Deepwater Horizon oil spill may have played role in dolphin deaths

mongabay.com
July 22, 2012

Researcher with dead dolphin calf. Photo courtesy of the University of Central Florida.

In the first four months of 2011, 186 bottlenose dolphins (Tursiops truncatus) were found dead in the Gulf of Mexico, nearly half of them dolphin calves many of whom were perinatal, or near birth. Researchers now believe a number of factors may have killed the animals. Writing in the open-access journal PLoS ONE, scientists theorize that the dolphins died a sudden influx of freshwater from snowmelt after being stressed and weakened by an abnormally cold winter and the impacts of the BP oil spill.

According to researchers, oil leaking from the BP-leased Deepwater Horizon could have decimated the dolphin's prey base, leaving a larger than usual number of dolphins suffering from malnutrition.

"Declines in planktivorous fishes over the shelf in summer and fall 2010 and evidence of genetic and physiological impairment of nearshore fishes support the hypothesis that bottlenose dolphins' forage base may have been reduced," the scientists write.

An harsh winter along the Gulf likely worsened matters. Then came high volumes of freshwater snowmelt into the Gulf of Mexico, which was the last straw for many dolphins and their calves, according to the paper.

"Unfortunately it was a 'perfect storm' that led to the dolphin deaths," explains co-author Graham Worthy in a press release. "The oil spill and cold winter of 2010 had already put significant stress on their food resources, resulting in poor body condition and depressed immune response. It appears the high volumes of cold freshwater coming from snowmelt water that pushed through Mobile Bay and Mississippi Sound in 2011 was the final blow."

However, the study argues that the dolphins would likely have survived the freshwater snow melt influx, if they weren't already stressed and in poor condition. The question remains: just how responsible was the oil spill for the dolphins' deteriorated health?

Bottlenose dolphins are listed as Least Concern by the IUCN Red List with a global population of over 600,000.

CITATION: Carmichael RH, Graham WM, Aven A, Worthy G, Howden S (2012) Were Multiple Stressors a ‘Perfect Storm’ for Northern Gulf of Mexico Bottlenose Dolphins (Tursiops truncatus) in 2011? PLoS ONE 7(7): e41155. doi:10.1371/journal.pone.0041155.

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