is that (background hydrocarbons) weren't from the spill - they were from natural sources. They don't like to admit that oil is a natural product. It is part of the Alaskan maritime environment."

The reply from NOAA? "I am personally convinced that what's on the bottom of Prince William Sound is coal," Short said. "Those seeps are negligible."

"I can't believe that Exxon and Exxon contractors won't back off on this," added Wright, who coordinates NOAA's oil spill research. "It's really important to them to be able to say that Prince William Sound was already contaminated with oil, and the spill just added a little bit more."

In any case, another line of research shows the danger of making assumptions about sources, even in a place as pristine as the Sound. USGS scientists have analyzed 61 samples of weathered tar balls found in the Sound after the spill and identified a startling source - California-based oil possibly spilled in Valdez or Whittier during the 1964 earthquake. In a study published in 1995, geochemist Keith Kvenholden and seven co-authors said that these "Non-Exxon Valdez tar balls are widely dispersed throughout the northern and western parts of Prince William Sound, over an area larger than that in which Exxon Valdez residues can still be found."

From one perspective, the spill is the story of a war followed by an uneasy peace. If the spreading slick could be likened to an invading army, then the defenders might be an irregular alliance of weather, water, natural chemistry and the surf of winter storms. Sometimes helping, sometimes hindering was the frenetic scramble of the $2 billion human-sponsored cleanup paid for by Exxon - people with shovels and skimmers, hot water hoses and chemical sprays. Tracking the progress has been an armada of scientists and observers.

Pink salmon would become one of the keys.

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'After you perform your experiment, (pink salmon) all swim back to you and tell you how they did.

In the aftermath of the United States' worst marine oil spill, biologists and fishermen feared that some wild runs of pink salmon might suffer. And with good reason. Within a couple weeks, oil had washed ashore on beaches containing almost a third of the spawning streams among the Sound's southwestern islands.

Fed by the region's ample rain and snow, the often unnamed creeks rush from lush forests to meet the ocean in tiny deltas inundated twice each day by the tide. It's there - in the fertile shallows shared by stream and sea - that pink salmon mature during the first half of their two-year lives. And it was along this rich intertidal zone (though not in the stream beds) that the heaviest slicks made landfall.

As oil piled up - 18 inches deep on rocky beaches like Sleepy Bay of Latouche Island - millions of pink salmon fry were about to wiggle from gravel to emerge into water laced with hydrocarbon runoff. That fall, another population of mature pinks laid eggs in streams that cut through oiled beaches.

What would happen to those pink salmon?

Similar questions were being asked about scores of species and habitats throughout the region, mobilizing what might be the most intense biological field investigation in history. But pink salmon - which return to spawn at two-year intervals in distinct genetic identities - were critical to understanding the spill's effects.

"People have asked me if pink salmon are the 'canary' of Prince William Sound," said Short. "They're not the 'canary' so much as the 'white rat.' " And in some ways, pinks are even better than the standard laboratory rat, Short said.

"After you perform your experiment, they all swim back to you and tell you how they did. And they come back at the same time and at the same age."

Field surveys had found that salmon embryos in 10 oiled streams died at greater rates than eggs in 15 unoiled streams between 1989 and 1993. Studies found evidence of continuing exposure to hydrocarbons in young fish. Especially significant were failures among eggs in gravel above the spill's "bathtub ring" where they would have had no direct exposure to oil.

The continuing mortality through 1993 suggested a couple of possibilities, wrote state biologist Brian Bue and two other authors in an academic article. Either exposure to oil in 1989 and 1990 caused genetic damage that then was passed on for two and three generations, or the oil remained toxic enough to cause physiological damage to individual fish through 1991 and reduce the health of eggs they laid two years later.

Exxon-sponsored scientists have since criticized these results as misleading. Brannon, the salmon biologist from Idaho, argued that the oiled streams were oriented so that their eggs would be collected at a more vulnerable point in their life cycle than in unoiled streams.

"My concern is they're reporting mortalities not from the effect of the oil but from the effect of their sampling," Brannon said.

The results raised several questions. How could such small concentrations of oil cause death in salmon eggs on down to the third generation after exposure? How could eggs that had no direct exposure to oil be dying?

In response to the field studies, Auke Bay Lab launched a series of experiments, involving 75,000 salmon fry reared in oil-coated gravel and released to the ocean. Over several years, they repeated the study three times and tested herring, all with similar results, according to Heintz.

"We found bad things happening to salmon and herring at 1 part per billion of PAH," Short said. "At 19 ppb, we found you would kill half of the exposed population."

That level of toxicity was particularly alarming, Short said. Earlier studies had found that it took concentrations of 1 part per million to kill half the exposed population. But those studies - performed in the 1970s by Auke Bay Lab - had measured what lighter and more volatile components of oil did to fish over a relatively short period.

But the new experiments focused on different factors. They measured how the heavier, long-lived PAHs gradually accumulated in the fish eggs, leading to damage and mortality over the life of the fish, Short and Heintz explained.

Then, in 1995, further field studies found PAH in sediments near the salmon streams. "We found there was still sufficient oil out there in these stream deltas to account for the mortality," Heintz said.

The death rate again increased in these oiled streams in 1997 - eight years after the spill. They included the creek at Sleepy Bay, which had been cleaned again by crews only that summer. As a result, pink salmon - tentatively listed this year as "recovered" by the Exxon Valdez Oil Spill Trustee Council - will now be listed among species that are still healing.

Brannon said he objects to the decision - both because he believes the original field studies are flawed and because the Sound's overall pink salmon population was never significantly damaged.

"The spill had no detectable effect on pink salmon," he said. "No recovery was necessary because there was no real loss that they had to recover from."

Biologists working for the Trustee Council agree that the spill interfered with only a fraction of the Sound's overall pink salmon run. But that doesn't mean certain wild runs - each with unique genetic heritage - weren't damaged and still need to recover.

"I don't believe there's a widespread population problem, but I think there are some real indicators that there's this "land mine' thing out there," said Bue, chief investigator of the pink salmon study. "(The Auke Bay) laboratory experiment suggests there's a real problem out there."

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'... it's important that people know there's oil still out there.'

If salmon still face oil "land mines," what about other animals in other locations? Unearthing the remaining oil to answer the question involves looking beyond the veneer of health on the surface.

"I think it's important that people know there's oil still out there," said biologist Pat Harris, who has been performing surveys in the Sound since the spill. "But it may not be visible, it may not be in a picture that Exxon would show you of a beach. It may be in stranded, isolated pockets."

But once identified, the remnants of crude have been surprisingly persistent and difficult to disarm, according to scientists.

Marine ecologist Irvine surveyed oiled sites along the Kenai Fjords and Katmai national parks throughout the early 1990s. As the National Parks Service's primary oil spill scientist, Irvine helped perform beach surveys and analyzed recovery methods.

Now working as a USGS biologist, Irvine and co-workers visited six sites in 1994 that had been studied off and on since the spill. What they found at five of the sites surprised Irvine - globs of a water-oil emulsion called mousse trapped behind boulders. When disturbed, it looked and smelled fresh.

"Five years after the spill, that oil was chemically like 11-day-old oil," she said. "To me that's pretty phenomenal, that at a distance of 500 kilometers and five years after the spill, you have oil (in that condition)."

In an article later published in Marine Pollution Bulletin, Irvine, Daniel Mann and Short pointed out that scientists had once assumed surf and storms would remove oiling from such exposed beaches. Instead, the mousse had been protected from breakup by the boulders, then preserved chemically by its own external film.

"The transport of mousse may allow for the long-distance dispersal of less weathered, and hence, still toxic oil," they concluded. If disturbed, this oil still has the potential to harm life.

The findings suggest several lessons, according to Irvine. First, people shouldn't assume "high-energy" rocky beaches - exposed to high surf and storms - are going to clean themselves, as they believed in the first year or two after the spill. Second, because the mousse has the capacity to preserve its contents from chemical degradation, the water-oil emulsion may be a kind of oil pollution "land mine," far more dangerous than once thought.

Still, Irvine said, she suspected these isolated sites pose only a "slight" biological threat. But their existence raises questions about how much unknown oil might remain in sediments.

Some 200 miles to the northeast, along the shores of Knight Island in Prince William Sound, the lesson may be even more dramatic.

Biologist David Sale, who worked on some of the original damage assessments in 1989, returned to the spill zone last summer as a guide and teacher. He escorted a photographer from National Geographic magazine to several places that had experienced some of the worst oiling.

"I hadn't been back to these spots for a number of years," Sale said. "I expected to find oil - and I did."

In the so-called "Death Marsh" - a high-tide lagoon in the Bay of Isles that was never treated - Sale and his companions easily found evidence of the original spill.

"It's basically saturating the peat," he said. "Turn over a rock, and there's a rainbow sheen."

Sale said he wasn't surprised. After all, Death Marsh was a lagoon, with little wave action and no cleanup. But his next stop was Rua Cove - a rocky beach exposed to the full brunt of the Sound's northeast gales. Gaping right into the spreading slick, Rua Cove had been hit bad in 1989. As he expected, Sale found weathered tar and asphalt among the rocks. But then, as Irvine had discovered out on the Alaska Peninsula, he uncovered significant amounts of fresh mousse in the "wave shadows" behind the large boulders.

What does it mean?

"Even now, it's a difficult question, the 'so what' question," Sale said. "Because you turn around and look the other direction down the beach, and it's verdant.

"The short answer is that the Sound is recovering, but there still seems to be some long-term impacts," he said.

About 20 miles farther down Montague Strait, Sleepy Bay forms the concave northern lobe of Latouche Island, yawning into the path of the 1989 oil slick like a mouth. In the weeks following the spill, Sleepy Bay swallowed thousands of gallons.

"By late May of that year, the shore was as black as outer space," wrote Anchorage assemblyman Charles Wohlforth when he was then a Daily News reporter. "Every rock and pebble was thickly coated with crude oil so thickly that the petroleum odor was sharp hundreds of yards offshore."

When then-Gov. Steve Cowper visited as part of the one-year anniversary in March of 1990, he observed pools of liquid oil and tar in the gravel. "If this was in West Texas, you could sell it," he said.

Sleepy Bay became ground zero for some of the most aggressive clean-up. Crews worked its beaches. Portions of its salmon stream were oiled, then excavated and replaced with fresh gravel. It contained four of the eight beaches cleaned again in 1997, using a citric-acid-based chemical solvent called PES-51R in an experimental treatment.

Harris and Christine Brodersen were on the team of scientists who sampled more than 50 sites in Sleepy Bay and two other bays before, during and after the latest clean-up work. In the spring of 1998 - after a winter's worth of storms - they returned for a final look.

In general, they said, the clean-up project appeared to have fulfilled its general goal - removal of about half of the remaining oil. Yet there remained trouble spots.

"You have to know where to look for it," Brodersen explained. "You could walk all over that beach and you would just think there was dirt beneath these rocks and you wouldn't realize what you were looking at. But if you would just stick the toe of your boot in it or a stick into it, the oil is there."

Where car-sized boulders had rolled around over the winter - in some cases covering up their survey sites - fresh oil was exposed to the surface. The phenomenon suggested a disturbing possibility, the scientists said.

"It indicated that no matter how much you clean it up, the next year, you're going to have more oil," Brodersen said.

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'...a lot of money has been spent on these studies and we still don't know the answer.'

Does this remaining oil continue to harm marine life? Or is it too limited and too isolated to matter?

Sprow and Exxon-sponsored scientists insist that the remaining oil does not hurt the Sound's marine life to a significant degree.

"Oil is a naturally occurring material," Sprow said.

Federal and state scientists report evidence of disturbing connections. Yet conclusive proof of the link between Exxon Valdez oil and damage to specific animals is elusive.

Studies of herring, mussels and sea otters illustrate the problem.

"I put a lot of emphasis on herring because it is so important," Senner said. Full recovery of the species in the Sound might be a good indication of overall recovery.

A key species in the ecosystem's food web, herring were on the brink of spring spawning when the oil swept through in 1989. Although herring continued to come back in healthy numbers over the next three seasons, Senner explained, "those are fish that were somewhere else when the oil hit the beaches."

Then, in 1993 and 1994, when the 1989 generation should have arrived in force to spawn, the population crashed. Many survivors returned with lesions and deformities, suffering from viral and fungal diseases.

Later laboratory experiments showed that exposure to oil can stress the fish, which can then develop the same diseases, Senner said. The recent research into the toxicity of PAHs further showed that extremely low concentrations can harm herring too.

But these findings still don't prove that the oil spill caused the herring to crash, Senner said. They only show correlations. Other environmental stresses - lack of food, changes in temperatures - may be factors too.

"We've tried to be very careful to not say that, unequivocally, that the oil spill caused the disease outbreak or the diseases," Senner said. "(But) that's an obvious possibility."

As much as any species in the Sound, mats of blue mussels sustained heavy oiling when the slick swept ashore. Fearing that aggressive clean-up would kill or damage mussels, their intertidal habitats were left largely alone during the clean-up. Scientists also thought that the beds would gradually clean themselves.

But by 1991, it had become apparent that the mussel beds were insulating and protecting oil from the elements. In a study of 88 mussel beds throughout the spill zone in 1992 and 1993, scientists found some of the highest levels of Exxon Valdez oil since the spill itself.

One of the authors, Harris, has continued to survey the mussel beds in Prince William Sound since then, some as small as 20 square meters, others as large as 700 square meters.

"In some cases, we're finding some fairly high levels of contamination," she said. "In general it's gone down since the spill. But there are still some areas that we're thinking are hot."

And yet - what are the effects? Certainly the mussel beds, by shielding oily sediments from the natural forces of waves and weather, preserve these pockets of sediment containing oil - "land mines" that could release still toxic oil into the water if disturbed. But what about the mussels themselves?

"I don't think we can say that oiling has been proven to have any effect on the beds," Harris said. "Mussels are just really tough animals. ... It's a very dynamic system and there are changes in the population that have very little to do with oiling or nonoiling."

Other creatures - like sea otters - may eat mussels that live over oiled sediments. Do the mussels introduce Exxon Valdez oil into the food chain? Studies aimed at measuring oil levels in mussel beds don't directly answer the question.

"We're not linking to impacted species - that's always been the weak part of the mussel bed project," Harris said. "And it hasn't become any clearer since the spill. ... They've found evidence of exposure in the animals, but we can't positively say that the pathway is through those mussels."

Farther up the food chain, biologists studying sea otters say the connections are just as hard to prove.

The oil spill was especially damaging to sea otters - more than 1,000 carcasses were recovered after the spill. Biologists later estimated that as many 5,500 sea otters may have died from exposure to oil. The specter of dead and oiled sea otters - gregarious and intelligent creatures that appeal to people - came to symbolize the spill's capacity to damage.

In the years after the spill, sea otters rebounded in much of the Sound and Gulf. But the recovery didn't include certain heavily oiled zones, especially bays along northern Knight Island.

Worried that certain populations of Harlequin ducks, pigeon guillemots and river otters also were suppressed by ongoing exposure to oil, the Trustee Council four years ago launched a multiyear study called the Nearshore Vertebrate Project. Among other angles, the project monitors otters and their food supplies in several oiled bays on Knight Island and compares them to unoiled bays on Montague Island.

Some results show disturbing correlations between otter populations and proximity to the remaining oil, according to biologist Jim Bodkin, who's studied sea otters for years.

"We're seeing a significant increase in the abundance of sea otters in most of western Prince William Sound, but we're not seeing any increase in areas of Knight Island that was most heavily oiled and mortality was up to 90 percent," Bodkin said. "There's evidence that there are fewer animals there now than in '89."

Blood samples of the otters in the study area also have contained a form of Cytochrome P450 - an enzyme that appears in animals that have ingested hydrocarbons. In theory, animals metabolizing hydrocarbons don't have as much energy to spend on gathering food, Bodkin explained. That can weaken them, lowering their chance of survival.

But those observations don't prove that remaining Exxon Valdez oil is continuing to damage sea otters around Knight Island, he said.

It's possible that other sources of hydrocarbons in the study area are damaging the animals, Bodkin said. "I'm not saying it's likely - I think it's unlikely - but you can't discount those concerns."

In the end, scientists say, the connections between the spill and its effect on the Sound may always be circumstantial.

"It's something we live with," said Trustee science coordinator Senner. "And it's frustrating to the public to know that a lot of money has been spent on these studies and we still don't know the answer. ... All we can do is tell people what we know and do it with integrity."

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"How many times do they put a stop light in before you kill two or three people?'

But the knowledge recovered from Prince William Sound could arm science and industry against any future spill.

"It's been an incredible opportunity," said the government fishery service's Wright. "It would have taken us 30 to 40 years to do all this research."

One line of research tracked a naturally occurring hydrocarbon in mussel beds called pristane. The nontoxic substance is produced by tiny animals called copepods, which are consumed by pink salmon fry. Pristane in pink salmon excrement is then absorbed by mussels. Scientists are testing whether pristane levels could offer clues to how many pink salmon will return.

"The fact that pristane is even a possible tool for monitoring pink salmon is a new discovery, and a very creative one," Senner said.

The Trustee Council has produced a list of 38 other "practical" developments from oil spill research. They range from advanced techniques for identifying the chemical "fingerprint" of crude oil to using inner ear bones to track hatchery raised pink salmon in the ocean.

Exxon also plans to continue performing research in the Sound, Sprow said. In particular, the corporation's scientists want to investigate the new findings on pink salmon and toxic effects of PAHs. "I think we're going to be very interested in helping to work to resolve some of these issues," Sprow said.

In the perfect world, such research would have been done in advance. But in the real world of limited funding, that's not how it works, Wright said.

"How many times do they put a stop light in before you kill two or three people?" Wright said.

"The longer legacy of this whole process is by studying the spill effects as closely as we have, we've actually learned a tremendous amount about how the ecosystem works," said Trustee Council chief scientist Spies.

* Reporter Doug O'Harra can be reached at do'harra@adn.com

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