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Mass Die-Off of Farmed Salmon Linked to Climate Change

Scientists have attributed an algal bloom that killed off $800 million worth of salmon in Chile to rising ocean temperatures, and they say other aquaculture operations around the world are at risk.

Written by Matthew O. Berger Published on Read time Approx. 4 minutes
Cages belonging to a seawater salmon farm in Puerto Montt, in southern Chile, in 2016.Martin Bernetti/AFP/Getty Images

Wild fish aren’t the only ones feeling the squeeze from climate change.

As warmer, lower-oxygen waters spread, domesticated fish are suffering as well.

In Chile, for instance, an algal bloom in 2016 killed nearly 12 percent of the country’s farmed salmon at a cost of $800 million. Scientists have now attributed that bloom to conditions that arose as a result of climate change, according to a new study published in the journal Scientific Reports.

But it won’t be the last time climate change is tied to die-offs in aquaculture operations, scientists say, and harmful algal blooms may be the prime cause of those losses.

“Climate is changing globally, and we have all experienced those changes in our lifetimes,” said Mauricio Urbina, a biologist at Chile’s University of Concepcion and a co-author of the study. “Therefore, these new and unexpected interactions or synergies are expected to occur more frequently.”

Urbina added that not all parts of the world are experiencing the same climatic changes so global warming won’t necessarily lead to more algal blooms everywhere. But in northern Patagonia, he said, “harmful algal blooms could occur more frequently.”

Chile is the second-largest producer of farmed salmon, with operations tucked in the sheltered bays and fjords of the South American country’s rugged coastline. But those locations can also make fish farms susceptible to deadly algal blooms.

In Patagonia, Urbina said, “we have predicted that in a ‘dry’ year the risks of harmful algal blooms will increase, while in ‘wet’ years the risk will decrease.”

A strong El Nino event during the austral summer of 2015-16, combined with weaker winds due to changes in another climate pattern, the Antarctic Oscillation, made the region much hotter and drier than normal. That, the researchers found, reduced ocean circulation, increased surface temperatures, reduced freshwater flows into the sea and led to increased volumes of nutrient-rich water reaching surface waters, where the Pseudochattonella verruculosa algae bloomed.

Once that algae started dying, its decomposition consumed so much oxygen that oxygen levels in the waters plummeted, suffocating salmon and other marine life – both wild and those living in pens – over the course of two weeks.

The bloom was one of several extreme algal events to take place in recent years. The study notes that such “super blooms” along the Pacific coasts of North and South America and in lakes have recently “exhibited an unprecedented extent and intensity, suggesting that climate change and other drivers are already increasing the risk of these events,” which scientists call a “harmful algal bloom” (HAB).

“Most HAB experts believe that changing climatic conditions are a factor in the observed increase in HABs,” said Jack Rensel, a biological oceanographer and aquaculture expert who is a principal investigator with the firm AquaModel.

Though he notes it’s difficult to determine the precise causes of blooms, in general, warmer waters, less mixing in the water column and slower ocean circulation – phenomenon all linked to climate change – will increase the occurrence of certain types of harmful algal blooms.

“We expect more HAB events not only in Chile but throughout many of the world’s oceans,” Rensel said.

Increased nutrient runoff from cities and farms doesn’t help. Some blooms, such as those that lead to annual dead zones in the Gulf of Mexico and elsewhere, are caused largely by agricultural fertilizer runoff or sewage. But that’s not the case in some of the sparsely populated waters of southern Chile where blooms have frequently occurred, according to Rensel.

In other places, blooms can be caused by nutrient pollution from the aquaculture operations themselves, as fish waste and uneaten food can accumulate in concentrated areas if the currents aren’t strong enough to flush out pens.

“It doesn’t take rocket science to figure out you’re going to get nutrient enrichment in some ways from these operations,” said Hans Paerl, a marine science professor at the University of North Carolina-Chapel Hill’s Institute of Marine Sciences who specializes in nutrient cycling and harmful algal blooms.

But climate change adds new dimensions to the problem as increasing water temperatures benefit cyanobacteria and other algae. “With cyanobacteria, the hotter it gets they’ll just keep increasing their growth rate, so the max rate is well above what extreme climate conditions are now,” said Paerl.

A shorter period of ice cover in many places also opens up a bigger window for algae growth and record rains, storms, droughts all “essentially play into the playbook of these harmful algal bloom species,” according to Paerl, by either increasing runoff of nutrients or decreasing water flows through coastal systems that would be needed to flush blooms out.

The booming aquaculture industry is fighting back.

Rensel said that in parts of coastal China, “where scallop farms and other aquaculture stretch as far as you can see,” ships spread a processed clay in the ocean that aggregates algal blooms. Other fish farms, he said, will pump air below blooms to aerate the water and cause algae-free water to circulate up.

But, he said, “most mechanical or chemical methods to remove HABs range from inefficient and expensive to harmful,” such as copper sulfate-based algaecides that have been used on freshwater blooms but can themselves be toxic to other marine life.

Locating farms in areas with good water circulation and not overloading the area with farms and nutrients is key, according to Rensel.

“It does not take a biological oceanographer to fly over the northern parts of the Chilean Inland Sea to observe that there are a lot of salmon farms but also many mussel farms interspersed,” Rensel said.

“In the past,” he added, “the approach had been to add more farms until there was a problem and resulting fish die-off, maybe not intentionally but that is how it worked out.”

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