A harmful algal bloom (HAB) event that started in March 2025 is ongoing along the coasts of the state of South Australia . It has caused fish kills impacting the marine environment and associated fishing and aquaculture industries, as well as beachgoers and tourism operators along the affected coasts. The state government has set up an information website and hotline dedicated to the event, and the state and federal governments are working together on mitigation of the bloom. Temporary fishing restrictions on certain species, starting 1 November 2025, have been imposed to allow them to recover. The environment ministers responsible are Lucy Hood (SA) and Murray Watt (Federal).
The causes of the bloom are attributed to flooding of the River Murray in 2022-23 that brought extra nutrients into the sea; a cold-water upwelling in summer 2023-24 that lifted more nutrients to the surface; and a marine heatwave that has occurred since September 2024. These changes, particularly the latter, have been attributed to climate change. The algae are not considered a significant health concern, although people who experience an allergic reaction such as asthma, coughing, or itchy eyes should not stay in close proximity to foamy or discoloured water.
Background and extent
In March 2025, a marine heatwave-induced bloom, initially identified as consisting mainly of the dinoflagellate Karenia mikimotoi, was blamed for the deaths of thousands of marine creatures, as well as coughing, sore eyes, and blurry vision in humans, along stretches of beach along the south coast of the Fleurieu Peninsula in South Australia. Despite hopes that winter storms would cause the bloom to dissipate,Horn, Caroline; Boisvert, Eugene; O'Brien, Samuel; Bermingham, Kathryn (30 May 2025). Hopes high after ocean temperatures off SA coast drop amid algal bloom outbreak ABC News. Retrieved 30 May 2025. relatively mild conditions (potentially attributed to climate changeHaynes, Jessica; MacLennan, Leah; Chorley, Che (30 May 2025). An ecological disaster has been unfolding on Australia's coast ABC News. Retrieved 30 May 2025.MacLennan, Leah (5 July 2025). SA's toxic algal bloom is unlikely to end soon so what are the consequences? ABC News. Retrieved 5 July 2025.) caused it to persistHorn, Caroline (20 June 2025). Micro algal bloom off SA persists as commercial fishers seek economic relief ABC News. Retrieved 20 June 2025. and spread along adjacent coasts of Kangaroo Island and Gulf St Vincent (including Adelaide's metropolitan beachesLandau, Sophie; Mason, Olivia; Horn, Caroline (28 June 2025). 'Heartbreaking' algal bloom hits metropolitan Adelaide beachesABC News. Retrieved 28 June 2025.Horn, Caroline; Landau, Sophie; Brine, Josh (3 July 2025). Dead marine life from algal bloom collected from Adelaide beaches amid local extinction fears ABC News. Retrieved 3 July 2025.) and into the Coorong.Horn, Caroline (4 June 2025). Marine algal bloom spreads to SA's Coorong as fishers find dead creatures in North Lagoon ABC News. Retrieved 4 June 2025.
By the middle of the year, the state government estimated the size of the bloom as around the same size as Kangaroo Island, which covers over (about 80 times the size of Sydney Harbour). By October 2025, the bloom had spread on the currents, into both Gulf St Vincent (flanking the Yorke and Fleurieu Peninsulas) and Spencer Gulf, along the coast of the Eyre, as well as to Kangaroo Island itself. Marine biologist Mike Steer, chief executive of the South Australian Research and Development Institute (SARDI), estimates that it has affected around 30 per cent of the state's coastline, roughly equivalent to . He said that its extent and duration is unprecedented in both South Australia and Australia, and is probably one of the top ten recorded blooms in the world. The densest occurrence of the bloom, manifesting in foam, was occurring on the stretch down the coast from the top of Gulf St Vincent down to Brighton Beach.
Species composition Besides K. mikimotoi, other Karenia species including K. brevisulcata and K. longicanalis have been found to be present in the bloom, as well as others yet to be identified.Phiddian, Ellen (29 August 2025). Scientists race to find weapons against South Australia's toxic algal bloom ABC News. Retrieved 29 August 2025. Collaborating with SARDI and independent local consultants, marine biologists from Flinders University, University of Technology Sydney and the University of Tasmania are involved in studying the bloom, with specialist marine toxicological expertise being sought from the Cawthron Institute in New Zealand.
In November 2025 researchers announced that they had tentatively identified K. cristata as having been the dominant species present for much of the time in the bloom, and that this little-known species (one of five Karenia species present, including K. papilionaceaHaynes, Jessica; Keane, Daniel; and Horn, Caroline (6 November 2025). What is the new species detected in SA's algal bloom? ABC News. Retrieved 6 November 2025.) is responsible for the brevetoxins detected in shellfish, and the probable cause of the respiratory symptoms experienced by surfers when the first reports of the bloom were made in March.Horn, Caroline (5 November 2025). Karenia cristata species source of toxins in SA's algal bloom, scientists believe ABC News. Retrieved 5 November 2025. K. cristata is the only species of the five present in the bloom which has been found capable of producing brevetoxins. As it was previously unknown in Australian waters – the only other reports having come from South Africa and Newfoundland – there were suggestions that it may have been introduced through ballast water, although this is considered to be highly unlikely, due to its physical fragility. In July 2026, new research found that laboratory-grown strains of K. cristata were exceptionally toxic to marine organisms, even in much lower concentrations than had been found in the ocean.
DNA sequencing has been used to identify the species present in the bloom, and K. brevis, the only other known brevetoxin-producing Karenia species, has not been found.
Possible causes According to the Australian government website in May 2025, the bloom has been building up over time, and its causes may be attributable to the following events: Floodwaters from the River Murray in 2022-23 bringing extra nutrients into the sea, a cold-water upwelling in summer 2023-24 that lifted more nutrients to the surface and a marine heatwave since September 2024, raising water temperatures by about 2.5C. However, scientists later said that this was an unlikely cause, as the time lag between the events was too great.
According to the not-for-profit Biodiversity Council, marine ecologists have warned that ocean warming and marine heatwaves associated with climate change, as well as nutrient pollution, have led to a rise in the number of algal blooms, and this is likely to continue. The Australian Marine Conservation Society agrees with the assessment that climate change has driven the bloom, calling the event a "crisis" and a "warning we can’t ignore".
Claims that the bloom may have been caused by the Adelaide Desalination Plant have been dismissed by researchers as misinformation.
Impact As of November 2025, 20,000 km and about 30% of South Australia coastline are impacted.
Although not produced by K. mikimotoi, brevetoxins produced by other Karenia species have been detected in commercial shellfish farms, leading to temporary closures,Yankovich, George and Graham, Narelle (8 May 2025). Toxin caused by algae forces quarantine of oysters on Yorke Peninsula ABC News. Retrieved 8 May 2025 and in foam washed up on beaches, with health authorities issuing advice for asthmatics.
The bloom has killed more than 500 speciesKeane, Daniel (13 October 2025). SA Premier says algal bloom is not 'toxic', despite word appearing in official government advice ABC News. Retrieved 13 October 2025. of fish, penguins,Pedler, Emma and Bailey, Duncan (9 May 2025). Dead little penguins washed up on multiple Eyre Peninsula beaches ABC News. Retrieved 9 May 2025. and marine mammals, and has also caused health problems such as asthma, skin and eye irritation, and coughing in humans. It has affected the aquaculture, fishing and tourism industries, especially local fishers, as well as the many residents who live along the coastline and surfers who frequent popular surfing spots such as Port Elliot. Several groups of First Nations peoples have called the ongoing bloom a "cultural emergency". The Narungga people of Yorke Peninsula have been heavily affected by the bloom. The Goolwa Pipi Co., which harvests a small edible saltwater clam known as the Goolwa pipi, was no longer able to harvest and sell their shellfish. Large areas of kelp, seagrass, and shellfish reefs, which tend to filter nutrients, have been lost. In addition, the gulfs around the South Australian peninsulas, especially Gulf St Vincent, are more susceptible to HABs because they have less water exchange than open ocean coastlines, and the water is shallower, which increases its heating capacity.
It was feared that the bloom would affect the breeding season of the unique population of giant cuttlefish in the northern Spencer Gulf, in a sanctuary area that received National Heritage status in 2023, the Cuttlefish Coast Sanctuary Zone. This population of the species is even distinct from the population in the southern Spencer Gulf, and the only population of giant cuttlefish in the world to breed in a mass event, in a spectacular display of colours, including blue, purple, green, red and gold, which change as they mate and lay eggs. This makes it a popular tourist attraction. See below for the device created by SARDI to protect some of the cuttlefish, an air-bubble curtain generator.
In the seven days preceding 3 October 2025, clearing of beaches along the coast collected around of marine animals, including longsnout boarfish, whiting, and leatherjacket fish. The previous week of hand-clearing netted a total of two tonnes of dead sea life.
According to head of SARDI Mike Steer on 3 October 2025, the algae and the brevetoxins they produce are "not significant health concerns", and there is no need to stay away from the water. He himself has continued to surf. However, if the water is discoloured or foamy, and a person is susceptible to itchy eyes, coughing, or other irritation, they should move away.
The local seafood industry has suffered a downturn, despite assurances that fish caught in SA waters are safe to eat, and many fisheries have been completely unaffected by the bloom; consumer confidence dropped owing to the news describing the bloom as "toxic" or "harmful". The shellfish-harvesting region around Stansbury on the Yorke Peninsula was closed for nearly eight months.
It was discovered in late October that, according to SARDI data, that the numbers of squid, garfish, and King George whiting had been severely depleted in Gulf St Vincent and around Kangaroo Island, while some other species, such as blue crabs, had also declined.
Early predictions Early model predictions by oceanographer Jochen Kaempf of Flinders University, reported on 22 July 2025, suggested that the algal bloom may shrink over winter but then spread to new areas in summer, including both Gulf St Vincent and Spencer Gulf. This modelling scenario was also featured in Robert Simms' motion for the establishment of a Joint Committee on the Algal Bloom by the South Australian Parliament.
Responses and suggestions Government responses The state government briefed federal Environment Minister Murray Watt on 11 June 2025, who engaged with then state environment minister Susan Close during the following months. At that time, the HAB was expected to dissipate over the winter months.