Algal bloom linked to surge in shark sightings
An algal bloom that has killed dozens of species of fish and sharks amid a lengthy marine heatwave, is being linked to a rise in shark sightings.
A sharp rise in shark sightings could be linked to an algal bloom that has killed about 100 species of fish and sharks amid a nine-month long marine heatwave across southern Australia.
A shark attack at a popular Adelaide beach on Thursday, in which a swimmer aged in his 60s had his leg mauled, has focused attention on the growing number of sightings and strandings since the bloom formed in March.
South Australia’s shark sightings log shows 195 reports so far in 2025, compared to 313 in 2024, 148 (2023), 153 (2022), 264 (2021) and 84 (2020).
Since the microalgae bloom, karenia mikimotoi, was identified off the Fleurieu Peninsula in March, there have been hundreds of reports of marine deaths, ranging from sharks and penguins, to popular fishing species such as flathead, squid, crabs, and rock lobsters.
Environment Minister Susan Close said the bloom had grown to an unprecedented scale, close to the size of Kangaroo Island, which is 4405sqkm, and as deep as 20m.
You might like
“We’re talking about a very large algal bloom with a significant impact on marine life,” she said.
The state government was “aware of community concerns” over sharks being sighted near shore, a Primary Industries and Regions SA spokesperson said, but it was “not uncommon” for white sharks to hunt other fish in near-shore areas.
“Algal blooms of this size and nature are rare in South Australia – the last one was recorded in 2014 in Coffin Bay. As such, there is a lack of clear evidence of how algal blooms such as these affect the behaviour of marine life,” the spokesperson said.
Earlier in May, authorities removed a dead white shark washed up on Henley Beach in Adelaide and two other sharks were reported dead at Port Willunga in Adelaide’s south and at Port Moorowie, on the south coast of Yorke Peninsula.
Stay informed, daily
Flinders University marine scientist Charlie Huveneers said the increased number of sightings and strandings was “unusual”, and likely related to the biotoxins from the algal bloom.
“But we will only be able to confirm this and understand which toxin might have led to these strandings once toxicology results come back,” he said.
The bloom is being driven by the marine heatwave that began in September 2024, with temperatures up to 2.5C warmer than usual, as well as relatively calm marine conditions with little wind and small swells.
In its most recent update on May 13, the South Australian Research and Development Institute said there had been a slight decrease in sea surface temperatures across the state’s coastal waters, but they remained up to 2C above average.
An Environment Department spokeswoman said “nothing can be done” to dilute or dissipate the bloom.
“This is a dynamic situation as the bloom travels depending on weather and water conditions and the impact on people in these areas can therefore be unpredictable,” she said.
The bloom, which is not toxic to humans but can cause skin irritations, is expected to remain until the arrival of strong westerly winds.
