Climate change is an increasing threat to coastal and estuarine dolphin populations, with extreme weather events disrupting their habitats, potentially culminating in fatal outcomes for dolphins.
Research involving Murdoch University has looked at two Australian dolphin mortality events, in which coastal bottlenose dolphins succumbed to Freshwater Skin Disease (FWSD), due to the detrimental effects of freshwater exposure.
Dr Nahiid Stephens, a lecturer in Veterinary Pathology at Murdoch University, said FWSD is an emerging cause of dolphin morbidity and mortality in many coastal estuarine regions around the world following extreme weather events.
“For the first time, we have been able to fully characterise the lesions and provide a case definition of this disease entity based on two such Australian mortality events,” Dr Stephens said.
“One event occurred in Victoria’s Gippsland Lakes in 2007 and the other in Western Australia’s Swan-Canning Riverpark in 2009.
“While these events are historical, they enabled us to conduct post mortems on the dolphin carcases to identify the cause of their death, characterise the severe skin lesions typical of the disease, and to see if there is a correlation between these events and others around the world.”
Dr Stephens said that in the same year of the Gippsland Lakes outbreak, a similar outbreak was reported for the first time in US waters when common bottlenose dolphins succumbed to what is now believed to have been FWSD in Lake Pontchartrain, Louisiana, following Hurricane Katrina in 2005.
We have found that FWSD occurs when there is a sudden and steep decrease in salinity occurring in mere days, with hypo-saline conditions then persisting for a number of weeks or months thereafter,” Dr Stephens said.
“The salinity reduces as a result of a sudden, significant influx of fresh water, normally following extensive rainfalls, often following a period of relative drought.
“With the two Australian events, the waters inhabited by the dolphins were intensively monitored for physical and chemical parameters before, during and after the events; and when mortalities occurred, thorough and systematic post-mortem examinations were carried out, including investigation and characterisation of the skin lesions with histopathology, electron microscopy, and molecular techniques to rule out infectious causes.
“We were fortunate that the dolphin populations in Australia were well documented by long-term and ongoing field ecology, population, and behaviour studies which provided crucial contextual information. Fortunately, the monitoring continues via Citizen Science projects such as WA's River Guardians and Dolphin Watch programs.
As a result of FWSD, skin lesions appear on the dolphins that progress to areas of ulceration and then opportunistic colonisation by algae, diatoms, fungi and bacteria.
“From this, death may ensue from fluid loss and electrolyte imbalance - because the ulcerative skin lesions are akin to severe third degree burns and often affect a large percentage of the body surface. Secondary infections also play a role.
“We identified that in the Gippsland Lakes, the outbreaks followed resumption of seasonal rainfall following a prolonged drought that flooded what is normally brackish to marine water with fresh water.
“In WA, unusually high winter-spring rainfall in the river catchments similarly turned a normally marine/brackish habitat to freshwater.
Moreover, similar events in the United States have followed the heavy rainfall and storm surges in the aftermaths of Hurricanes Katrina and Harvey.”
“Based on these findings, we are concerned Freshwater Skin Disease is an emerging disease of cetaceans which we are likely to see increasing in frequency in vulnerable estuarine and coastal habitats globally that continue to be affected by worsening climate change,” Dr Stephens concluded.
The research paper is titled ‘Fresh water skin disease in dolphins: a case definition based on pathology and environmental factors in Australia’ published in Nature Scientific Reports. Read the full paper here: https://doi.org/10.1038/s41598-020-78858-2