Tech & Science Marine heatwaves threatening oyster industry and affecting Great Barrier Reef, scientists warn

21:45  24 november  2019
21:45  24 november  2019 Source:   abc.net.au

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a man standing next to a body of water: The CSIRO's Alistair Hobday says the ocean is ringing © Provided by Australian Broadcasting Corporation The CSIRO's Alistair Hobday says the ocean is ringing "alarm bells". (ABC News: Brendan Esposito)

Waters off parts of Australia are warming at some of the most rapid rates in the world, threatening the future of some of the country's most important marine industries, scientists say.

According to new research, the warming is being driven by climate change and is creating an increase in the frequency, duration and severity of what are known as marine heatwaves (MHWs).

Scientists say the heatwaves are having a severe impact on oysters — and threaten the future of the industry — as well plants and creatures that rely on the ocean for life, pushing some into new areas, while killing others.

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"The oceans are really ringing the alarm bells," said CSIRO biological oceanographer Alistair Hobday, a leading expert on MHWs.

"[The oceans] are telling us we've got big problems and those problems are not going to go away."

A MHW is defined as a period of warm water that lasts five days or longer, where temperatures are in the top 10 per cent of events typically experienced in that region.

They are graded in severity — similar to how cyclones are — with category five being the most intense.

The heatwaves lead to outbreaks of diseases that can be fatal to oysters and other molluscs, and reduce the reproduction rates of species such as salmon and abalone as well as killing seagrass and kelp.

"[We thought] marine heatwaves were an example of what the climate would look like in 100 years time," Dr Hobday said.

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"But we [are] getting it today."

Tropical fish in Tasmania?

In 2016, a MHW struck in waters off Tasmania.

It was a category three, but lasted for 251 days.

Unlike their land-based cousins, marine heatwaves can linger for months.

So much so, local fishermen started catching tropical fish in what is considered to be the gateway to the Antarctic.

Even today, fish usually found in warmer climates are being found in the Tasman.

The impact of the 2016 MHW was seen first hand by oyster farmer Cassie Melrose.

She runs the Melshell Oyster farm at Swansea, Tasmania, with her husband Ian.

It has been in their family since 1984.

When the MHW hit, it led to an outbreak of Pacific Oyster Mortality Syndrome (POMS), a highly contagious virus that can be deadly to oysters, but has no impact on human health.

Oysters on Ms Melrose's farm were diagnosed with the disease, but, miraculously, none died.

"Thirty kilometres south of us, 80 to 90 per cent of their stock was completely wiped out overnight," she said.

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"We were going out to our lease daily and just wondering what we were going to find each day, and getting reports back from the other farmers."

Ms Melrose said it was a stressful time accompanied by an overwhelming sense of guilt that they had escaped unscathed while most of their neighbours suffered.

The 2016 MHW was the longest and most severe MHW on record with the fisheries and aquaculture industries still recovering.

"Surprisingly, South Australia is struggling more than Tasmania," Ms Melrose said. "South Australia didn't have the virus, but they relied on us for our baby oysters.

"So they completely lost supply. They're still recovering four years later."

'Adaption is evolution'

Marine heatwaves can happen in waters anywhere and at anytime.

But they are particularly intense in western boundary currents, where water is flowing from the equator to the polar regions — hence Australia is especially susceptible to them.

However, scientists can now predict when a MHW will strike up to two months in advance.

Scientists and oyster farmers are working together to develop strategies to cope with MHWs.

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They include sea rangers alerting those in fisheries industries to relocate stock if possible, and plan for possible food shortages.

"Adaptation is really evolution," Ms Melrose said.

"I think the oceans warming, or climate changing, is just another step that oyster farmers [will have to] get used to."

Ms Melrose suspects her oyster farm survived POMS because their oysters allowed water to flow between the baskets and lines where they grow.

When the waters warm, they remove flotation devices attached to the baskets, and sink them deeper into cooler water.

So far, it has worked.

But Dr Hobday warned that method would not work forever.

"Heatwaves are actually a chance to practice how you're going to respond to long-term climate change," he said.

"The more you can practice the things that you're going to do, the better you'll be when you're really hit by the big events."

What about the reef?

More than 2500km north of Swansea, the Great Barrier Reef is also feeling the impact of the MHW phenomenon.

So scientists, such as Associate Professor Jody Webster, are looking at ways to help.

The University of Sydney's School of Geosciences professor runs experiments off One Tree Island — a scientific zone of the reef off the coast of Gladstone — to try to understand the reef's evolution.

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He does it by studying coral cores and examining the existing reef.

By mapping its past behaviour, Professor Webster believes he will be able to better predict how it will respond to climate change and — importantly — which corals have the best chance of survival.

He said the rate of change the reef was experiencing was "the real crux of the issue", particularly following the back-to-back coral bleaching events in 2016 and 2017.

"The great fear is if this happens again at the same frequency or increased frequency in the next 10-20 years, that just doesn't allow the reef enough time to recover," he said.

The Great Barrier Reef Marine Park Authority released its summer outlook this week, warning it expects to record one of its warmest years on record and lead to localised coral bleaching.

However, it is not expecting a MHW in the region.

New category heatwaves

Dr Hobday said as the ocean continues to warm, additional categories of MHW severity would need to considered.

And so in the future, we might be describing category seven or eight; MHWs that are seven times as intense as we see today.

Dr Hobday predicts the first category seven MHW to strike by 2050.

"We need coordinated action around dealing with the amount of greenhouse gases entering the atmosphere," he said.

Jody Webster agrees.

"We have to ask the question, are we hastening its (the Great Barrier Reef's) current demise?

"And can we do anything about that? And I guess my view is, we can."

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