Sharks seek sea change as ocean temperature rises with climate change

Many sharks in Australia have the same migration pattern as grey nomads.

In winter, they journey northward to escape the cold. In summer they reverse their trajectory, heading south to flee excessive heat.

This migration is especially important for ectothermic or "cold-blooded" sharks and rays that rely on external temperatures to regulate their body temperature.

So how will climate change, and rapidly warming waters on Australia's east coast affect shark population and distribution?

Marine ecologist and professor Jessica Meeuwig, from the University of Western Australia, said it was clear from research that as oceans warmed, marine megafauna moved towards the Earth's poles.

"So in Australia's case that means south," she said.

"And so that means that we have to think about the implications for how we manage these animals and our interactions with them."

Bull sharks summering longer in Sydney

Bull sharks (Carcharhinus leucas) can be found in coastal waters and rivers from the south west of WA, around the top of Australia and down to the bottom of NSW.

But generally this species prefers more tropical settings and water temperatures over 22C.

On the east coast, the shark is often found north of Sydney but some will venture as far as the Paramatta River during summer.

Nicolas Lubitz, an ecologist at James Cook University, tracks bull sharks on the east coast from Cape York down.

Dr Lubitz said Cape York had resident sharks that stayed in the region all year round, but other animals were seasonal.

"They sort of do this whole thing where they go to southern NSW, Sydney area, in the summer," he said.

Five years ago there was research that predicted there would be a three-month increase in seasonally suitable habitat by 2030 for bull sharks in NSW.

Separate research by Dr Lubitz that looked at patterns over 15 years found migrating bull sharks added an extra day each year to their summer stays in Sydney.

This meant sharks stayed an extra 15 days in 2024 compared to 2009.

Dr Lubitz stressed that didn't mean there's currently more sharks, just that the ones that already visited the region stayed longer.

And bull shark nurseries could be expanding south. 

Dr Lubitz said the Hunter River used to be the southern limit for baby bull shark sightings, but in the past 10 to 15 years there had been increasing reports of pups in the Hawkesbury River.

How climate change might impact the resident sharks of Cape York as waters in northern Queensland get hotter is currently unclear.

Tiger sharks in Tasmania

Similarly to bull sharks, tiger sharks (Galeocerdo cuvier) are found from south west WA around the tropics and down to southern NSW.

Their distribution on the east coast largely depends on their age and gender.

Female sharks (both juvenile and adult) tend to stay in Queensland waters — the most thermally suitable — according to a study that tracked tiger sharks from 2002 to 2020.

Juvenile males ranged further — from Queensland down towards Victoria. What happens with male adults is unclear as not many are ever tagged.

The study's lead author Yuri Niella, a science officer at the IMOS Animal Tracking Facility in Sydney, said modelling suggested the tiger shark's range would expand to Tasmania's east coast by 2030.

But technology used to detect tagged marine animals has already found tiger sharks that far south, he said.

"We last year serviced our line down in Maria Island and for the first time in 20 years of this program we got tiger sharks detected."

"These guys are expanding their distribution range in face of climate change."

Dr Niella said future tiger shark distribution could also depend on changes to prey distribution, such as sea turtles, which appear to be moving southwards into NSW.

Young white shark habitat could be squeezed

Unlike bull sharks and tiger sharks, white sharks (Carcharodon carcharias) are regionally endothermic which means they can create warm blood in certain muscles that can then be pumped around the body. 

Australia's white sharks can be found in cooler southern waters from north west WA to Tasmania and up to southern Queensland.

On the east coast, juvenile white sharks (under 3 metres) and sub-adults (3 to 4.8 metres) have been tracked travelling thousands of kilometres every year.

They were generally within 20 kilometres of a coastline and preferred waters between 16 to 24 degrees Celsius, according to an analysis of six years of satellite tracking data from 41 sharks.

In summer these sharks hang around south-west Victoria at Corner Inlet and an area of water near the NSW-Victoria border (known as Twofold Shelf), and in winter they shift up towards Queensland and northern NSW.

But study lead author and marine ecologist Adrienne Gooden, from Southern Cross University, said there could be a loss of wintering habitat in coming decades because of a warming ocean.

"So it is likely that we will see a compression of habitat where they can't move as far north, but they also are not getting increases in available habitat in that southern region."

Ms Gooden said this meant the Twofold Shelf region could become more important in the future for the young sharks, and we may see shifts in the abundances and distributions of the sharks along the NSW coast.

Not every species can move southwards

There are plenty of other large species like scalloped hammerheads (Sphyrna lewini), zebra sharks (Stegostoma tigrinum), and manta rays (Mobula alfredi) also moving south.

But what happens to populations of cold water species along Australia's south coast that mightn't be able to just swim southwards to Antarctica?

Louisa Graf, a marine scientist a Deakin University, studies the potential impacts of climate change on smaller shark and ray species around Victoria's temperate waters.

Ms Graf recently led a study that suggested there could be a decline in suitable habitat for Victorian shark and ray species by 2090 under a moderate warming scenario.

This could lead to an overall drop in the abundance of fiddle rays although some species like gummy sharks (Mustelus antarcticus) might get a boost. 

"Our research did indicate that marine protected areas may serve as potential refuge sites in the future," Dr Graf said.