At least 70 common and bottlenose dolphin carcasses were found on South Australian beaches in 2025, the highest mortality rate in 12 years, according to long-term data presented at a scientific conference. The deaths coincide with a devastating algal bloom that began in March 2025 and a concurrent marine heatwave affecting southern Australia since September 2024.
Highest dolphin mortality since 2013
Dr Catherine Kemper, former curator of mammals at the South Australian Museum, analysed reports from citizen scientists and government staff, alongside museum data from 2001 to 2024 and government-commissioned postmortems. She presented the findings at the Australian Mammal Society and Australasian Bat Society conference on Thursday. The 2025 mortality in Gulf St Vincent, a large marine zone west of Adelaide heavily impacted by the bloom, was the highest since 2013, when morbillivirus killed dozens of dolphins.
"We suspect strongly that for common dolphins the underlying cause was a food shortage," Kemper said, "because one of their major prey is southern calamari, and southern calamari populations were just decimated in Gulf St Vincent during the algal bloom." Government research found southern calamari populations were 80% below baseline levels in Gulf St Vincent and Spencer Gulf.
Emaciation and food chain disruption
Many dolphins found in Gulf St Vincent were severely emaciated. While published postmortems did not indicate direct effects of algal toxins, Kemper said emaciation could be linked to the bloom's impact on food sources. Dr Mike Bossley, a dolphin researcher working with Kemper, noted that dolphins die from various causes, including boat strikes, fishing gear entanglement, and shark attacks. However, many in 2025 were very skinny, suggesting a lack of prey, particularly squid for common dolphins.
"We know that the algal bloom really hit squid populations very hard. It seems likely that most of the increase in deaths can be attributed to a reduction in food availability," Bossley said. The marine heatwave may have been an additional stressor.
Additional deaths and ongoing impact
At least 20 more dolphin carcasses have been reported in the first five months of 2026, including the recent death of a popular Port River dolphin known as Zoom. The bloom of Karenia cristata algae has also affected hundreds of other marine species. Chronic weight loss has been found in postmortems of dolphins, seals, birds, little penguins, and turtles tested since the bloom arrived.
A spokesperson for South Australia's Department for Environment and Water said disruption to marine food chains was potentially a contributing factor to increased dolphin deaths. "While dolphins do not have gills and are therefore not directly affected by the algal bloom in the same way as fish, experts believe that some marine wildlife may have been affected by disruption to marine food chains and the effects of algal biotoxins," the spokesperson said. "Findings of weight loss suggest indirect effects of the algal bloom on the animal's food sources as a cause of illness or death."
Call for reinstated postmortem program
Kemper said the 2023 disbanding of a decades-long marine mammal postmortem program at the SA Museum hampered analysis of the bloom's effect on cetaceans. She and Bossley have called for systematic postmortems to be reinstated to better understand and respond to such events.
