GREEN

The Northern Australian Die-Off That's Going To Bite Us Where It Hurts

If you love barramundi, you have to love mangroves.

14/03/2017 2:45 PM AEDT | Updated 14/03/2017 5:32 PM AEDT

Nobody loves mangroves. They're stinky, they're muddy, they're not particularly nice to look at and really, if a few thousand hectares of mangroves died on a remote part of the NT coastline, why should we care?

We'll tell you why. Because mangroves are the breeding ground for some of our favourite local marine species.

Do you like eating mud crab? How about barramundi, mangrove jack (AKA red snapper) and banana prawns?

These are just four of the species whose future likelihood is imperilled due to the mangrove die-off which occurred about a year ago on the coastline of the Gulf of Carpentaria in the Northern Territory and Queensland.

James Cook University

BUT WHY DID THE DIE-OFF OCCUR?

You won't believe this, but the mangroves died of thirst. Now, that might sound like an unlikely thing to happen to a water-dwelling plant, but two forces combined to make that happen.

Firstly, El Nino temporarily altered the winds in the Gulf region. This caused sea levels to drop by about 20 centimetres for long enough to deprive the mangroves of their usual tidal flows. There was also a drought on the land, which meant they were also starved of fresh water.

And as Dr Norm Duke from James Cook University explained, although mangroves live in salt water (because taller plants upstream in the fresh water zone steal their sunlight, forcing their retreat to the coast), they actually prefer fresh water.

"Let's get really frank about it, mangroves don't like salt water. If salt gets too far into any plant, it kills it," he said.

before
after

SO WILL THEY GROW BACK?

Dr Duke is not sure when that will happen.

"These trees [yep, mangroves are trees] can be up to 200 years old and to replace those trees is going to take at least a decade and probably two. So we're going to see some erosion and things like that.

The loss is particularly sad given that the Gulf coastline is considered home to the least altered mangrove systems in the world.

"Very few places on earth and have such a largely pristine unmodified estuary," Duke said.

I'M SAD ABOUT THE MANGROVES. BUT ARE THERE WIDER RAMIFICATIONS?

There are. As we mentioned at the top of this story, this has the potential to impact the habitat of some of our favourite marine species. This may threaten their viability as commercial fish into the future as numbers drop.

Professor Marcus Sheaves is the head of Marine Biology and Aquaculture at James Cook Uni. As he explained, these and many oterh species breed and feed in the nutrient-rich mangrove environment. Take that away and the whole food chain falls apart.

"If you lose that structural complexity, you end up with a mudbank once the dead wood has washed away," he said.

"For example, mangrove jacks eat crabs that live on mangrove leaves that have fallen into the water. There's a direct pathway where the nutrients get turned into fish biomass. You can't mess with mangrove chemistry."

SO WHAT NEXT?

It's taken a year for pictures revealing the full scale of the die-off to be available, and according to Dr Duke, it took several months for the die-off to come to the attention of mangrove tidal wetland specialists and managers.

To help deal with such situations in the future, he wants to see Indigenous rangers and community volunteers trained and equipped to do rigorous and repeated shoreline assessments.

"We cannot afford to be caught out like this again," Duke said. "The Gulf die-back has been a wake-up call for action on shoreline monitoring.

"We urgently need a national shoreline monitoring program. We have the specialists, we have the resources, and we know there is interest and concern among the Australian public."

James Cook University

DARE WE MENTION THE C WORD?

The changing climate is a factor in just about every natural phenomenon on earth these days. But did climate change have anything to do with this natural die-off?

It's inconclusive at this stage. But as Duke reminded us, scientists now know that mangroves, like coral reefs, are highly vulnerable to changes in climate and extreme weather events. The scientific world is now watching this remote part of Australia closely.

Australia's top mangrove men and women will review the situation at a dedicated workshop at next week's Australian Mangrove and Saltmarsh Network annual conference in Hobart, hosted by the University of Tasmania and CSIRO.

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