Unprecedented Death of Hundred of Dolphins After BP Oil Spill

A dead dolphin washed up on shore. Photo courtesy: NRDC

It has now been nearly two full years since the Deepwater Horizon rig exploded in the Gulf of Mexico, loosing a massive geyser of oil into the marine ecosystems. Since then, biologists have been working diligently to understand the full impact that the disaster has wrought on the region's wildlife — even as the spill and its fallout slide further from the public's view.

Of course, much of that fallout still has conservationists deeply concerned. One of the most worrisome phenomena to emerge in the wake of the spill is the mass die-off of hundreds of bottlenose dolphins, which the NRDC calls "unprecedented". Since the spill, over 600 dolphins have stranded themselves in the region hardest hit by oil, and 95% of them have died.

In a fact sheet it just released on the topic, the NRDC elaborates:

Over the last two years, an unusually high number of bottlenose dolphins have beached along the shores of Louisiana, Alabama, Mississippi, and western Florida, raising enormous concern; and, for good reason. Even in a species known to experience mass mortalities — from brevetoxin and disease — the current die-off is unprecedented in its duration and magnitude.

And when they say "unprecedented," they mean it: "Never before have the dolphins in the Gulf experienced a die-off that has 1.) lasted as long, 2.) involved as many animals, or 3.) afflicted as many calves." The die-off has lasted a staggering 25 months now.

A larger than normal amount of breached, or "stranded", dolphins began washing up on the shores around the Gulf even before the spill began. But the mass die-off continued throughout the year, and worsened dramatically in 2011. A primary concern of this die-off—the species is known to be prone to occasional events of this nature — is the unusual number of stillborn calves showing up.

Chart showing rise in dolphin deaths. Image courtesy: NRDC

During the spill, dolphins were spotted swimming in the middle of the slick — they're not well-equipped to detect toxins on surface water. But most of the die-offs didn't take place during the event itself, when the sheens were thickest, "meaning that the vast majority of known mortalities cannot be attributed to the immediate, acute effects of exposure."

Theories abound. # 1: "We know that some coastal areas in the northern Gulf were degraded by the BP spill, and when storms come, toxics that have accumulated in the bottom sediment could reenter the water column, as has been observed decades after the Exxon Valdez disaster." And dolphins don't ditch even highly degraded habitats.

#2: "... dolphins feed high on the food chain, and toxins absorbed through fish consumption can bioaccumulate in their tissue. Some toxic compounds found in oil such as polycyclic aromatic hydrocarbons have been shown to cause birth defects in humans; it is conceivable that they are responsible for some of the recent stillbirths in dolphins."

Or maybe, #3:

"perhaps dolphins’ exposure to oil or dispersants compromised their immune response and left them susceptible to the bacteria. Or perhaps the impact was indirect. Loss of prey can also undermine the immune system and can deplete the energy that mothers need to calve and nourish their babies, resulting in reproductive failure, lower birth weights, and smaller chances of survival."

The NRDC admits that any causal links between the mass die-offs and the BP spill are still largely inconclusive, but maintains that there is now enough evidence to suggest that the oil disaster is probably a contributing factor to the dolphins' calamity. The report makes a strong case for continued vigilance, especially by government agencies tracking fallout from the spill, and for looking carefully for other long-term impacts on ecosystems. After all, it notes, it took decades for the oil from the Exxon Valdez to work its way fully into the food chain, and for the gravest impacts to be made manifest. The same may be true with the BP spill — and on an even larger scale.