The tranquil waters of South Australia's Gulf St Vincent have turned into a grim scene as a devastating algal bloom continues to wreak havoc on marine life. The consequences are stark: at least 70 carcasses of common and bottlenose dolphins were discovered in the gulf last year, with another 20 found so far in 2026 – including that of a well-known Port River dolphin nicknamed Zoom.
Experts point to a critical link between the algal bloom, food shortages, and the surge in dolphin deaths. Dr Catherine Kemper, former curator of mammals at the South Australian Museum, suspects that the depletion of southern calamari – a key food source for common dolphins – is behind the alarming rise in mortalities. The calamari populations were severely depleted due to the algal bloom, with research confirming they fell by 80% below baseline levels in Gulf St Vincent and Spencer Gulf.
The harmful Karenia cristata algal bloom, which began in March 2025, has had far-reaching consequences for marine life. Citizen scientists and government staff have recorded hundreds of species deaths alongside the dolphins, while a concurrent marine heatwave has been affecting southern Australia since September 2024, adding to the pressure on vulnerable ecosystems. Although direct algal toxins were not detected in published post-mortems, the widespread emaciation of dolphins suggests an indirect impact through the decimation of their food sources.
Dr Mike Bossley, a dolphin researcher, agrees that while various factors can contribute to dolphin deaths, the extreme thinness of many animals in 2025 strongly points to prey scarcity. He highlights the severe impact of the algal bloom on squid populations, concluding that a reduction in food availability is the most probable cause for the increased mortalities. Kemper and Bossley's analysis, based on reports from citizen scientists, government staff, museum data, and state government-commissioned post-mortems, was presented at a recent conference.
The algal bloom has also brought into sharp focus the need for systematic post-mortem analysis of marine mammals in South Australia. Dr Kemper and Dr Bossley are advocating for the reinstatement of the long-standing marine mammal post-mortem programme at the SA Museum, which was disbanded in 2023. A spokesperson for South Australia's Department for Environment and Water acknowledges that disruptions to marine food chains may be a contributing factor to the increased dolphin deaths.