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South-east Australia marine heatwave forecast to be literally off the scale

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Patch of Tasman sea expected to warm over spring and summer to temperatures that risk significant losses to sea life

Australia’s south-east could be in for a marine heatwave that is literally off the scale, raising the prospect of significant losses in fishing and aquaculture.

The Bureau of Meteorology has forecast a patch of the Tasman Sea off Tasmania and Victoria could be at least 2.5C above average from September to February, and it could get hotter.

Oceanographer Grant Smith said the colour-coded scale the bureau uses to map forecast sea surface temperature anomalies stops at 2.5C.

“We didn’t account for anomalies that high when we developed this ... it could be 3C, it could be 3.5C, but we can’t see how high it goes,” he said.

Smith said he could not be sure that it was the first time forecast temperatures had gone beyond the scale’s upper limit, but it was the first time he had seen it.

South-eastern Australia is a known climate change hotspot with its waters warming about four times faster than the global average.

“The east Australian current brings warm water south and then also the rising atmospheric temperature cooks it at the same time,” CSIRO research director Alistair Hobday said.

In 2016, the region suffered its longest ever marine heatwave, which ran for about 250 days. The effects on marine life were profound.

Abalone fishers reported a rise in catch mortality, Tasmania’s salmon farming industry lost stock to the warm water, tropical fish moved in and there was an outbreak of the highly contagious viral infection Pacific oyster mortality syndrome.

Hobday said there could be similar sorts of impact this summer if the forecast heatwave crashed into the coast.

“Then we would expect to see impacts on aquaculture, we’d expect to see new species showing up in southern south east Australia, we’d expect to see impacts on remaining kelp forest in the region,” he said, noting that Tasmania’s giant kelp species had already lost 95% of its historical range.

Salmon farmers may opt to harvest early, try to boost oxygen levels in the water, or change their feed mix.

Hobday said he would publish a paper in September advising researchers, industry and others how to prepare for the hotter, drier weather expected with the likely onset of El Niño conditions after three relatively wet, cool years in Australia.

Rich Little, also from the CSIRO, is part-way through a project to determine how marine life has changed in south-eastern waters over recent decades.

Little and a team of scientists spent July aboard a research vessel that travelled up Tasmania’s east coast to near Eden, on the New South Wales south coast, and back again. The journey mimicked a marine survey done over a similar area in the mid-1990s.

Two more runs are planned by November 2024, allowing scientists to analyse what is being caught in nets compared with a couple of decades ago.

Little said it was too soon to draw conclusions, but there was solid anecdotal evidence that the mix of marine life had changed.

“Anecdotally ... we caught a lot, lot more mackerel than we know they caught in the 1990s. Tonnes of mackerel,” he said.

“At the same time we saw a lot of things that feed on mackerel – more fur seals that were feeding on them.”

He said the number of seals in the area, as well as other marine mammals, had increased. Other species that were commonly caught in the 1990s were relatively scarce, including blue warehou and red fish.

Little said scientists were examining the extent to which observed changes in in the ecosystem were due to the climate crisis caused mostly by burning fossil fuels and other local factors

“We really can’t make a definitive conclusion at this stage, but I think it’s most likely to be a combination of both,” he said.

Source: The Guardian