“There’s nothing left here,” sighs Javier Franch as he shakes the heavy strand of mussels he’s just pulled to the surface in northeastern Spain. they are all dead

After a long and brutal heatwave ravaged the country this summer, water temperatures in the Ebro Delta, the main mussel-growing area in Spain’s Mediterranean Sea, have reached 30 degrees Celsius (86 degrees Fahrenheit).

And any breeder who did not remove his molluscs in time has lost everything.

But that’s not the worst of it: Most of next year’s harvest also died in one of the most intense marine heatwaves in the Spanish Mediterranean.

Experts said the western Mediterranean experienced an “extraordinary” marine heatwave in late July, with persistently hotter-than-normal temperatures threatening the entire marine ecosystem.

“The high temperatures shortened the season,” says Franch, 46, who has worked for nearly three decades at the company his father founded, whose production has fallen by a quarter this year.

The relentless sun has been heating up the mix of fresh and salt water along Catalonia’s delicate coastal wetlands, where the Ebro River meets the Mediterranean Sea.

On a scorching summer morning in Deltebre, one of the delta’s communities, the clam rafts – long wooden structures with ropes, each capable of carrying up to 20 kilograms of clams – should be teeming with workers scurrying about during the peak season.

But there is little movement.

“We lost the remaining yield, which wasn’t much because we were working to move forward so we don’t go through that,” explains Carles Fernandez, who advises the Ebro Delta’s Mollusc Producers Association (Fepromodel).

“But the problem is that we’ve lost the young stock for next year and we’re going to have a pretty big cost overrun.”

The heat has wiped out 150 tons of commercial mussels and 1,000 tons of juveniles in the delta, according to initial estimates.

And producers calculate their losses at over a million euros ($1,000,000) as they now have to buy young mollusks from Italy or Greece for next year.

“If you have a week with temperatures above 28C there may be some mortality, but this summer it’s lasted almost a month and a half,” with peak temperatures approaching 31C, says Fepromodel boss Gerardo Bonet.

Normally the two bays of the Ebro Delta produce around 3,500 tons of mussels and 800 tons of oysters, making Catalonia the second largest producer in Spain, although it lags far behind the production of Galicia, the north-western region on the colder Atlantic coast.

For years, the Delta harvest has been brought forward, shortening a season that once ran from April to August.

Affected by coastal erosion and a lack of sediment supply, the rich ecosystem of the Ebro Delta – a biosphere reserve and one of the most important wetlands of the western Mediterranean – is particularly vulnerable to climate change.

And that extreme summer, when Spain suffered a 42-day heatwave — a record three times the average for the past decade, says AEMET’s national forecaster — has also left its mark beneath the water’s surface.

“Some marine populations that cannot withstand such high temperatures for a long period of time will suffer the so-called mass extinction,” says marine biologist Emma Cebrian of the Spanish National Research Council (CISC).

“Imagine a forest where 60 or 80 percent of the trees die, with the resulting impact on associated biodiversity,” she says.

The succession of heatwaves on land has triggered another at sea in what could turn out to be “the worst” in that area of ??the Mediterranean since records began in the 1980s, by the time all data is analyzed in November.

Although heat waves in the sea are not a new phenomenon, they are becoming more extreme with worsening consequences.

“If we compare it to wildfire, it can have an impact, but if you continue to have it, that probably means the affected population can’t recover,” Cebrian said.

Experts say the Mediterranean is becoming ‘tropical’ and mollusc farmer Franch is struck by the mounting evidence as his boat slips between empty rafts of mussels in a breezeless bay.

It is considering increasing its production of oysters, which are more resilient to high temperatures but currently only account for 10 percent of its production.

But he hopes it will help secure his future in a sector that directly or indirectly employs 800 people in the Ebro Delta.

“(The sector) is threatened because climate change is a reality and what we are seeing now will happen again,” he says with concern.