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updated 4/10/2009 6:33:44 PM ET 2009-04-10T22:33:44

Birds are the most high-profile victims of oceanic oil spills, but fish suffer from these messy accidents, too. Even worse, a new study suggests, the chemicals commonly used to clean up oil spills make oil far more toxic to fish, particularly for eggs and young fish.

Scientists already debate about how best to clean up spills. The new work makes those decisions even more complicated and controversial.

"While you can see the risk on the surface, appreciating risk under the surface is much more difficult," said Peter Hodson, a fish toxicologist at Queen's University in Kingston, Ontario. "You're trading off one set of risks that are fairly clear for another set of risks that are not so clear."

Oil and water don't normally mix. So, when a truck, train, or ship accidentally dumps its cargo into a lake, stream or sea, the oil sits on top of the water and spreads across its surface. The slick substance then flows with the currents and tides, poisoning the animals it encounters along its way.

On the scene of a spill, difficult decisions need to be made quickly. In an ideal world, the rescue team would simply skim all the oil off the surface, said Nancy Kinner, co-director of the Coastal Response Research Center at the University of New Hampshire in Durham.

In reality, even the best equipment leaves a lot of oil behind. What's more, most spills happen during storms, making cleanup dangerous and difficult. According to policy, Kinner said, crews simply won't go out on the water if there is a risk of capsizing.

Another way to get oil off the surface is to use a chemical dispersing agent. These detergent-based substances cause oil to bead up into tiny droplets that can mix into the water and disperse into deeper layers. Underwater currents can then theoretically dilute the oil and its risk to the environment.

Dispersion spares surface-dwelling animals, such as birds and otters. But as oil drifts downward, it falls on fish and on the eggs that are stuck to surfaces or buried in the sediment.

To find out just how dangerous dispersed oil might be to fish, Hodson and colleagues performed a series of laboratory experiments with beakers that were meant to simulate contaminated lakes. In all of the beakers, the scientists mixed water with diesel oil, then added newly hatched trout embryos. In some beakers, the scientists added a dispersing agent.

Their analyses, published in the journal Environmental Toxicology and Chemistry, showed that dispersants greatly increased the amount of hydrocarbons that could affect fish. In turn, that extra dose of exposure made the oil 100 times more toxic to the animals. Toxicity was measured as an elevated enzyme response in the livers of the fish.

Exposure to dispersed oil doesn't kill a lot of fish, Hodson added. Instead, it either kills eggs before they hatch or leads to damage or deformities in juvenile fish. Compared to the horrifying appearance of oil-drenched birds on beaches, it can be hard to catch the attention of the public -- or even of cleanup managers -- with such subtle and hidden health effects.

"What he's saying, and he's correct, is that it could be way more fish fingerlings or eggs that are impacted than you'd ever impact birds," Kinner said. "It kind of adds fuel to the discussion."

Another message of the study, Kinner added, is that, when it comes to accidents that involve oil, there are no easy answers and no happy endings.

"Once the petroleum product is spilled, there is going to be damage no matter what," she said. "It's already a bad situation. The question is: How are we going to minimize the risks as much as possible?"

© 2012 Discovery Channel

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