Anna Salleh ABC 15 Apr 13;
Dugongs in one of Australia's largest populations appear to be getting sick and dying as a result of exposure to cold water, say researchers.
If confirmed, the findings may have implications for dugong conservation strategies.
The findings are reported in a recent issue of the journal Diseases of Aquatic Organisms.
Dugongs (Dugong dugon) are a member of a small group of aquatic mammals called sirenia -- or sea cows -- that don't like cold water, says marine epidemiologist, Dr Mark Flint, of the University of Queensland.
In the US, the closely related endangered manatees Trichechus manatus is known to gather around warm water that comes out of power plants when it gets cold.
In the past decade, manatees in Florida have been dying from a condition known as cold stress syndrome (CSS), which occurs when the animal is exposed to water temperatures lower than 17 or 18°C.
The animals' skin becomes cracked and susceptible to opportunistic infections and they can die within a few days, says Flint, who has worked on manatees suffering CSS in Florida.
Animals with chronic CSS can die after several weeks, during which time they lose weight and their body fat atrophies.
"When you cut them open [they're body fat is] just like water instead of being that nice white stuff you expect to see," says Flint.
Now, Flint and colleagues have documented signs of what could be CSS in dugongs, recovered from Moreton Bay in southeast Queensland, which is home to one of the largest populations of the animals in Australia.
"It's exceptionally important because it's a large population. We know there's about 1000 dugongs in Moreton bay," says Flint. "It's probably about the best-studied group of dugongs in the country."
Post mortems
Between 2010 and 2012, the team, which included veterinary pathologist Dr Helen Owen, carried out post mortems on 14 dugongs collected from Moreton Bay by government authorities.
While there were obvious explanations for four of the deaths, ranging from a twisted gut to a blunt trauma likely from a boat, 10 of the dugongs had more mysterious symptoms, says Owen.
"They were emaciated and had increased burdens of parasites, and most had thickened fissured skin," she says.
This looked suspiciously like cold stress syndrome, which Owen says is thought to be due to a suppressed immune system, making the animal prone to opportunistic infections.
The CSS hypothesis was supported by the record of water temperatures in areas where the dugongs are known to graze.
These records showed that during the year, water dropped below 20°C, sometimes to less than 15°C.
There is some evidence that dugongs head out of Moreton Bay into the warmer coastal waters of the East Australian Current.
But, say the researchers, even outside the bay, water could still get below 20°C in the coldest months of the year, and this would prevent dugongs from escaping persistent exposure to cold water and the risk of CSS.
"Based on this study, like Florida manatees, it appears the dugongs of southeast Queensland develop clinical signs consistent with CSS when water temperatures drop below 20°C," write the researchers.
Although they note that while most dugongs died in winter, some of them also died in summer, which could be due to chronic cases of CSS persisting into the warmer water periods.
Seagrass factor
Research shows water temperatures at the level of seagrass, is lower than surface temperatures which means dugongs would be exposing themselves to colder water to feed.
The researchers say habitat disruption could force dugongs to feed on seagrass in deeper and colder water and increase their risk of CSS.
They say maintenance of seagrass pastures in the warmer areas of Moreton Bay during winter may aid in the conservation of this vulnerable species.
Flint says researchers are not sure if CSS a recent phenomenon and is on the increase.
It could have always have been there, but only now be being picked as a result of greater surveillance. Alternatively, CSS could be increasing due to environmental factors, he adds.
