Experts say nutrients from the land flushed into the sea by heavy rains can attract fish and other animals to feed – this can then attract sharks
Wild weather leads to murky water and shark warnings at popular Sydney beaches – video
Experts have urged Sydneysiders to avoid swimming at murky beaches due to increased risk of shark encounters following torrential rain that has caused devastating damage along Australia’s east coast.
There have been numerous shark sightings in Sydney in recent days. On Tuesday, the Northern Beaches Council issued an alert that a shark more than 3 metres long had been spotted off Manly Beach, which was subsequently closed.
The Manly Observer reported that it had been “receiving reports from local surfers and swimmers all week”, and Surf Life Saving NSW also reported an unidentified 2.5 metre shark off the beach on Monday morning.
A spokesperson for the Northern Beaches Council said there had been no further sightings since Tuesday.
A spokesperson for the NSW Department of Primary Industries said there was no evidence to suggest that shark numbers were increasing in the coastal and estuarine waters of Sydney, but that the risk of shark encounters increased in murky waters after heavy rainfall.
“Run-off and nutrients from the land run into our waterways and can attract fish and other animals to feed – this can then attract sharks,” the spokesperson said. “Reduced water visibility can make conditions ideal for ambush predators, like sharks, to hunt in. Our recommendation is to avoid the water during these conditions.”
Prof Nathan Hart of Macquarie University said the increased rainfall was pushing effluent and rubbish into waterways.
“It delivers a lot of nutrients into coastal waters, and that can sometimes attract particular prey species into the area,” he said. “Inevitably, you might get some predators coming in to feed on those fish as well.”
A modelling study Hart co-authored in 2019 found that the risk of shark attacks by certain species was linked to wet weather. “Tiger sharks, for instance – their attack risk increases with rainfall,” he said.
Hart’s research found the risk of great white shark encounters increased with mean monthly rainfall up to a limit of 100mm, but dropped above that – a finding Hart attributes to a “complex interaction between season, the type of stuff that’s being washed down, and just how much has been washed down in those rivers”.
Great white shark attack risk was also significantly higher with warmer sea surface temperatures, the study found.
A key driver of the wet 2021–22 summer in New South Wales has been an ongoing marine heatwave in the Tasman Sea.
Sightings of large great white sharks were rare, Hart said, adding that during marine heatwaves great whites seem to move into waters closer to the coast, where upwelling currents bring cold water up from the deep.
“When the water gets warm … white sharks actually seek out those cooler waters,” he said. “Whether that’s because their prey are going there, or just because physiologically the water is too hot and they’re trying to find colder pockets, we don’t really know.”
Dr Vanessa Pirotta, a marine predator researcher, said the higher than usual temperatures could have implications for where sharks moved as a result of following prey.
“We have seen higher temperatures, persisting for a longer period of time on the east coast, and it seems to be strengthening in some areas. This is likely to have changes for the way in which animals are distributed in the marine environment,” she said.
“It would be very tricky to tie all the environmental variables to why we’re seeing sharks when we are,” she said. “It’s a big ocean and these animals are capable of large geographical movements.”
Research from the NSW Department of Primary Industries has found that sharks are present in the state’s waters year round.
Great white sharks were mostly present in sea surface temperatures between 18 and 24C in NSW, the department said. “Bull sharks are more numerous in the estuaries and coastal waters of Sydney over the summer and autumn months, when water temperatures increase.”
“By tagging and tracking the movements of sharks, we know that sharks are not resident to any particular location along the NSW coast. White sharks tagged off the NSW coast have been tracked to Western Australia, Papua New Guinea, Tasmania, sub-Antarctic islands, New Zealand, and New Caledonia.”
“Data from our tagged sharks indicates that most juvenile white sharks move northward along the NSW coast in late autumn and early winter.”