A combination of factors probably led to this year’s rash of dolphin deaths, although plumes of cold water that entered Mobile Bay in January and February may have delivered the killing blow, according to one of the Gulf’s foremost dolphin scientists.
Graham Worthy, a University of Central Florida researcher who ran the state of Texas’ dolphin research program for a decade, said it may be impossible to pinpoint the exact cause of death for the 90 animals that have been recovered this year.
But, Worthy said, basic tests looking at the fat composition of the animals’ blubber layer and the condition of their skin, would reveal much about the overall fitness of the creatures before they died.
“I suspect what we are seeing is several things coming together to form a perfect storm,” Worthy said. “The cold was a very unusual circumstance, but I think we may also be seeing an indirect effect stemming from the BP oil spill, from the way it may have disrupted the food chain.”
The Gulfport-based Institute of Marine Mammal Studies reported only one new animal found in either Alabama or Mississippi over the weekend. About half of the 90 dead dolphins found so far were calves.
Federal officials said they would be supplying Worthy with tissue samples from the dead dolphins and relying on his laboratory analysis as they try to determine what killed the animals. It is unclear when Worthy and other scientists will receive tissue samples so that they can begin their research.
Temperature and river flow gauges show that melting snow flowing into the rivers that drain into Mobile Bay caused a sudden and sharp drop in water temperatures shortly before dolphin carcasses began washing ashore, first in Alabama, then Mississippi.
Scientists from the Dauphin Island Sea Lab have speculated that one particular 15-degree drop in water temperatures within a 24-hour period may have killed many of the dolphins.
“Wow. That’s a significant drop,” said Blair Mase, the senior National Oceanic and Atmospheric Administration scientist investigating the dolphin deaths. She called the relationship between the cold plumes and the dolphin deaths “interesting” and said it was among the factors researchers would probe.
However, Mase said, the region has experienced cold winters in the past, such as last year, without an associated rise in dolphin deaths.
Worthy said the key this year might be in the way water temperatures may have worked in concert with other factors.
“In terms of the absolute temperature, would that be enough to kill an otherwise healthy, adult bottlenose dolphin? Probably not in and of itself,” Worthy said.
“Basically, had the dolphins been fully healthy, I don’t think we would have seen the mortality. The critical part of this picture is what condition were the animals in before the cold?”
Much of Worthy’s prior research has centered on the analysis of both blubber and skin tissue in marine mammals. He said the blubber layer of dolphins changes during the year, with a much higher fat content in winter compared to summer.
“The winter fat content, think of it like putting high-quality foam insulation in your house. That will keep you much warmer than if you put crumpled newspaper in your walls,” Worthy said, explaining that in the summer, dolphins let the fat level drop by about half to help them shed heat.
He said that disruptions in the summer and fall feeding routine this year might have left the animals with insufficient fat layers. Among the possible disruptions, Worthy said, were impacts related to the BP spill.
“A lot of the things these dolphins feed on spawn offshore. Were they exposed to oil? Were parts of their normal diet unavailable?” Graham asked. “If you’ve got a dolphin that is feeding in the fall on poorer quality prey, it hasn’t had time to develop that blubber it needs for the winter. If it gets cold, they may not be able to survive.”
Worthy and Mase both said closing Gulf waters to fishing during the spill may also have played a role. With shrimp boats and commercial and recreational fishermen off the water, two of the primary food sources for local dolphins would have been removed.
For instance, in Mobile Bay, pods of dolphins follow closely behind shrimp boats, both as they drag their nets and as they cull their catch. Ultimately, each boat amounts to a free seafood buffet for any dolphins in the area. Likewise, federal web sites document the growing problem of dolphins stealing fish from rod and reel fishermen, either while they are being reeled in or just after they are released.
Worthy speculated that such a shift will ultimately be revealed.
“It makes sense. Any kind of prey shift will impact the animals,” Mase said of the fishing closure. “That’s why Graham’s work is so important. We want to know if there was a prey shift.”