In the years since its Deepwater Horizon oil spill befouled huge stretches of the Gulf of Mexico, oil giant BP has honed its skill at cherry-picking scientific studies to duck responsibility for the spill’s environmental impacts.

Its latest effort concerns a study of a massive die-off of bottlenose dolphins in the gulf from 2010 through June 2013, occurring mostly after the April 2010 Deepwater Horizon blowout caused the worst oil spill in history. The peer-reviewed study, led by Stephanie Venn-Watson of the National Marine Mammal Foundation in San Diego, was funded by the National Oceanic and Atmospheric Administration and published last week in the open-access journal PLoS One.

The study analyzes the strandings of 1,305 dolphins on the beaches or shores of the Gulf of Mexico from February 2010 through the present. About 94% of the stranded mammals were dead. This is the longest marine mammal die-off in the gulf -- known under federal law as an “unusual mortality event " or UME -- on record. The 2010 and 2011 figures for Louisiana are the highest annual numbers ever recorded for that state; for Mississippi and Alabama the 2011 figure is among the highest ever in those states.

BP, in a statement on its website responding to the study, asserted that it “reiterates what other experts, such as NOAA, have stated: the UME started three months before the Deepwater Horizon spill, and the cause or causes have not been determined. The study does not show that the accident adversely impacted dolphin populations.” (A slightly different BP statement on the study, making essentially the same points, is here.)


This is a quintessentially selective, and highly misleading, interpretation of the PLoS study. But it’s conventional tactics for BP, which has been desperately minimizing data pointing to serious environmental damage related to the oil spill. Its most flagrant effort in this line was an op-ed by one of its chief PR men, Geoff Morrell, published by Politico in October and headlined “No, BP Didn’t Ruin the Gulf.” (We questioned Politico’s role in providing BP with this priceless PR platform here.)

BP’s defense against the dolphin study requires close examination; we asked BP for a fuller response to the study, but the company referred us to its published statements. So let’s take a look.

BP is correct in observing that the UME started in February 2010, three months before the Deepwater Horizon blowout launched a six-month torrent of oil into the gulf. But that’s more a technical designation of the study period, rather than the identification of a starting point for an event that continued from before the spill to years afterward.

A look at the monthly data on dolphin strandings collected by NOAA researchers reveals how misleading the BP interpretation is. Of eight data-collection sites stretching from the Texas gulf shore to Florida, the dolphin strandings in the three months before the oil spill were below average. (Strandings happen all the time, but only rarely in clusters attracting notice.)


The exceptions were above-average readings in March 2010 at stations in Northern Louisiana and Western Mississippi, in and around Louisiana’s Lake Pontchartrain. But the NOAA team believes those resulted from a separate event involving the incursion of fresh water into the lake, unrelated to the post-spill strandings. “Cold weather and fresh water may have contributed to this stranding cluster,” the report says.

Outside those two sites, all data cited by the report show that the run of above-average strandings started in May 2010, after the spill.

It’s also true, but misleading, that the causes of the stranding and deaths “have not been determined.” Here, BP is taking advantage of scientists’ natural inclination to interpret data cautiously. The researchers acknowledge that many factors may be contributing to the dolphin die-off. But what the oil company doesn’t tell you is that the scientists have pointed the finger of suspicion pretty firmly at oil as a major culprit.

The scientists observe, for example, that the most severe run of strandings -- an above-average rate in 14 of the 16 months from August 2010 through November 2011 -- in Barataria Bay, La., “a coastal area heavily impacted by the spill.” They say the timing and location of the Barataria stranding cluster is “consistent with the spatial and temporal distribution of oil to bay, sound, and estuary habitats in that region during and after the DWH oil spill.”


They add that Barataria Bay dolphins displayed conditions, including lung disease, “consistent with adverse health effects that might be expected following oil exposure based upon the literature of documented effects in other animal species.”

Outside Barataria Bay, the researchers note, “the location, timing, and magnitude of dolphin stranding trends observed following the DWH oil spill, particularly statewide for Louisiana, Mississippi, and Alabama, overlap with the location and magnitude of oil during and the year following spill.”

By comparison, the gulf coasts of Florida and Texas, which “experienced little to no oiling,” also lack “significant annual, statewide increases in stranded dolphins.”

You can call this circumstantial evidence. But it’s a powerful indication of cause and effect.


One of BP’s last defenses is to try linking the dolphin die-off to brucella, an infectious bacteria: “Nearly one-third of the dolphins tested through November 2014 tested positive for brucella,” the oil company says.

NOAA implies that this is a red herring. “While Brucella has been confirmed in many marine mammal species globally,” the agency says, “to date it has not been confirmed as causing epizootics [that is, unexpected increases in disease] or die-offs of multiple age classes in marine mammals, including dolphins.”

Is the new study a smoking gun? No, because scientists tend to avoid ruling out even peripheral causes for complicated environmental events. But hundreds of dolphins are dead, and since BP has been caught in the vicinity wielding what could be the murder weapon, it would be a stretch to assume the company is an innocent bystander.

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