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Gulf spill harmed small fish, studies indicate

Effects vary but dire impacts seen with some very low exposures

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LONG BEACH, Calif. — Two years after BP’s Deepwater Horizon well blowout, laboratory studies are finally offering clues to the spilled oil’s impact on sea life. Brief, very low exposures to oil were capable of killing many fish embryos and hatchlings, new studies show. Those that survived often exhibited major deformities that would diminish an animal’s fitness.

Affected species ranged from the young of large open-ocean denizens, such as tuna, to minnow-sized coastal homebodies — the tiny fish that serve as lunch for everyone bigger. Researchers shared their findings in mid-November during a symposium at the Society of Environmental Toxicology and Chemistry’s annual meeting.

Among oil constituents that most threaten sea life are polycyclic aromatic hydrocarbons, or PAHs. Through a process known as weathering, lighter-weight chemicals evaporate off of fresh oil, rendering what’s left progressively heavier and sludgy. New chemical analyses show that weathering reduces oil’s propensity to shed PAHs into water, finds Damian Shea of North Carolina State University in Raleigh. The bottom line, he concludes: Weathering reduces oil toxicity.

“But in our hands, weathered oil is more toxic,” said Andrew Esbaugh, a fish physiologist at the University of Texas at Austin’s Marine Science Institute who directly tested the effects of weathered oil on fish. Along with a broad team of researchers from several universities and the National Oceanic and Atmospheric Administration in Seattle, he compared the relative toxicity of weathered oil to fresh BP oil in newly hatched, lab-reared Gulf cobia and mahi mahi.

To simulate how waves disperse oil constituents, his team put oil and seawater into a blender and then spun the mix. They measured the amounts and types of PAHs that entered the water in each batch before exposing hatchlings to the mix for 48 to 96 hours.

Half of the baby fish died after exposure to water containing 5 to 10 micrograms per liter of weathered oil and its PAHs. To achieve the same kill rate with water hosting fresh BP oil, Esbaugh reported, it took concentrations four to eight times higher.

Sunlight also boosted the toxic effects, another study found. PAHs enhance the ability of tissues to absorb harmful ultraviolet radiation, explained Aaron Roberts of the University of North Texas in Denton. “We like to think of PAHs as being the anti-sunblock,” he said.

To probe this phototoxicity in egg fertilization, Roberts’ group collected newly spawned eggs and sperm from clean, lab-reared sea animals and then mixed the cells into oiled seawater. For the most sensitive species, blue crabs and mahi mahi, it took just 2 to 6 micrograms of PAHs per liter of water to diminish the eggs’ normal rate of fertilization — at least when solar UV radiation was present. Even after blocking 90 percent of the UV rays, enough still got through to diminish fertilization success beyond the harm caused by oil alone, Roberts said.

In these species, “Photo-enhanced toxicity can account for up to a 20-fold higher sensitivity,” observed coauthor Martin Grosell of the University of Miami in Florida.

Lab-reared juvenile tuna and mahi mahi that survived brief early-life exposures to the oiled seawater often exhibited deformities that could jeopardize their long-term survival, another study found. For instance, many hatchlings exhibited subtle heart abnormalities after trace oil exposures in the egg that lasted only a day or so, Grosell reported. A month later, when his team put these youngsters into the equivalent of an aquatic treadmill, the fish could only swim about 70 percent as fast as those that had developed in clean water.

This is “severely reduced swimming performance,” Grosell noted, and could compromise the ability of wild fish to catch sufficient prey or travel the extremely long distances that highly migratory tuna typically navigate each year. Prompting his team to conduct such tests were data showing that many commercially important deep-water species, especially tunas, typically spawn in the Gulf during the months the Deepwater Horizon well was spewing oil. In previous years, that spawning often occurred directly in the spill zone.  

Finally, a team of researchers reported changes in the activity of a broad range of genes in minnow-length Gulf killifish retrieved from oiled sites on the Gulf Coast. This fish is the most abundant vertebrate in Louisiana’s marshy waters. Neither water at these sites nor the animals’ tissues hosted detectable traces of PAHs, reported Andrew Whitehead of the University of California, Davis. Still, the animals’ genes exhibited a clear fingerprint of exposure to PAHs.

This fingerprint of toxicity, he concluded, “shows these fish have been exposed to biologically relevant concentrations.”

In a related lab experiment, colleague Fernando Galvez of Louisiana State University in Baton Rouge has incubated eggs from clean killifish with clean water combined with sediment from marsh edges tainted by the spill. Many eggs refused to hatch, even though their embryos hadn’t died. Of those that did hatch, many of the fish exhibited heart, spine and other defects. The heart rate of hatchlings raised with heavily oiled sediment was only 60 percent of normal, Galvez’s team found.

Whitehead cautioned against diminishing the importance of these effects as only impacting killifish. The marshland oiled by the spill is a nursery for plenty of commercial fishery species as well, he said. “So what’s happening in the killifish is going to be happening in other animals that share the same habitat.”

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