Dolphin deaths in South Australia spike after algal bloom decimates food sources

The number of dead dolphins washing up on South Australian beaches spiked in 2025, according to long-term data that reveals mortalities during the state’s devastating algal bloom were the highest in 12 years.

Last year, at least 70 carcasses of common and bottlenose dolphins were found across South Australia, with a further 20 reported in 2026, including the recent death of a popular Port River dolphin known as Zoom.

Many of those found in Gulf St Vincent, a large marine zone west of Adelaide, which was heavily impacted by the bloom, were severely emaciated.

Dr Catherine Kemper, who previously worked as a curator of mammals at the South Australian Museum, said dolphin mortalities in the gulf in 2025 were the highest since 2013 – when dozens of animals were affected by morbillivirus, a type of disease found in cetaceans.

“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.”

Dead dolphins have been recorded by citizen scientists and government staff since the devastating bloom of Karenia cristata algae began in March 2025, along with hundreds of other marine species. A concurrent marine heatwave has affected southern Australia since September 2024.

Kemper, working with the dolphin researcher Dr Mike Bossley, analysed those reports together with museum data from 2001 to 2024 and dolphin postmortems commissioned by the state government, presenting the results at the Australian Mammal Society and Australasian Bat Society conference on Thursday.

While published postmortems into dolphin deaths did not indicate the direct effect of algal toxins, many of the animals were emaciated, Kemper said, which could be linked to the effect of the bloom on their food sources. Southern calamari populations were 80% below baseline levels in Gulf St Vincent and Spencer Gulf, according to government research.

Dead dolphins do wash up for a range of reasons, Bossley said. “Dolphins get hit by boats, get tangled up in fishing gear, get attacked by sharks,” he said, but many in 2025 were very skinny, suggesting a lack of prey, particularly squid in the case of 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.”

The marine heatwave may have been an additional stressor, he said.

Kemper said the 2023 disbanding of a decades-long program for marine mammal postmortems at the SA Museum had hampered analysis of the bloom’s effect on cetaceans. She and Bossley have called for systematic postmortems to be reinstated.

A spokesperson for SA’s Department for Environment and Water said disruption to marine food chains was potentially a contributing factor to increased dolphin deaths in 2025 and during the first five months of 2026.

“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.”

Chronic weight loss has been found in many postmortems for animals tested since the bloom arrived, including dolphins, seals, birds, little penguins and turtles.