Fears blue whale decline signals changes in Bonney Upwelling ocean current

Every summer, the coast near the Victorian and South Australian border becomes a rich feeding ground for marine wildlife.

A strong ocean current, the Bonney Upwelling, drags cold water and deep-sea nutrients to the ocean's surface, attracting marine mammals, migratory birds and fish to the region from November to May.

Over the past few years, marine scientists have noticed shifts in the ecosystem, prompting fears the upwelling could be producing less food for wildlife.

The ocean current helps drive two key industries in the region, fishing and tourism, and is so important for locals that they celebrate it with the annual Bonney Upwelling Festival, which takes place this weekend in Portland.

Changes to the upwelling have some people wondering what impact it might have, not only on the region's environment, but also on its economy.

Whales in decline

Pete Gill, who has monitored pygmy blue whales along the coast near Portland for 28 years, said whale sightings had halved in that time.

The endangered pygmy blue whale migrates to the waters from Cape Otway, in Victoria, to Robe, in South Australia, to feed every summer.

"Back in the early 2000s, we'd be seeing 12 to 18 whales each survey on average, but now we're seeing five or six," Dr Gill said.

When he compared the results of aerial surveys he conducted in 2000-2004 and 2020-2024, he found that blue whale sightings had dropped by 51 per cent.

Dr Gill said it was not yet clear whether the population had dropped or if the whales had shifted their migration habits, but it seemed the region no longer offered the rich food source it once did.

"We have to assume that there's something going on with climate change,"

Dr Gill said.

"Where there's food, whales will come.

"If there's a big supply of krill, we think that they use their deep voices to call other whales in.

"It's like saying, 'OK, there's a lot of food here, come and join in'.

"But if there's only scattered patches, they will probably stay silent."

Food source disappearing

The upwelling sweeps large swarms of krill closer to the coastline, where they thrive in the nutrient-rich cold water, and become a feast to fish, seabirds and marine mammals.

In the past, whales and other filter feeders would have been able to find krill easily near the surface of the ocean.

But Dr Gill said he did not see many swarms of krill near the surface anymore, forcing wildlife to venture further and use more energy just to catch a meal.

"If it's at the surface, the whale doesn't have to go to all the effort of diving down 200 metres or deeper looking for krill," he said.

He has observed a 65 per cent decrease in surface krill swarms since the early 2000s.

"If there's no krill for blue whales, then there's no krill for food for other species as well," Dr Gill said.

A colony of Australian fur seals that feed on fish in the Bonney Upwelling system has also been found to be suffering from malnourishment and population decline.

A festival of the sea

Portland's annual Bonney Upwelling Festival kicks off this weekend, marking the start of the seven-month-long current, bringing anglers, whale watchers, and other tourists to the region.

Glenelg Shire Mayor Karen Stephens said the upwelling was the main driver of tourism in Portland.

"The Upwelling Festival is all about our whales and encouraging people to come and experience that," Cr Stephens said.

"It brings people from outside of our area into it and it gives them an opportunity to explore."

The upwelling also makes Portland a prime location for tuna fishing, which Cr Stephens said was essential to the local economy and another drawcard for visitors.

"It's really important to us that we do protect the environment," she said.

Impact of climate change

Marine ecologist and Deakin University PhD candidate Alex Shute has been studying the past 30 years of trends in the Bonney Upwelling.

He said that in the past five years, there had been greater variability in the strength of the Bonney Upwelling current, and believed climate change could be the cause.

"We have found there are some years where it's fairly weak and then years where it is really quite extreme," Mr Shute said.

"With climate change, it is generally expected you would have a stronger upwelling."

In 2024, the Bonney Upwelling was the strongest on record, but Mr Shute said a stronger current did not lead to more nutrients and krill.

"It's all about balance," he said.

"If you're having a very extreme year, that may go beyond a temperature threshold of a certain zooplankton, and then that will generally affect the entire food chain."