A report by biologists in Alabama and Mississippi suggests a "perfect storm" -- a discharge of cold freshwater, stress from the BP oil spill and unusual winter conditions -- could play a part in the usually high number of dolphin strandings that occurred in 2011.
Scientists have studied the unusual mortality event along the northern Gulf of Mexico since the National Oceanic and Atmospheric Administration first began tracking it in February 2010.
Dr. Ruth Carmichael, senior marine scientist from the Dauphin Island Sea Lab and assistant professor of marine sciences at the University of South Alabama, joined with biologists from the University of Southern Mississippi and the University of Central Florida to examine the dolphin stranding data.
According to their report, 186 dolphins that washed ashore from Louisiana to western Florida from January to April 2011.
They say the number of calves stranded during that four-month period was six times higher than the average number of newborn strandings in the region since 2003 and nearly double the historical percentage of strandings.
By looking at the data available concerning the condition of the carcasses and where and when the dolphins were found, they have concluded that while the exact cause of death remains undetermined, a contributing factor could be a collision of environmental factors -- the cold water, the unusual winter and the stress from the BP oil spill.
A sudden drop in temperature as snowmelt water pushed through Mobile Bay and Mississippi Sound occurred in the days before discrete peaks in numbers of stranded dolphins, particularly newborn dolphins. "We think that the dolphin population was already distressed and in poor condition when hit with cold freshwater pulses during the perinatal period," Carmichael said.
The pulses of cold water that "came like a freight train," Carmichael said, adding to the stressors already affecting the dolphins. "We know from work by colleagues at NOAA's National Marine Fisheries Service that some adult dolphins were in poor condition after the unusually cold winter and oil spill. Some had bacterial infections," she said. "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 cold or oil spill, which made them susceptible to assault by the high volumes of cold freshwater coming from land and resulted in distinct patterns in when and where they washed ashore."
She said NOAA's investigation is continuing and a direct cause of death has not been established. "At the very least, we know it's related to when and where these animals stranded. We're hoping that we'll just provide part of the puzzle so that once more of the biological date becomes available this will help NOAA and everyone understand the bigger picture," she said.
However, Moby Solangi, executive director of the Institute for Marine Mammal Studies in Gulfport, said it's too early to speculate as to the causes of the dolphins' deaths because toxicology results have not been released.
He said turtles and fish would have been the first affected by the pulses of cold freshwater, and that did not happen.
"This is a different phenomenon," he said. "It has nothing to do with cold water. You would have seen turtles die first. You would have seen fish die first."
He added, "All the animals that died were responded to by us, necropsied by us and checked out by us and the government has all the tissues and will be conducting the results. They are the ones who should be writing this thing rather than somebody who doesn't have that data."
Carmichael, who is the study's lead author, said targeted analyses of tissues from stranded dolphins are essential in assessing a cause of death. However, the research team hopes their findings highlight the importance of considering the environmental data along with biological samples to interpret stranding patterns.
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