Scientists fear toxic algal bloom has suffocated SA’s great white sharks as Port River dolphin tragedy sparks calls for action

Experts fear a deadly algal bloom may have “suffocated” South Australia’s great white sharks. It comes as a much-loved Port River dolphin was found dead.

A dead great white shark found on a rocky stretch of Southern Yorke Peninsula coastline. Picture: Shark Watch SA

A deadly algal bloom devastating South Australia’s coastline may have “suffocated” some of the state’s most iconic ocean predators, scientists have warned, as investigations into a spate of great white shark deaths continue.

Necropsy reports of great white sharks that washed up dead on SA shores have been released as the state’s algal bloom continues to affect beaches and estuaries along parts of the state’s coastline.

The reports – released on the Department of Environment and Water website – include details of nine white sharks that washed up on South Australia’s beaches from April to July.

The dead sharks ranged in length from over four metres to one just 1.5 metres.

A dead great white shark at Port Broughton being taken away by a Fisheries officer. Picture: Danielle Griffin

SARDI executive director Professor Mike Steer said three sharks examined showed clear signs of damage to their gills – a hallmark of exposure to Karenia species algae – while five others were too decomposed to confirm a cause but likely suffered the same fate.

“Out of the nine, three of them were clearly impacted by the algae,” Prof Steer said.

“Five were too decomposed to tell, but given the timing, there’s a likelihood they too were affected.

“The bloom effectively impacts the gills and prevents the animal from breathing appropriately.”

The toxic bloom can also strip oxygen from the water as it dies off, putting further stress on gill-breathing marine life.

“Effectively, they suffocated,” Prof Steer said.

“It’s unfortunate to see, but the biology matches up – sharks are fish, they have gills.

“From a biological perspective, it makes sense.”

The unprecedented bloom, which has killed vast numbers of fish since March, could also be triggering knock-on effects throughout the food chain, potentially starving larger predators by disrupting their prey base.

“There’s clearly a cascading impact across the food web,” Prof Steer said.

“The large amount of fish species washing up will affect higher predators that rely on them for food.”

While six of the sharks tested had no cause of death identified, three had physical symptoms consistent with a cause of death related to exposure to high levels of Karenia.

Brevetoxins – a group of neurotoxins produced by Karenia species – have been identified as part of SA’s devastating bloom event.

In low levels, the karenia species does not pose serious problems, but when environmental conditions favour its growth, it can become a harmful algal bloom.

Prof Steer urged the public to immediately report any future shark standings, as decomposition can quickly destroy key evidence such as gill tissue.

“If a shark does wash up, we need to know quickly so we can collect the best information possible,” he said.

“Once they decompose, we lose the value of determining cause of death.”

Shark Watch SA – an online, interactive service providing coastal water users with up-to-date information about shark sightings – is in the process of co-ordinating an independent shark dissection/necropsy project.

A great white shark washed up on Henley Beach on May 5. Picture: Benjamin Neville/@offpiste4wdtours

The group plans to increase the amount of available resources examining the biological impacts of the algal bloom on different shark species affected by the algal bloom.

“Our Shark Dissection/Necropsy Project is ideal for marine biologist students and postgraduates looking to refine their hands-on examination work on these precious species during this volatile time,” a post on social media said.

“The more volunteers we have on-board equates to more specimens being dissected and a wider network of available resources conducting vital independent research at any one time.”

Heartbreak as beloved Port River dolphin found dead

In a separate blow to SA’s marine environment, a much-loved Port River dolphin known as Rocket was found dead this week, sparking renewed calls for stronger protections in Adelaide’s Dolphin Sanctuary.

Rocket – orphaned as a calf after her mother Ripple died from an unknown condition – survived against the odds, reaching several years of age despite losing her mother when she was just seven months old.

Beloved Port River dolphin Rocket with his mum Ripple. Picture: Supplied.

But she was found dead on Thursday night with a large gash severing her spine.

A necropsy will determine the cause of the fatal injury, though authorities say there is no suggestion it is linked to the algal bloom.

Rocket was seen hours before her death feeding and playing with other dolphins.

“I am angry and grief-stricken,” said Marianna Boorman, who has documented Rocket’s life since birth.

“Rocket was a symbol of the many hazards facing dolphins in the Adelaide Dolphin Sanctuary, and more needs to be done to protect them.”

Much-loved Port River dolphin Rocket died Thursday night. Picture: Supplied.

Whale & Dolphin Conservation’s Declan Andrews said Rocket’s death highlighted the urgent need for more rangers and safety measures such as propeller guards on boats.

Veteran dolphin researcher Dr Mike Bossley, whose work led to the sanctuary’s creation, said the risks were already well understood.

“Improving protection for these dolphins is not a matter of more research,” he said.

“The hazards are well known. What is needed is more rangers on the water, more often, to educate people and ensure compliance with boat speed rules.”

The organisation will seek a meeting with the Minister and the Department of Environment and Water to discuss stronger protections for the dolphin population.

Dolphin sanctuary advocates say more rangers are needed to enforce boat speed rules. Picture: Supplied.

White shark death reports

April 18: Port Moorowie, 2.7m. Physical symptoms, including gill damage, were consistent with a cause of death related to exposure to high levels of Karenia.

April 18: Port Willunga, 1.5m, no cause of death was identified.

May 2: Ardrossan, 3.2m, no cause of death was identified.

May 5: Henley Beach, 3m, physical symptoms including gill damage, were consistent with a cause of death related to exposure to high levels of Karenia.

May 13: Port Wakefield, 3m, no cause of death was identified.

May 16: Sultana Point, 3.2m, no cause of death was identified.

May 30: Stansbury, 4.2m, no cause of death was identified.

June 27: Port Broughton, 3.5m, physical symptoms (including inflamed gills) were consistent with a cause of death related to exposure to high levels of Karenia.

July 15: Port Turton, no cause of death was identified