Cold weather, the Deepwater Horizon oil spill and an influx of cold fresh water combined to cause record cetacean mortality
Melissa Gaskill Scientific American 19 Jul 12;
From Nature magazine
Bottlenose dolphins in the northern Gulf of Mexico were hit by a triple whammy of events, leading to an unusually high death rate in early 2011, a paper published in PLoS ONE suggests.
Between January and April 2011, 186 bottlenose dolphins (Tursiops truncatus) washed ashore in the northern Gulf of Mexico. Of these, 86 were near-term or newborn, nearly double the historical average. Dolphin deaths are being monitored by an ongoing Unusual Mortality Event (UME) survey, which began in response to high numbers of adult dolphins dying during a period of sustained cold weather in early 2010.
The PLoS ONE study suggests that the cold weather was the first of three factors that weakened the dolphin population and contributed to the high death rate. The second was the Deepwater Horizon oil spill that followed in April. And the third was large volumes of cold fresh water from melting snow entering Mobile Bay — an inlet in the Gulf of Mexico — in 2011.
Dolphins naturally encounter seasonal temperature and freshwater fluctuations, says lead author Ruth Carmichael, a marine scientist at Dauphin Island Sea Lab in Alabama, and neither factor alone would necessarily cause strandings or death. But the cold freshwater pulses may have been the proverbial straw that broke the camel’s back, if the dolphin population was already weakened due to depleted food resources caused by preceding events, such as oil in the northern Gulf food chain, or bacterial infection.
Between June 2010 and January 2012, 12 of the 51 stranded dolphins tested for Brucella bacteria gave positive results, according to Teri Rowles, coordinator of the National Oceanic and Atmospheric Administration (NOAA) Marine Mammal Health and Stranding Response Program in Silver Spring, Maryland. Brucella, which is present in some healthy animals, can cause pneumonia, encephalitis, skin and bone infections, and abortion in dolphins.
“Studies show that dolphins were in poor condition after Deepwater Horizon and some particularly cold winters, and we know from theNOAA analysis that some had Brucella,” says Carmichael. “For animals already stressed and in poor condition, this freight train of cold fresh water could certainly have affected the timing of mortality.” The cold water pulsed into Mobile Bay during the spring birthing period, and the greatest number of newborn strandings were found close to this area. Since dolphins have a gestation period of 12 months, some of the stranded newborns would have been conceived during the oil spill, which could have affected their ability to survive.
“It’s a common fact that animals in good shape nutritionally are much more able to withstand change and stress,” says marine biologist Moby Solangi, executive director of the Institute for Marine Mammal Studies in Gulfport, Mississippi. “With multiple environmental challenges, we may not be able to say it was one thing or another. We do know that dolphins in the northern Gulf have been subjected to a number of environmental challenges in the past few years, and we do know that each one of those challenges will have affected their ability to deal with the others.”
As is often the case with strandings, few of the dolphins were recovered in good enough condition to determine the cause of death, Rowles says. Nor do scientists have full data on the causes of death from those that have been analyzed. The UME survey is still ongoing, and final data and analysis are typically not available until 18 months after an event.
Carmichael hopes that the study will encourage scientists to consider the physical and chemical environments in which events such as strandings happen, as these could affect the way species respond to stresses.
High Dolphin Deaths in Gulf of Mexico Due to Oil Spill and Other Environmental Factors, Study Finds
ScienceDaily 18 Jul 12;
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.
Journal Reference:
Ruth H. Carmichael, William M. Graham, Allen Aven, Graham Worthy, Stephan Howden. Were Multiple Stressors a ‘Perfect Storm’ for Northern Gulf of Mexico Bottlenose Dolphins (Tursiops truncatus) in 2011? PLoS ONE, 2012; 7 (7): e41155 DOI: 10.1371/journal.pone.0041155
