What’s causing the fish kills in South Australia?
Since March 2025, a harmful algal bloom has impacted coastal waters across the St Vincent Gulf and South Australia’s southeast coast. It’s one of the largest events of its kind in the state, and it’s having a serious impact on local fish and marine life.
Here’s what’s going on — and what it could mean for the future.
What is a fish kill?
A fish kill is the sudden death of fish and other aquatic animals in large numbers, often caused by extreme environmental changes. These can include drought, disease outbreaks, pollution, or — as we’re seeing now — harmful algal blooms and marine heatwaves.
Most fish kills are short, sharp events, but the long-term impacts can ripple across ecosystems and food chains.
How marine heatwaves and algal blooms stress fish
A marine heatwave is a period of unusually warm ocean water. These warmer temperatures hold less oxygen and can disrupt habitat, prey availability and fish behaviour. When a heatwave coincides with other stressors, like a harmful algal bloom, the result can be lethal.
Algal blooms occur when microscopic algae multiply rapidly in warm, still water rich in nutrients. Some blooms are harmless, but others — like the current one in SA — produce toxins that can damage fish gills, impair their nervous systems, or deplete oxygen in the water. That combination is deadly, especially for species that can’t move far to escape.
What’s happening in SA?
The 2025 event began in mid-March when warm temperatures, still conditions, and nutrient-rich upwellings created the perfect storm. By April, the algal bloom stretched over 4,400 square kilometres — an area the size of Kangaroo Island.
Water testing has confirmed the presence of Karenia mikimotoi, a toxic algae linked to past fish kills in SA and around the world. Oxygen depletion and high concentrations of toxins are affecting a wide range of species, particularly bottom-dwellers and reef-associated fish like leafy sea dragons, leatherjackets, and pufferfish. Shellfish, crabs, stingrays and cuttlefish have also been impacted.
Fortunately, popular fishing species like whiting, salmon and tuna appear to be avoiding the worst-affected areas so far.
Citizen science in action
OzFish members and other citizen scientists have been documenting what they’re seeing in real-time. More than 1,400 observations have been submitted to iNaturalist since March, covering over 200 marine species. This data is helping build a clearer picture of the bloom’s impact and supports long-term monitoring and recovery.
Can you still go fishing?
Yes, but only in unaffected areas. Avoid fishing or swimming where fish have washed up or where water looks or smells off. Algal toxins can be harmful to people too — especially through contact, breathing in particles, or eating contaminated fish or shellfish.
What’s next?
There’s no quick fix for an event of this scale. Algal blooms this large typically break up naturally when conditions change — cooler weather, rougher seas, and lower nutrients will eventually help. But as climate change increases the frequency and severity of marine heatwaves, events like this may become more common.
That’s where long-term restoration comes in. Restoring shellfish reefs, seagrass meadows and kelp forests can improve water quality, provide refuge for fish, and help build resilience against future impacts. Efforts upstream — like stabilising banks, reducing nutrient runoff, and tackling pollution — also play a key role in keeping our coastal waters healthy.
Feature image is a Southern fiddler ray submitted by S. Hartley at iNaturalist.org.