A dead sea lion is discovered on one end of the coast, and not far away an entire pod of long-beaked common dolphins lay stranded.
It is becoming a familiar scene for those living along the coast of Southern California.
And much like in South Australia, where thousands of dead sea creatures have washed ashore since March, a harmful algal bloom is to blame.
Warning: This story contains content that some readers may find distressing.
Worst bloom in Southern Californian history
In California, this is the fourth consecutive year of death and destruction along the coastline.
There have been scores of marine mammals and sea birds injured and killed, with US rescuers dealing with “by far historically the largest mass stranding event” relating to algal blooms.
A common dolphin being rescued on a Californian beach in April. (Supplied: Pacific Marine Mammal Center )
In South Australia, the Karenia mikimotoi bloom, kills fish and other sea creatures by impacting their gills, causing them to drown.
In the US, authorities have been dealing with different types of algae, including one that produces domoic acid which can cause mammals like sea lions to experience seizures, behave aggressively or die.
The Pacific Marine Mammal Center (PMMC) said the harmful algal bloom on the Southern California coast had significantly impacted sea lion and common dolphin populations and had killed two humpback whales and a minke whale.
A minke whale found this year at Long Beach, California, died from ingesting fish with domoic acid toxin. (Supplied: PMMC and OARRA)
“In 2025, we’ve experienced the worst domoic acid algae bloom that we’ve ever experienced in Southern California history,” Alissa Deming, PMMC vice president of Conservation, Medicine and Science told ABC News.
“We saw over 2,000 marine mammal strandings related to this bloom and that is by far historically the largest mass stranding event that we’ve ever experienced related to one of these harmful algal blooms along our coast.”
Dr Deming said the PMMC hospital had taken in hundreds of sick mammals, which had been “physically demanding and very emotionally taxing” on staff and volunteers trying to help while protecting the public from aggressive animals.
“A number of the sea lions were also pregnant, and that resulted in reproductive losses of over 85 fetuses born and lost in our hospital as a result of this bloom,” she said.
Sea lions being treated to flush out domoic acid toxin at the Pacific Marine Mammal Center. (Suppleid: Pacific Marine Mammal Center)
The veterinarian said there were likely similar drivers for its bloom to the one happening in South Australia including increasing marine temperatures and more nutrients from cold water upwelling.
“By combining the warm sea surface temperatures with increased nutrients, that has resulted in us having a bloom event every year since 2022, with them seemingly becoming larger, longer lasting and covering a lot larger geographic region,” Dr Deming said.
Investigations are underway if the destructive Los Angeles wildfires in January and the associated run-off have contributed to the bloom.
Damage caused by a wildfire in the Pacific Palisades neighbourhood of Los Angeles, California. (ABC News: Cameron Schwarz)
The US National Office for Harmful Algal Blooms said a conservative estimated cost of harmful algal blooms for the the US was about $50 million, but experts say expenses are hard to quantify.
A 2024 study estimated losses to tourism-related businesses during the 2018 Florida red tide bloom at $US2.7 billion.
Dr Alissa Deming says their center hospital was at capacity with sick mammals. (Supplied: Pacific Marine Mammal Center)
Dr Deming said the US government agencies continued to have monitoring and management practices in place to support the commercial seafood industry.
“Being able to have good federal and state agencies that can do surveillance to test to ensure seafood is safe for the public, as well as trying to come up with preventative measures or best management practices to prevent blooms, … are really high priority items,” she said.
“We’re happy we have some of those systems in place, but there’s absolutely room for improvement I think all around the world to help prevent the conditions that support these really long lasting and devastating blooms.”
The public is warned not to approach stranded sea lions poisoned by the harmful algal bloom in Southern California. (Supplied: Pacific Marine Mammal Center)
Experts say warming climate making blooms worse
Other algal blooms have also wreaked havoc in other parts of the world.
An algae surge choked to death an estimated 40,000 tonnes of salmon in Chile in 2016 while hundreds of elephant deaths in Botswana in 2020 were linked to cyanobacteria, often called blue-green algae in Australia.
The scale of each harmful algal bloom event seems to be increasing, professor Gretta Pecl from the University of Tasmania’s Institute for Marine and Antarctic Studies told the ABC.
“If there isn’t something done to mitigate, to reduce climate emissions, warming will continue and these kinds of events will be more and more likely,” she said.
Kathi Lefebvre (left) and Emily Bowers (right) prepare a whale faecal sample for algal toxin extractions in a Seattle lab. (Supplied: NOAA Fisheries/Amelia Brower)
US research biologist at NOAA Fisheries Kathi Lefebvre, who has been studying harmful algal blooms for 25 years, said the blooms were getting worse and reaching areas “where we used to not have problems”.
“We’re basically seeing that the warming climate is changing the frequency and length and locations and toxicity of harmful algal blooms,”
she said.
“It is turning out to be a major impact of climate change.”
Authorities in South Australia have linked South Australia’s algal bloom to climate change, and state and federal governments have pledged a collective $28 million for a suite of measures to tackle the algal bloom, including more funding for testing.
Dr Lefebvre said authorities needed to take action now to mitigate the effects of future blooms.
“What’s going to happen if these blooms are going to be continuing is that there’s going to need to be ways to test for toxins … to protect human health,” Dr Lefebvre said.
“The more information we have, the better decisions we can make.
“We could start now, but it’s going to take a while for that to have any influence on what’s happening.”