When the Deepwater Horizon oil head blew in April 2010, it caused the largest oil spill in U.S. history, gushing 4.6 million barrels of oil into the Gulf of Mexico. The immediate environmental impacts were obvious, with birds, sea turtles and fish coated in oil and dying. The dolphins living in the areas were also hit hard, with the following year seeing a record number of dead animals, especially calves, washing up on the beaches.
Five years on, and it seems the dolphins are still feeling the effects, as a new report suggests they are facing massive rates of reproductive failure. Published in the Proceedings of the Royal Society B, researchers detailed the reproductive and survival rate of common bottlenose dolphins living in Barataria Bay, Louisiana, an area hit particularly hard by the spill. The study reports that only 20% of pregnant dolphins sampled have produced viable calves since the spill, compared to an average of 83% in an unaffected reference population, and that overall survival rate of the dolphins is down roughly 10%.
A pod of dolphins swimming through oil after the 2010 blowout. NOAA
Since the blowout, researchers have found that dolphins sampled, both dead and alive, show an increase in pathologies related to exposure to petroleum and the toxic substances used to disperse it as a cleanup attempt. They’ve found animals with drastically low weight, low blood sugar, and lesions in the lungs, as well as documenting hypoadrenocorticism – worryingly low levels of the stress hormones responsible for the fight-or-flight response – for the first time in dolphins.
Hypoadrenocorticism is known to be life-threatening for both mother and child in humans, and experiments have shown that when other animals such as mink are exposed to oil, they also show the symptoms of this condition. In addition to this, after the Exxon Valdez oil spill in 1989, sea otters were observed to have high rates of failed pregnancies. It seems that the dolphins in the Gulf could be experiencing something similar, limiting their ability to adapt to stress and possibly leading to states of shock.
At least half the pregnant dolphins sampled also suffered from lung disease, also thought to be related to toxic chemicals found in petroleum. The researchers compared the pathologies and calf survival rate with other populations of dolphins found within the Gulf which were not impacted by the 2010 oil spill, and found that they were unaffected during the same period. Previous research has also found that dolphins from the spill zone were four times more likely to die of infectious diseases than dolphins from elsewhere.
It seems that the effects of the Deepwater Horizon spill are far from over, and only in the coming years will we see what impact lower birth and survival rates will have on the wildlife populations still living in the Gulf.
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