In just two years, more than 600 bottlenose dolphins have washed onto U.S. shores from the Gulf of Mexico, about 95 percent of them already dead. Such "unusual mortality events" have hit Gulf dolphins before, but this one is different. In terms of total deaths, calf deaths and overall duration, it's unprecedented in recorded history.

And it conspicuously parallels another unusual event for the Gulf Coast: the BP Deepwater Horizon oil spill, which also began two years ago.

Scientists still can't definitively link the two disasters, and large numbers of dolphins actually started washing ashore in February 2010, two months before the spill began. But the problem has grown much worse in the two years since, and given its historic severity — especially in Louisiana's Barataria Bay, which was heavily oiled in 2010 — there is concern on the Gulf Coast that oil is at least playing a role.

"The Barataria Bay dolphins have severe health problems that are not showing up in dolphins from the un-oiled area, and have not been seen in previous studies of dolphins from other sites along the Atlantic coast or the Gulf of Mexico," explains a fact sheet from the National Oceanic and Atmospheric Administration, which is studying the die-off. These dolphins "are underweight, have low hormone levels, low blood sugar, and some show signs of liver damage," NOAA reports, adding that their symptoms "are consistent with those seen in other mammals exposed to oil."

Not only do the symptoms raise red flags, but so do the numbers and locations of dying dolphins. From 2002-2009, Louisiana averaged 20 dolphin strandings per year, but then had 139 in 2010, 91 of which occurred after the BP spill began. 2011 was even worse, with 159 strandings statewide — almost eight times the '02-'09 average. Alabama and Mississippi had four and five times the normal stranding rate in 2011, respectively, and 100 cetaceans have already washed up across the Gulf Coast in 2012. Yet strandings are back to normal in Florida, which is farther from the spill site.

"This magnitude of strandings in the northern Gulf is unprecedented," NOAA explains. "Further, there is no evidence that two of the most common causes of previous dolphin die-offs in the Gulf, morbillivirus and marine biotoxins, are the cause of this UME." NOAA does note that 11 stranded dolphins have tested positive for Brucella bacteria, but as veterinary epidemiologist Stephanie Venn-Watson told the New Orleans Times-Picayune last year, that doesn't necessarily rule out oil. "The dolphins may be more susceptible to severe infection because their immune system is not working well," she said, "or the pathogen ... becomes stronger and thus are able to do more damage."

Here's a NOAA bar chart of all marine-mammal strandings along the U.S. Gulf Coast since 2010, compared with the 2002-'09 baseline (note that April 2012 doesn't have a full month's worth of data yet):

And here's a NOAA map of all the 2012 strandings through April 15 (click the image to enlarge, and compare to similar maps for 2011 and 2010):

If these "severely ill" dolphins really were sickened or at least weakened by BP oil, NOAA lists the following ways it could have happened:

  • Inhaling vapors at the water's surface.
  • Ingesting oil from the sediment or water while feeding.
  • Eating whole fish, including internal organs and fluids, such as liver and bile, which can harbor chemical contaminants.
  • Absorption through their skin.

There are nine different dolphin species in the Gulf of Mexico, plus Bryde's whales, sperm whales, orcas and several other types of smaller toothed whales. But while many of these cetaceans gravitate toward deep waters, bottlenose dolphins are the only ones found in all Gulf coastal habitats — NOAA estimates their overall population to be around 10,000. A tenth of those are thought to live in Barataria Bay.

Of course, dolphins and whales aren't the only ones affected by the Gulf oil spill. Eleven people lost their lives when the Deepwater Horizon exploded, and countless more have suffered from the spill's ecological and economic effects. More than 200 million gallons of crude flowed into the Gulf between April and July 2010 — followed by thousands of gallons of chemical dispersants — wreaking havoc with a wide range of sea creatures, including plankton, fish, crabs and coral. A three-year, $112 million research project is now examining how much lasting damage this has done.

And, as Al Jazeera English reported this week, Gulf animals are already showing strange deformities — even those that weren't alive in 2010. Young killifish are growing up with misshapen hearts, for example, while shrimp whose grandmothers were exposed to the oil spill are being born without eyes. See the video report below:

The dolphin deaths have proved especially compelling, though, both because dolphins are a sentinel species and because they're smart, charismatic mammals like us. On top of that, their current plight is worse than anything the Gulf has seen in decades — and it might be even worse than we realize. Matthew Huelsenbeck, a marine scientist with Oceana, tells the Sarasota Herald-Tribune that the 600 dolphins stranded since April 2010 may represent just 2 percent of the total number killed.

NOAA plans to issue a final report on the Barataria Bay dolphins within the next six months, but cautions that "results for other areas will take longer, because new samples are still being collected." Meanwhile, if you're on the Gulf Coast and find any oiled, injured or dead marine animals, NOAA asks that you call one of these numbers:

Dolphins and whales

1-877-WHALEHELP (1-877-942-5343)


Florida: 1-888-404-FWCC (1-888-404-3922)

Louisiana, Mississippi and Alabama: 1-904-731-3079

Sea turtles

Texas: 1-866-TURTLE5 (1-866-887-8535)

Louisiana: 1-337-962-7092

Mississippi: 1-888-SOS-DOLPHIN (1-888-767-3657)

Alabama: 1-866-SEA-TURT (1-866-732-8878)

Florida: 1-888-404-FWCC (1-888-404-3922)

Also on MNN:

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

What's killing the Gulf of Mexico's dolphins?
Two years after the BP oil spill, the Gulf is still suffering a historic dolphin die-off. Experts say oil is a likely culprit, but evidence remains elusive.