By Roberta Attanasio, IEAM Blog Editor
During the past year, bottlenose dolphins – known sentinels of ocean health – have been dying at an alarming rate along the US East and Gulf Coasts, albeit for apparently different causes. Along the East Coast, a major cause of death is dolphin morbillivirus, a pathogen related to human measles and canine distemper, which is responsible for an epidemic probably fueled by current lack of immunity and perhaps aided by global warming and pollution – factors that may increase the dolphin susceptibility to infection.
The measles-like virus also caused an outbreak in 1987-1988, wiping out 50 percent of the coastal migratory dolphins. During the ongoing epidemic, more than 1,000 dolphins have already succumbed to the infection, although the toll may be higher because of uncounted dolphins that die offshore. The epidemic started in June 2013 and prompted the declaration of an Unusual Mortality Event – an unexpected event that involves a significant die-off and demands immediate response under the Marine Mammal Protection Act of 1972.
Additional causes of concern are the possible spread of dolphin morbillivirus from East Coast to Gulf Coast dolphin populations as well as its spread from dolphins to other marine mammals, such as humpback and pygmy sperm whales.
Meanwhile, on the Gulf Coast, from the Texas-Louisiana border through the Florida Panhandle, more than 1,000 dolphins have succumbed since the first deaths were reported four years ago, before the British Petroleum (BP) Deepwater Horizon oil spill occurred. However, the causes of these deaths, which have also been labeled as Unusual Mortality Events, are not clear.
The Deepwater Horizon spill does contribute to the current dolphin die-off, though, as shown by a study recently published (December 18, 2013) in the journal Environmental Science and Technology and entitled “Health of Common Bottlenose Dolphins (Tursiops truncatus) in Barataria Bay, Louisiana, Following the Deepwater Horizon Oil Spill.”
The research was carried out by a team of government, academic, and non-governmental scientists as part of the Natural Resource Damage Assessment (NRDA), in which response agencies work in cooperation with BP to identify potential injuries to natural resources as well as loss of services resulting from the spill, to develop a restoration plan, and to implement and monitor its effectiveness.
For the study, scientists captured and examined bottlenose dolphins from Barataria Bay, Louisiana (an area that received heavy and prolonged oiling), as well as dolphins from Sarasota Bay, Florida, which represented the reference site (no oil was observed in this area.) All dolphins were released after the examination.
Results show that dolphins from Barataria Bay present evidence of hypoadrenocorticism, which indicates adrenal toxicity – a finding previously reported for laboratory mammals exposed to oil. In addition, Barataria Bay dolphins were 5 times more likely to develop moderate-to-severe lung disease when compared to Sarasota Bay dolphins. In total, 29 dolphins were examined in Barataria Bay – of these, 48% were given a guarded or worse prognosis, and 17% were not expected to survive.
Many disease conditions observed in Barataria Bay dolphins are considered uncommon but are consistent with exposure to petroleum hydrocarbon.
Lori Schwacke, the study’s lead author and an experienced investigator of dolphin health, said in a public release, “I’ve never seen such a high prevalence of very sick animals — and with unusual conditions such as the adrenal hormone abnormalities.” In addition, she told NPR “Many of the Barataria Bay dolphins were underweight. Their blood tests showed abnormalities such as elevated liver enzymes or markers of inflammation. Many were hypoglycemic, which is low glucose or low blood sugar, and several were anemic.”
The scientists examined alternative hypotheses to explain the conditions observed in Barataria Bay dolphins, as for example exposure to other toxic chemicals known to induce adverse health effects in marine mammals. However, blubber samples from the Barataria Bay dolphins contained relatively low concentrations of these toxic chemicals – including polychlorinated biphenyls (PCBs) and pesticides – as compared to other coastal bottlenose dolphin populations.
Erin Fougeres, a marine mammal biologist with the National Oceanic and Atmospheric Administration’s fisheries service, told the New York Times “We can’t conclusively say what role oil played at this time, but the event is pretty unprecedented in terms of how long the die-off has been occurring and how many have died as part of that event.”
Scientists are currently carrying out studies based on continued photographic monitoring – as part of the Deepwater Horizon NRDA – to understand the impact on dolphin reproduction and long-term survival.