Four years after the Deepwater Horizon well blew, coating the Gulf of Mexico with the worst offshore oil spill in U.S. history, University of Miami and federal scientists have determined that the slick drifting across miles of ocean took a toxic toll on one of the region’s most valuable fisheries.
The study found that crude collected from the spill so damaged the hearts of young tuna in a lab that it likely killed them in the wild — or left them terminally slow and unable to hunt.
“This is basically the first study to come out directly assessing the impact of oil collected from the Gulf,” said Martin Grosell, a professor at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, where five researchers teamed up with scientists from the National Oceanic and Atmospheric Administration and Stanford University.
While the scientists can’t yet say for certain what the findings mean for the future of Gulf tuna populations already in decline from overfishing, they know its suggestions are grim.
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“It’s hard to imagine you can significantly injure embryos without damage to the whole population,” Grosell said.
The BP well, which failed in April 2010, spewed more than 4 million barrels of oil before it was capped five months later. Chemicals dumped near the wellhead kept much of the crude from spreading across the gulf’s floor, but left it freely drifting to the surface where “it formed a large and dynamic patchwork of slicks” covering about 6.5 square miles, the researchers said.
The disaster occurred just as bluefin and yellowfin tuna were starting their spring spawning, sending their fragile embryos to grow in warm surface waters. From the beginning, marine biologists and environmentalists worried about the health of the fish, particularly bluefin whose numbers have dropped by as much as 75 percent in the last 40 years. Individual fish, which can weigh hundreds of pounds, are prized in Japanese sushi restaurants and can fetch tens of thousands of dollars or more. But because the fish mature late and live long, they could only speculate about their fate. Juveniles at the time of the spill would take years to reach catchable sizes.
“The spill occurred in 2010 and we’ve been working ever since to establish the effect levels of the crude spilled at the time,” Grosell said.
Bluefin, yellowfin and amberjack are pelagic fish, meaning they are large predators found in open oceans. The Gulf of Mexico is one of only two breeding grounds in the world for bluefin, which are being considered for the endangered species list. It is also an important fishery for yellowfin: commercial fisherman hauled in nearly 50 million pounds in 2010.
Scientists know from studying the 1989 Exxon Valdez spill in Alaska that young fish are particularly vulnerable to oil spills. The herring population has yet to recover in Prince William Sound. Another blow in the Gulf, they concluded, could be fatal for the bluefin population.
Adding to the difficulty for researchers was the scarcity of freshly fertilized tuna embryos.
Finding them in the wild, even in healthy waters, is like “finding a needle in a haystack,” Grosell said. But they finally located specimens of bluefin at a single hatchery in Australia, and yellowfin and amberjack in Panama.
Once in their labs, scientists exposed the embryos to two samples of oil collected during the spill. One came from surface skimming operations and the second was collected from a pipe attached to the wellhead during the effort to shut down the well. Crude oil, Grosell said, is a mix of about 50 chemicals, some of which are known to be toxic to marine animals.
When the team examined the tiny embryos, they found that even low concentrations of oil caused defects, producing watery sacks around their hearts, bad eyes and ill-formed fins. Some of the concentrations were well below those taken from surface waters in the Gulf, researchers said, meaning the young fish in the ocean at the time of the spill likely fared far worse.
BP, in a statement, said that only a small number of samples taken exceeded the low end of concentrations that the researchers tested. Just last week BP, after earning back the right to drill in 2011, returned to the Gulf market by winning drilling rights on two dozen bids at one of the Interior Department’s regular billion-dollar auctions for the area.
“The paper provides no evidence to suggest a population-level impact on tuna, amberjack or other pelagic fish species in the Gulf of Mexico,” spokesman Jason Ryan said in a statement. “The oil concentrations used in these lab experiments were rarely seen in the Gulf during or after the Deepwater Horizon accident.”
But the researchers believe that the findings not only shed light on the Gulf tuna and amberjack, but show the threat oil poses to all fish.
“The heart abnormalities occur in herring, pink salmon and many species exposed,” Grosell said. “So it looks like whether they’re warm water or cold water, salt or fresh, it applies to all fish species,” he said.
The team is also conducting additional studies. While they are incomplete, he said so far none contradict these findings.
“Efforts are ongoing to assess the situation of the fish population and the impact of this loss and the size and what that would do long-term to the fish,” he said. “Are we going to know exactly how many tuna we lost in the Gulf because of this mess? No. Are we going to have a good idea about what it did to the population in general? I would say yes, but that is to come.”