Why do sharks bite humans? As four attacks devastate Australia in just 48 hours, scientist reveals exactly what might be behind the spate of incidents

As four shark attacks devastate Australia in just 48 hours, a scientist has revealed what could be behind the spate of incidents. 

Swimmers in Sydney have been warned to avoid the water after a surfer, 39, was mauled off the coast of Port Macquarie, following three earlier attacks.

Reports suggest that bull sharks – a notoriously aggressive and common species – might be responsible for all four attacks.

Speaking to the Daily Mail, Professor Shokoofeh Shamsi of Charles Sturt University has revealed the likely reasons for the attacks – and says the sharks are not entirely to blame.

From floods of raw sewage and toxic pesticides to rising noise pollution, human activity has disrupted the waters where sharks live, breed, and hunt.

Some of these changes might result in sharks becoming more stressed, more aggressive, bolder, and more likely to bite a human, according to the expert.

'There is increasing research showing that our environment has changed a lot, including the ocean,' she told the Daily Mail.

'Because of this, they are extremely stressed, and of course, you get this aggressive behaviour.'

On Sunday afternoon, a 12–year–old boy was attacked by a shark while jumping off rocks near East Sydney's Nielsen Park.

Then, on Monday morning, an 11–year–old boy was knocked into the water by a shark that bit a chunk out of his board.

Later that evening, a male surfer suffered life–changing injuries after being bitten on the leg by a shark at North Steyne.

With the latest attack off Port Macquarie bringing the total to four, this is now one of Sydney's worst spates of shark attacks on record.

Professor Shamsi claims the increased risk of shark attacks should have been predictable.

The four attacks came shortly after a period of heavy rainfall, which washed a large amount of runoff into the sea.

As the city's sewage washes into the sea, it attracts schools of bait fish that lure freshwater–resistant bull sharks closer to the shore.

At the same time, sediment reduces visibility to the point where humans can't see the approaching threat, and sharks can't see what they are biting.

The fact that shark attacks increase after heavy rainfall is extremely well known, but Professor Shamsi says something else might be going on.

A rainstorm doesn't just change the water's visibility; it massively alters the entire chemical makeup of the area.

She explained: 'Just imagine it: You're walking down the street, and the clean air suddenly changes to become very smoky.

'It smells different, you don't have the visibility, you cannot do your daily activities, you get restricted in the places you can go, and the food you can eat.

'That happens in the ocean when all this fresh water enters, and just like we would get stressed and become a bit jumpy, why shouldn't the same thing happen to sharks?'

Although sharks don't normally attack humans intentionally without provocation, increased stress could mean that accidental attacks and aggressive behaviour become more likely.

On top of that, there is increasing evidence to suggest that chemicals in runoff from the land might make sharks even more likely to attack.

Heavy rainfall carries pharmaceutical products from sewage, pesticides from farmland, and even industrial chemicals from factories into the water.

These chemicals are absorbed into the bodies of marine animals, disrupting the healthy function of their nervous systems.

For example, studies have shown that rats display significant long–term increases in depression, anxiety, and aggressive traits after being exposed to a common chemical used in pesticides.

Likewise, there is growing evidence that pollutants and pharmaceutical contaminants can affect aggression, memory, and stress in freshwater fish such as Nile tilapia.

Professor Shamsi says: 'Based on research on terrestrial animals, and also on humans, we know that a lot of these environmental pollutants can be involved in a lot of neurological symptoms and diseases.

'Marine animals are basically the same cells and same systems as terrestrial animals and humans, so why shouldn't it happen in them?'

Additionally, alongside these harmful chemicals, heavy rainfall also carries parasites and infections from human settlements into the water.

Some of these can have profound effects on the behaviour of the animals they infect.

The parasite Toxoplasma gondii, for example, has been shown to reduce fear responses, increase risk–taking, and change how the brain processes threats.

While researchers haven't started looking for these infections in sharks, the parasite has been found in species ranging from fish and dolphins to sea otters.

'These parasites have evolved over millions of years to manipulate the host's behaviour to get what they want,' says Professor Shamsi.

'Together, all of these things, parasites, microorganisms, environmental pollutions, everything together, add to the stress of these creatures.

'When they are stressed, they do things that distressed animals do; and that includes biting.'

Professor Shamsi says that this idea is currently a hypothesis in need of concerted further research, but adds that scientists should be paying more attention to this potential issue.

If this theory is true, the solution is to avoid the water shortly after heavy rainfall when sharks are most likely to attack.

However, it might not just be the rain that is making sharks more stressed and potentially dangerous.

There is increasing evidence to show that noise pollution from shipping, industrial activity, and military sonar has a profound effect on marine wildlife.

Water conducts sound about four times more effectively than air, meaning a noise which is quiet above the water can be deafening beneath the waves.

Sharks, like most fish, are extraordinarily sensitive to sound and changes in water pressure, using this ability to detect prey over long distances.

Studies have shown that the noise of military sonar, pile driving and seismic explorations produces an acute effect on normal behaviours like hunting.

Likewise, a 2025 study found that sharks exposed to recordings of human noises showed significantly increased stress responses.

Professor Shamsi says that this could 'absolutely' add to the risk of shark attacks, but cautions that more research is needed to be certain.