The largest oil spill on open water to date and other environmental
factors led to the historically high number of dolphin deaths in the
Gulf of Mexico, concludes a two-year scientific study released July 19.
A team of biologists from several Gulf of Mexico institutions and the
University of Central Florida in Orlando published their findings in
the journal PLoS ONE.
For the past two years, scientists have been trying to figure out why
there were a high number of dolphin deaths, part of what's called an
"unusual mortality event" along the northern Gulf of Mexico.
Most troubling to scientists was the exceptionally high number of
young dolphins that made up close to half of the 186 dolphins that
washed ashore from Louisiana to western Florida from January to April
2011. The number of "perinatal" (near birth) dolphins stranded during
this four-month period was six times higher than the average number of
perinatal strandings in the region since 2003 and nearly double the
historical percentage of all strandings.
"Unfortunately it was a 'perfect storm' that led to the dolphin
deaths," said Graham Worthy, a UCF provosts distinguished professor of
biology and co-author of the study. "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."
The cold winter of 2010 was followed by the historic BP Deepwater
Horizon disaster in April 2010, which dumped millions of gallons of oil
into the Gulf of Mexico, likely disrupting the food chain. This was in
the middle of the dolphins' breeding season. A sudden entry of high
volumes of cold freshwater from Mobile Bay in 2011 imposed additional
stress on the ecosystem and specifically on dolphins that were already
in poor body condition.
"When we put the pieces together, it appears that the dolphins were
likely weakened by depleted food resources, bacteria, or other factors
as a result of the 2010 cold winter or oil spill, which made them
susceptible to assault by the high volumes of cold freshwater coming
from land in 2011 and resulted in distinct patterns in when and where
they washed ashore," said Ruth Carmichael, a senior marine scientist at
the Dauphin Island Sea Lab, an assistant professor of Marine Sciences at
the University of South Alabama and the lead author of the study.
The majority of perinatal strandings were centered on the
Mississippi-Alabama coast, adjacent to Mobile Bay, the 4th largest
freshwater drainage in the U.S. The onshore movement of surface currents
during the same period resulted in animals washing ashore along the
stretch of coastline where freshwater discharge was most intense.
Others who contributed to the study include: William M. Graham and
Stephan Howden from the University of Southern Mississippi, Stennis
Space Center and Allen Aven from the Dauphin Island Sea Lab and the
University of South Alabama.
Worthy is the Hubbs Professor of Marine Mammalogy. He received his
PhD in 1986 from the University of Guelph in Canada and then completed
post-doctoral training at the University of California at Santa Cruz,
where he studied elephant seals, bottlenose dolphins and California sea
lions. He spent 11 years as a faculty member in the Department of Marine
Biology at Texas A&M University at Galveston and served as the
State Coordinator for the Texas Marine Mammal Stranding Network.
Worthy and his team at UCF have been studying dolphin populations in the Pensacola and Choctawhatchee bays for years.
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