A harmful algae bloom along South Australia’s coast has led to the deaths of hundreds of marine species, in what experts describe as one of the region’s most devastating marine die-offs to date.
The bloom, caused by the algae Karenia mikimotoi, has now spread over 150km of the southern coastline since first appearing in March. The affected zone spans an area nearly the size of Kangaroo Island.
The algae has been linked to widespread deaths of marine life, including sharks, rays, leafy sea dragons, octopuses, cuttlefish, and numerous fish and shellfish species. Local reports and scientific surveys confirm the scale of the ecological damage.
South Australia’s environment minister Susan Close stated that the outbreak is the largest ever recorded in the region. The algae thrive under the current marine heatwave conditions, with sea surface temperatures reported to be 2.5°C above normal and weather conditions remaining calm — both of which contribute to the bloom’s growth.
The algae produce toxins that harm fish and other sea creatures by attacking their gills and nervous systems, leading to haemorrhaging and disorientation. The phenomenon has been described by marine experts as resembling a “horror movie” for aquatic species.
There is little intervention possible at this stage. The minister said a change in weather, particularly the arrival of strong westerly winds, is the only force likely to disperse the algae and break the bloom.
An evaluation of over 1,400 citizen-submitted reports revealed that about half of the dead marine animals were ray-finned fish, while over a quarter were sharks and rays. Cephalopods and crustaceans have also been severely affected.
Although not directly harmful to humans, the bloom has caused skin irritation and respiratory discomfort for some beach visitors. Authorities have shut down several oyster farms and banned pipi collection in certain zones to limit exposure.
The first signs of trouble appeared in March, when beachgoers noticed dense foam and dead sea life along the Fleurieu Peninsula’s Waitpinga and Parsons beaches. Subsequent scientific tests confirmed the presence of K. mikimotoi.
According to marine biologist Professor Shauna Murray, the algae produce reactive oxygen compounds that can quickly suffocate marine life, leading to mass deaths.
Experts point to the persistence of high-pressure systems delaying the winds needed to clear the algae — a symptom tied to ongoing climate shifts.
Environmental officials say the long-term effects on marine biodiversity and local economies are still being assessed. However, conservationists are calling for stronger measures to monitor and respond to marine heatwaves, which are becoming increasingly frequent due to warming ocean temperatures.
South Australia is currently experiencing one of its worst droughts, compounding the environmental crisis. The minister warned that these marine heatwaves are not isolated incidents but a sign of escalating climate disruption in coastal ecosystems.
She described the situation as a clear sign of a climate emergency playing out beneath the surface, with extreme events like this bloom becoming the new normal.
