There’s an account of a fifth-century marker that stood on the border between what is now Algeria and Tunisia. On it was recorded the taxes levied on the different kinds of goods that passed its way: dates, grain and, critically, sponges.

Sponges continue to be harvested in Tunisia. Sailing from the Kerkennah Islands and the southern fishing town of Zarzis, Tunisian sponge fishermen have been plying the high-risk trade carried on by their families for as long as anyone can remember.

However, a mysterious blight, similar to one that struck in the 1980s, has resurfaced, decimating the sponge crop and forcing Tunisia’s sponge fishermen to go to even greater depths and take more risks to harvest their catch.

Sponge boat skippers Khessi Mohsen and Kamel Ramdani. Photograph: Simon Speakman Cordall

Sitting in one of the empty classrooms of Zarzis’ fishing college, skippers Kamel Ramdani and Khessi Mohsen describe the dangers of diving for sponges.

“If you make a mistake, like you come up too fast, you can end up in a wheelchair,” Ramdani says through an interpreter. “The nearest hospital [where decompression injuries can be treated] is in Tunis, so people must wait till they get there.” Tunis is 340 miles from Zarzis.

Routinely diving to depths of 50 metres with no air supply other than that pumped through perilously thin tubes from their boats, accidents are commonplace and fatalities not unknown. Depending on the depth of their catch, divers can spend between one and three hours under water, passing their catch to the boat above via baskets hauled up by the crew.

“People get injuries to their ears and, if there is a problem with the mask, with their eyes,” Mohsen adds. “Sometimes people get drunk, but without wine [nitrogen narcosis], and they will take off the mask, or the air pipe [can become trapped]. Sometimes, people just get tired.”

Some boats now carry expensive decompression chambers, which help to speed up the work and reduce the risk, though accidents still occur.

Ramdani and Mohsen speak about how their grandfathers hunted sponges from small boats equipped with little but glass-bottomed buckets and harpoons, searching for changes in the colour of the long weeds on the seabed that indicated the presence of sponges among the rocks, a method still used in the shallow waters around Kerkennah today.

Fishermen have to dive ever deeper to reach the sponges. Photograph: DeAgostini/Alamy

In Zarzis, sponges so close to the shoreline are now the stuff of folklore. And the fields further out to sea – which have been maintained for decades – are now threatened by the current blight.

For Ramdani and Mohsen, this means heading out for weeks at a time, sailing into deeper and potentially more dangerous waters as they harvest the dwindling crop of sponges. Some boats have even strayed as far as the Italian island of Lampedusa. Both men acknowledge the strain this puts on their families, but that’s how it is.

Later, on board Ramdani’s vessel, Mohsen describes how he and his crew can work from sunrise to sunset, sleeping on deck and resting only when locked within the decompression chamber that sits on the stern of the boat.

A sponge diver with his catch off Tunisia’s Kerkennah Islands. Photograph: Joerg Boethling/Alamy

“There used to be about 50 sponge boats in Zarzis,” Mohsen explains, “Now there are only about 12.” With more and more sponges succumbing to the blight, much of the Zarzis sponge fleet is being forced to find new purpose amid the armada of fishing vessels that operates from the port.

In 2017, 17 tonnes of sponges were landed in Tunisia. In 2018, that figure dropped to just 6 tonnes. By September of this year, the catch had made something of a recovery, with 9 tonnes of sponges landed.

The disease wiping out Tunisia’s sponges remains a mystery. “We don’t know,” Mohsen explains. “It could be pollution from the gulf of Gabes,” where 13,000 tonnes of industrial pollutants are reportedly released daily into the sea, “it could be climate change heating the water. We don’t know.”

Karim Ben Mustapha of the Institut National des Sciences et Technologies de la Mer in Tunis is equally perplexed. Despite the advanced scientific tools at the institute’s disposal, the intricate nature of sponges, whose whole bodies are essentially highly localised ecosystems, as well as their role as marine filters, leaves them susceptible to any number of maladies.

However this isn’t the first time the sponges have succumbed to a blight. “There was a widespread disease across the Mediterranean between 1986 and 1987,” Karim says. “That killed off a lot of the sponge fields, but then they started to renew.”

However, while Ramdani and Mohsen are quick to draw comfort from the industry’s past recovery, this current blight is worryingly unique.

“This disease isn’t the same,” Karim explains. “This is something that’s only been documented in Tunisia.”

The Mediterranean sponge population was affected by a blight in the 1980s, but fishermen think this time is different. Photograph: Joerg Boethling/Alamy

According to Ramdani’s observations, sponges found below a depth of 40 metres are unaffected by the blight, leading Karim to conclude that the cause is both local and specific. “Normally, there’ll be a thermocline at about 20 or 30 metres that protects the deeper water [from surface heat]. If this thermocline disappears, then you have a big problem.” Establishing why a thermocline may have disappeared is no easy task, Karim says. “I cannot tell you if the main cause of the disease is pollution or global warming. I think it’s a mix.”

However, there is still cause for hope. Sponges are one of the oldest forms of life on the planet. They have adapted to ice ages and survived past plagues. The odds remain in their favour. For Ramdani, already training his seven-year-old son in the art of sponge fishing, the future may not be over, yet.