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Today, we're kicking off a special NPR series marking the 25th anniversary of the disaster. The environmental damage at the time was horrendous, but many scientists thought it would be short lived. Instead, it stretched out for many years.

NPR's Elizabeth Shogren reports that 25 years of research has now led to some startling conclusions about oil's persistent effects.

ELIZABETH SHOGREN, BYLINE: Cordova, Alaska, was the closest fishing village to the spill. Since then, it's become a hub for scientists. Researchers recently gathered in the town's library to talk about herring. It was the herring that tipped off scientists that oil's effects were more complicated than they imagined.

Here's what happened: Herring were spawning at the time of the spill. None of those herring eggs survived. But a year later, the herring population seemed to bounce back.

JEEP RICE: That was the nature of oil spills. You studied them for one or two years. When things were back on track, you walked away.

SHOGREN: Jeep Rice from the National Oceanic and Atmospheric Administration says herring seemed to match that conventional wisdom.

RICE: And so we walked away from herring. And all of a sudden, they crash to the bottom of the floor, and then we're scrambling trying to figure out why.

SHOGREN: In addition to the herring population crashing, other animals also unexpectedly suffered for years: killer whales, sea otters, harlequin ducks. It seemed it didn't take much oil to do a lot of damage, and do it in different ways.

For example, oil under rocks and in sediments contaminated clams that sea otters ate. It didn't kill the otters outright. Dan Esler is with the U.S. Geological Survey. He says it shortened their lives and suppressed the population for 20 years.

DAN ESLER: The understanding that lingering oil could have chronic effects on wildlife populations was a new and important finding, and one that we did not anticipate at the time that we started the research.

SHOGREN: Then there was another unexpected effect, on fish eggs. The clue came from pink salmon, which weren't doing well even years after the spill. To figure that out, NOAA's Jeep Rice's team exposed pink salmon embryos to tiny amounts of oil.

RICE: We were dosing them with oil that you couldn't see, you couldn't smell. But we're doing it for a long time. And six months later, you know, they had abnormalities. And, you know, kind of wow.

SHOGREN: The substances responsible were polycyclic aromatic hydrocarbons that are found in oil. Scientists hadn't realized they're toxic to aquatic life. They discovered these compounds persist long after other parts of oil evaporate. It took years to figure all this out. But still, no one knew how the oil was damaging the animals over the long term. One possibility was fish hearts. It's one of the first things that develops in a fish embryo.

Another NOAA scientist, John Incardona, tried a novel experiment with fish.

JOHN INCARDONA: We put them on a treadmill in essence for fish, a swim tunnel, and tested how fast they could swim for a prolonged period, and the oil exposed ones couldn't swim as fast for long. We found that the shape of their hearts were different.

SHOGREN: OK. So now, they had a good idea what organ was harmed. But they still didn't know what the mechanism was. Then came the spring of 2010 and the big BP oil spill into the Gulf of Mexico. Scientists focused on fish likely to be spawning, such as the blue fin tuna.

Barbara Block is a marine biologist at Stanford University who studies tuna. She feared the BP spill like the Exxon Valdez could have long-term effects.

BARBARA BLOCK: Survivorship of the eggs are critical for future generations, especially on a severely depleted population of the bluefin that breed in the Gulf of Mexico.

SHOGREN: Block collaborated with the NOAA scientists to answer a question that still puzzled them.

BLOCK: What would cause a fish heart to slow down?

SHOGREN: They showed that oil interrupts the electrical signals that are essential for fish hearts to beat effectively. So far, they think this can happen to embryos or young fish.

BLOCK: What we're doing is applying the science of the Exxon Valdez and taking it into 21st century methodologies.

SHOGREN: It's like a coroner pinning down a mysterious cause of death but taking 25 years to do it. And as in a criminal case, this knowledge could give scientists evidence to hold companies responsible for long-term damages no one ever knew oil spills were causing.

Elizabeth Shogren, NPR News.

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