Tasmanian wetland rebounds as invasive rice grass nears eradication

For two weeks, at the best time of year, not a single oyster is harvested from these rich waters in Tasmania's far north-west.

Nearby, a week-long mass herbicide spray is taking place to kill off rice grass, an invasive weed.

The chemical has never been detected in oysters here, but, to be safe, farmers hold off harvesting for another seven days.

"An oyster actually processes everything through its system within 48 hours, so we go up to nearly three times that long to allow anything to discharge," Mr Poke said.

Despite the financial hit, the oyster grower of decades is entirely onboard.

"We looked at what had happened in other estuaries, [and] for my own business to survive, we needed to make sure that the environment was looked after," he said.

The fight against the weed in the wetlands began about 15 years ago with an ambitious plan by Circular Head Landcare to not just control it, but wipe it out entirely.

Now an end is almost in sight.

Before: Rice grass in Duck Bay in February 2018.. . After: Duck Bay following years of restoration efforts.. .

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First planted in the 1940s to beautify Smithton's Duck River, the weed has colonised mudflats and saltmarsh across a 120-kilometre stretch from Woolnorth to Stanley.

Left to run rife, it would have continued preventing access to habitat and feeding grounds, and eventually impacted water flow in and out of the bay.

Birds traverse globe for wetland

Feeding into a network of sheltered embayments, estuaries and tidal islands, Duck Bay forms part of Tasmania's largest coastal wetlands.

Studies show it meets several of the criteria required to make the Ramsar Convention list, which would consider it a wetland of international importance.

During summer, thousands of shorebirds can be found at the state's far north-west tip — more than the rest of Tasmania combined.

Some have travelled from far flung places like Siberia, Alaska and the Arctic, including the far eastern curlew, the ruddy turnstone, and the tiny red-necked stint, which weighs just 30 to 35 grams.

Underwater, there are gobies, sardines, flounder, galaxiids, garfish, leatherjacket and Australian salmon.

Native plants 'start to recover'

With support from farmers, Landcare estimates it has brought 50 hectares of weed down to just five.

"All of these little sheltered bays were full of rice grass, which is a bright green, wavy grass up to your knee," volunteer Sue Jennings said.

The effort has involved spraying the grass with Fusilade Forte via quadbikes, utes with long hoses, and, more recently, a drone.

Meanwhile, volunteers scan the coastline, attacking any seedlings that have been missed or have cropped up.

"It kills pasture grass and it kills rice grass, but it doesn't kill the salt marsh plants," Ms Jennings said.

"So as we get rid of the rice grass, the salt marsh plants start to recover on their own."

Fish bounce back

A team of scientists from the University of Tasmania has tracked the impact on native saltmarsh vegetation and fish communities, and has uncovered "absolutely remarkable" findings.

"After two years, three years, you wouldn't even be able to say there was rice grass there, so all the native vegetation has come back," wetland ecologist Vishnu Prahalad said.

Fish species have more than doubled, jumping from 5 to 11.

Among those returning are the yellow-eye mullet, which are caught recreationally and commercially, and hardyheads, a small bait eaten by bigger fish.

Fish have been plentiful in number, too.

"The first year … I think we caught two fish, we struggled to look for them, and there they were, two fish across four nets, which is very unusual," Dr Prahalad said.

"After the rice grass was removed … in one net we had about 200 mullets.

"I've never seen anything like that in terms of the recovery."

The discovery had scientists wondering whether fish simply had easier access the area, or if there was more to it.

"And we found that they were all feeding there, and they were eating all the typical food that you find in a salt marsh," Dr Prahalad said.

Welcoming the return of mud

Depending on weather and funding, Landcare hopes to finish attacking meadows in the next two years.

After that, there's a decade of monitoring ahead, reducing the amount of spray along the way.

"This is the most important environmental thing we will ever do in our lives," Ms Jennings said.