The dolphins in the Gulf of Mexico are in the midst of a massive die-off. The reasons why remain a complicated and mysterious mix of oil, bacteria and the unknown.

Normally an average of 74 dolphins are stranded on the northern shore of the Gulf of Mexico each year, especially during the spring birthing season. But between February 2010 and April 1, 2012, 714 dolphins and other cetaceans have been reported as washed up on the coast from the Louisiana/Texas border through Franklin County, Fla., reported the National Oceanic and Atmospheric Administration. Ninety-five percent of the mammals were dead.

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Since many of the dead dolphins sink, decompose or are eaten by scavengers before washing up, NOAA believes that 714 may represent only a fraction of the actual death count. NOAA declared the die-off an “Unusual Mortality Event” as per the Marine Mammal Protection Act of 1972.

Although the timing of the die-off largely coincides with BP’s Deepwater Horizon oil spill and its aftermath, the deaths actually started increasing about two months before the April 20, 2010, explosion, which started the monthslong oil spill.

Before the spill, 112 dolphins had already been reported stranded on the shore.

In the summer of 2011, NOAA tested 32 live dolphins in Barataria Bay, an area heavily impacted by the oil spill. The dolphins were underweight, anemic, and had low blood sugar and/or some symptoms of liver and lung disease. Nearly half had abnormally low levels of hormones that help with stress response, metabolism and immune function.

The symptoms are consistent with those seen in animals exposed to oil.

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But the mystery remains as to why the dolphins started dying before the spill. One possible culprit is Brucella bacteria. Since an initial find of five infected dolphins, NOAA has found an additional 11 infected dolphins washed up on the Gulf Coast. But dozens more showed no signs of infection.

NOAA’s investigation into the dolphin tragedy continues.

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Photo: Bottlenose dolphin (Tursiops truncatus). A dolphin surfs the wake of a research boat on the Banana River near the Kennedy Space Center. Credit: NASA, Wikimedia Commons.