Hundreds of dugongs and thousands of turtles will likely starve to death in coming months after flood waters smothered Queensland’s seagrass meadows with sediment.
Col Limpus, the chief scientist for wildlife and threatened species in Queensland’s environmental department, is so familiar with the pattern of death that follows big floods he can provide a timeline of what is in store.
“We can guarantee that in the months that follow we’re going to see an elevation in mortality strandings,” he said.
“It’ll start to be obvious in about April and it will build to a peak that we’ll probably see in about September [and] October.
“I expect we will jump from tens of dugong a year dying, stranding along the east Queensland coast, to hundreds. I’m expecting our stranding of green turtles will jump from hundreds to the low thousands.”
That’s what happened after Queensland’s devastating 2010-11 floods.
It also happened in 1991, when two big floods about a month apart saw the Mary River dump vast amounts of fresh, sediment-laden water into Hervey Bay, killing about 1000 sq km of sub-tidal seagrass.
“That had a major impact on the dugong population and on the turtle population. We had the highest ever recorded mortality of dugong associated with that event.”
Those that survived moved out and it took several years for the habitat to recover enough for herds to return.
“The calving rate dropped from 20% of the big dugong with calves, to 2%. These sorts of things have big impacts.”
Limpus said dugongs and turtles typically carry a lot of fat and that’s why it takes a while for them to wither.
“You can take away the food supply now – as I expect is happening with these floods – but the strandings won’t start increasing until they use up those fat reserves.
“Then you start to get your old animals or the ones that are under stress – mums with calves that are feeding and so on – start to be impacted weeks, months later.”
Helene Marsh is an emeritus professor at James Cook University. She shared Limpus’ concerns and said dugong deaths would be very likely in the Hervey Bay area.
However, she said in previous years, when seagrass dieback had led to such mortality events, there had been a combination of both serious flooding and a cyclone.
“I’m very worried but I’m just a little less certain,” she said. “The difference this year is that the largest impact has been from flooding – rather than flooding and a cyclone.”
Marsh said in the short term it would be important to have large-scale monitoring of seagrasses in flood affected areas so that any declines could be tracked.
Limpus says environmental managers and scientists are in new territory, given the shrinking interval between major flooding events that used to have a probability of happening once every 100 years.
“We had it for the first time in decades with south-east Queensland rivers in 2010-11,” he said. “We had two floods, again a month or two apart. Then we had it again in 2013, and in 2017, and again now.
“At this point, we don’t understand well enough how we go about restoring the habitat but I think that’s a challenge for us now, with the increased frequency of these sorts of things.”