Bull sharks are spending longer in Sydney Harbour and other summer grounds. Here’s how you can stay safe

Four people have now been bitten by sharks in the last two days in New South Wales, including three in Sydney Harbour. Two people are in critical condition.

The shark species responsible isn’t yet known. But some of these incidents likely involved the highly adaptable bull shark (Carcharhinus leucas). This unique fish species can tolerate a wide range of water salinity, from oceans to brackish estuaries, and even freshwater rivers.

Bull sharks have long been found in warmer Australian waters, ranging from south-west Western Australia, all the way around the Top End and down the east coast as far as the New South Wales-Victorian border.

The movements of bull sharks in Sydney Harbour have been studied for several years. Their presence is more likely when waters are warmer over summer. But they’re staying longer than before. Last year, researchers found that bull sharks were spending on average an extra day per year in their summer grounds (shallower coastal waters, estuaries and rivers) as ocean temperatures rise due to climate change.

Record heavy rains in Sydney flushed plenty of nutrient-rich water from farms and wastewater treatment plants into the river system, including the harbour. This nutrient runoff attracts more prey such as baitfish and larger fish, and in turn larger predators such as sharks. Stormwater also makes harbour waters murkier, which means that bull sharks rely more on hearing and electroreception than sight to locate food sources. This can lead to bites due to mistaken identity.

Although human activity (noise and movements) in the water can attract sharks, humans are not a food source for bull sharks. Almost all encounters and negative interactions from these sharks come from an exploratory bite. Unfortunately for those affected, the bites can be very serious.

What could be behind these incidents?

Bull sharks are unique among sharks in that they can tolerate fresh, brackish and salt water. Most other shark species don’t use estuaries or rivers as part of their home range or lifecycle. This ability to tolerate and adapt to different salinity levels is one reason bull sharks are found in both coastal waters and river systems around the world, including estuaries.

Once mature, female bull sharks will return to their home rivers to give birth to live young. Newborns are small adult replicas. As they grow, juvenile and sub-adult bull sharks travel down river systems and tend to live in the lower estuaries for the first five years of their life to avoid larger predators. During that time, they opportunistically feed on a range of prey to get bigger before moving into the open ocean.

Bull sharks are very opportunistic feeders. Scientists have found an astonishing variety of things in bull shark stomachs, such as wood, metal and other inorganic matter, though fish are their prey of choice.

Estuaries and harbours tend to have murkier water than the open ocean, as rivers often carry plenty of sediment and nutrients. This means bull sharks have to rely on senses other than sight, such as sound, which travels well underwater, smell, as well as their close-range ability to sense weak electrical fields caused by the movements of living creatures. Many shark bites are likely due to the habit bull sharks have of opportunistically biting in case it might be food.

Over the last week, pulses of stormwater have made Sydney Harbour murkier and more nutrient-rich, attracting baitfish and the predators who follow them.

Bull sharks, like other sharks, learn patterns quickly. Many species of shark have learned to associate the specific sound made by fishing boat engines with food. When fish are hooked or trapped in a net, sharks may be able to get a free feed. Dolphins do the same thing.