Sharks are on the move, seeking a new home as the ocean's temperature rises due to climate change. It's a fascinating yet concerning phenomenon that's happening right before our eyes.
Many shark species in Australia share a similar migration pattern with the iconic grey nomads. During winter, they head north to escape the cold, and in summer, they reverse course, fleeing the excessive heat by heading south. This migration is crucial for ectothermic sharks and rays, as they rely on external temperatures to regulate their body heat.
But here's where it gets controversial: how will climate change and the rapidly warming waters on Australia's east coast impact shark populations and their distribution?
Marine ecologist and professor Jessica Meeuwig from the University of Western Australia sheds some light on this. She explains that research shows marine megafauna, including sharks, are moving towards the Earth's poles as oceans warm. In Australia's case, that means a southward shift.
"It's a global phenomenon," Meeuwig says. "Animals are seeking cooler waters as the tropics become too hot. This has implications for how we manage these animals and our interactions with them."
Let's take a closer look at some specific shark species and their changing habits.
Bull Sharks in Sydney:
Bull sharks (Carcharhinus leucas) are typically found in coastal waters and rivers, ranging from southwestern WA to the top of Australia and down to NSW. They prefer tropical settings and water temperatures above 22°C.
On the east coast, bull sharks are often found north of Sydney, but some venture as far as the Paramatta River during summer. Ecologist Nicolas Lubitz from James Cook University has been tracking bull sharks on the east coast from Cape York down.
Lubitz explains that while some bull sharks are residents, staying in the region year-round, others are seasonal visitors. "They migrate to southern NSW and the Sydney area in summer and then head back up to Cape York in winter," he says.
Research predicts that by 2030, bull sharks in NSW will have an extra three months of seasonally suitable habitat. Additionally, Lubitz's research over 15 years found that migrating bull sharks are adding an extra day each year to their summer stays in Sydney. This means sharks stayed an extra 15 days in 2024 compared to 2009.
But here's the catch: it doesn't necessarily mean there are more sharks; it's just that the ones that visit are staying longer. And it seems bull shark nurseries could be expanding south, with increasing reports of pups in the Hawkesbury River in recent years.
Tiger Sharks in Tasmania:
Tiger sharks (Galeocerdo cuvier) are found in a similar range to bull sharks, from southwestern WA around the tropics to southern NSW. Their distribution on the east coast depends on age and gender. Female tiger sharks, both juvenile and adult, tend to stay in Queensland waters, which offer the most thermally suitable conditions.
Juvenile male tiger sharks range further, from Queensland down towards Victoria. The distribution of adult male tiger sharks is less clear, as they are rarely tagged.
Yuri Niella, a science officer at the IMOS Animal Tracking Facility in Sydney and lead author of a study tracking tiger sharks from 2002 to 2020, predicts that the tiger shark's range will expand to Tasmania's east coast by 2030. However, Niella notes that technology to detect tagged marine animals has already found tiger sharks that far south.
"These guys are expanding their distribution range in the face of climate change. They're not concerned about Tasmania; they're seeking warm waters," Niella explains.
The future distribution of tiger sharks could also depend on changes in the distribution of their prey, such as sea turtles, which appear to be moving southwards into NSW.
Young White Sharks and Habitat Loss:
White sharks (Carcharodon carcharias) are regionally endothermic, meaning they can create warm blood in certain muscles and pump it around their bodies. Australia's white sharks are found in cooler southern waters, ranging from northwestern WA to Tasmania and up to southern Queensland.
On the east coast, juvenile and sub-adult white sharks have been tracked traveling thousands of kilometers annually, generally staying within 20 kilometers of the coastline and preferring waters between 16 and 24 degrees Celsius.
In summer, they hang around southwest Victoria at Corner Inlet and an area near the NSW-Victoria border known as Twofold Shelf. In winter, they move up towards Queensland and northern NSW.
However, marine ecologist Adrienne Gooden from Southern Cross University warns that there could be a loss of wintering habitat in the coming decades due to a warming ocean.
"We may see a compression of habitat, where they can't move as far north, and they're not gaining more habitat in the southern region," Gooden says.
This could make the Twofold Shelf region more important for young white sharks in the future, and we may see shifts in shark abundances and distributions along the NSW coast.
Not Every Species Can Move Southwards:
While many large species like scalloped hammerheads, zebra sharks, and manta rays are also moving south, what about cold-water species along Australia's south coast? Can they simply swim southwards to Antarctica?
Louisa Graf, a marine scientist at Deakin University, studies the potential impacts of climate change on smaller shark and ray species in Victoria's temperate waters. Her recent study suggests a decline in suitable habitat for Victorian shark and ray species by 2090 under a moderate warming scenario.
This could lead to a drop in the abundance of fiddle rays, while species like gummy sharks might thrive. Graf emphasizes the importance of marine protected areas as potential refuge sites in the future.
"More research is needed to fully understand the impacts of ocean warming," she concludes.
So, what are your thoughts on these changing shark migration patterns and their potential impacts? Do you think we should be concerned about the future of these species and their habitats? Feel free to share your opinions and insights in the comments below!
