September 24, 2022

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The sight of lifeless silvery fish will become more common, according to one marine biologist who believes La Nina will exacerbate mass fish kills in Australia.

Hundreds of dead fish found washed up at Lake Macquarie.
Hundreds of dead fish found washed up at Lake Macquarie. (NBN)

According to Dr Leonardo Guida, a shark scientist and conservationist at the Australian Marine Conservation Society, scenes like the photo above will become more common.

“I suspect these fish kills will increase in the future,” he said, adding that climate change and wet weather is a leading cause of die-offs.

“With climate change it’s quite likely, almost a guarantee, we will see more severe weather events, which can ultimately lead to fish deaths.

“Already we’ve seen some pretty horrible examples.”

Guida explained floodwater can decrease the amount of oxygen available in riverine and marine environments as it can generate bacteria and algal blooms.

Road signs are submerged under floodwater along the Hawkesbury River near the Windsor Bridge.
Road signs are submerged under floodwater along the Hawkesbury River near the Windsor Bridge. (Getty)

“In the context of floods you get massive run-off of water,” he said.

“Animal waste, chemicals, nutrients from fertilisers can get pushed into ecosystems and that can then cause algal blooms, which use up oxygen in the water – reducing the amount of oxygen for other marine life like fish.

“Because of flooding – whether it’s from nutrients, fertilsers, animal waste or decaying plants and animals – marine life is getting choked of oxygen.”

Guida pointed to the Lake Macqurie fish kill observed last weekend and admitted he was “surprised” to see a white-spotted eagle ray among the deceased.

Although it has not been formally confirmed he suspects recent wet weather and flash flooding contributed to the die-off.

"Thousands" of dead fish, including this white-spotted eagle ray, were found washed up on the shore of Manning Park over the weekend.
“Thousands” of dead fish, including this white-spotted eagle ray, were found washed up on the shore of Manning Park over the weekend. ( Jane Goldsmith / NBN news)

The NSW Environment Protection Authority (EPA) is ramping up efforts to determine the cause of the mass kill.

In a statement issued it said heavy metals were an “unlikely” cause of death – signs point towards oxygen depletion.

“The Department of Planning and Environment labs tested for a full suite of metals and found levels below the ANZECC marine water quality guidelines (where guidelines are available),” a statement read.

“Officers observed hundreds of dead fish of various species and sizes with greying around the gills, which suggested possible oxygen depletion.

“A decrease in oxygen content in water is a natural event that can cause rapid fish deaths.

“Residents are warned to stay out of the water until a cause is determined.

Devastating deluge impacts two states

Areas at risk of fish-kills

The entire coast of Australia is at risk of marine and riverine die-offs, Guida said.

With the east coast at the mercy of La Nina, the west coast is experiencing another weather phenomenon; a negative Indian Ocean Dipole, which is fuelling marine heatwaves.

Thousands of dead fish washed up onshore at Coral Bay in March 2022.
Thousands of dead fish washed up onshore at Coral Bay in March 2022. (DBCA Parks & Wildlife Service)

Locals described a coral spawn and fish die-off near Exmouth around Ningaloo Reef in March as “different to anything they’d seen before,” Guida said.

“We saw a coral die-off and a fish kill,” Guida said.

“It’s not definitive but one of the causes they suspect is because of the marine heatwave there were alterations in water currents and temperature.

“What supposedly happened is the coral spawn hadn’t been able to dissipate as effectively and essentially created coral spawn slicks that were trapped close to the coast

“You had this massive concentrated die-off of coral spawn that used up oxygen in the water as they decay leading to the subsequent fish kill.”

Guida said the modification of river systems, wetlands and coastal regions are a “compounding factor” for mass fish kills and called for greater environmental protections around waterways.

A main street is under floodwater on March 31, 2022 in Lismore.
A main street is under floodwater on March 31, 2022 in Lismore. (Getty)

“Coastal ecosystems are under more pressure than ever,” Guida said.

“Whether we modify these systems for residential purposes or farming we can increase the frequency and intensity of floodings because we’re changing the way water naturally flows.

“We need to have stronger environmental laws that support the protection and recovery of these ecosystems so they are more resilient to future impacts.

“That way we can have a healthier future.”

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