Five years after the Deepwater Horizon disaster began killing marine life in the Gulf of Mexico, the number of dead dolphins washing ashore has begun at last to taper off, scientists have found.
"The number of mortalities in the region has declined since the peak years of 2010 to 2014," Jenny Litz, a research biologist with the National Marine Fisheries Service, said Friday.
As of Oct. 11, she said, the total number of dolphins and whales that were killed by their exposure to Deepwater Horizon oil was 1,433, with about 87 percent — or 1,246 — being bottlenose dolphins. That works out to about 248 dead dolphins a year when the normal annual average along the gulf is 74.
Some dead and dying dolphins continue to wash ashore, she said, but not in the numbers that plagued the Gulf Coast beaches in prior years. Instead the numbers are "falling back closer to the usual averages."
As a result, scientists are consulting with their panels of experts on whether to declare the die-off that stretched from Texas to Florida officially over. No decision has been made at this point, however. When it is declared over, though, the Deepwater Horizon die-off will go down in history as "the largest, and longest, dolphin mortality event ever in the Gulf of Mexico," Litz said.
Litz emphasized that even if the scientists do declare this "mortality event" to be over, that doesn't mean they will stop investigating the long-term impacts the oil exposure might have had on the survivors.
BP declined to comment on the dolphin die-off.
The disaster began in April 2010 with an explosion that killed 11 crew members. The rig sank 5,000 feet to the bottom of the gulf and began gushing oil.
Because the leak happened so far from the surface, BP could not immediately figure out a way to shut it off, even as the oil created a slick on the surface that was large enough to see from space. The company was unable to close off the flow until July of that year.
As the months passed, oil washed ashore in marshes and on beaches from Louisiana across the gulf, reaching the Florida Panhandle in June.
BP sprayed a record amount of the chemical dispersant Corexit to try to stop the oil from tainting the beaches and marshes on shore. The company even sprayed the dispersant deep underwater as the oil shot out of the broken rig, something that had never been tried before. It created plumes of oil droplets that flowed through the gulf's deep canyons.
The combination of oil and dispersant has been blamed for sickening or killing a variety of marine species, ranging from tiny creatures wiped out wholesale on the ocean floor to shrimp and crabs that suffered deformities, and red snapper, which ended up with skin lesions.
But no species suffered more visibly than the charismatic dolphins and their marine mammal cousins, the whales. They began washing ashore across the gulf, reaching beaches as far east as Apalachicola. Some were premature newborns or stillborns. Some were coated with oil.
Some of the ones that died were suffering from a bacterial infection called Brucella, which scientists believe resulted from the oil suppressing the dolphins' immune system. Others suffered liver and lung damage.
Although no one knows how many dolphins live in the gulf, scientists estimate the number could be about 10,000. There are nine different species of dolphins in the gulf, with the bottlenose being the most common. The gulf is also home to sperm whales, Bryde's whales, killer whales and several other kinds of smaller toothed whales.
Both the dolphins and whales could have been exposed to the oil by inhaling the vapors on the surface, absorbing it through their skin, ingesting it from the water or from the sediment while feeding, or from eating the fish that were suffering from their own oil contamination problems, scientists said.
Now that BP has settled the claims for environmental damage, scientists are trying to come up with plans for restoring the gulf, including somehow coping with the loss of so many dolphins and whales that are so important to the food chain.
Contact Craig Pittman at firstname.lastname@example.org. Follow @craigtimes.