Samir Sdiri is insistent. “There are hardly any fish left. Those that they do catch are dirty. If you open up their gills, you can see that the inside is black.”

Against the cafe’s chatter, Lobna Ben Ali Bouazza nods in agreement. “When I was a child, my parents would let us play on the beach here all day, swimming in the sea – everything. These used to be the best beaches around. Now I take my children elsewhere.”

Outside the cafe, the residents of La Goulette, a small fishing suburb to the north of Tunis, go about their business. It’s the first few days of June and the beaches are relatively empty. A few couples drift across the sand, while upturned fishing boats lie idly in the noonday sun. These beaches will be unrecognisable in the summer, as families from throughout greater Tunis, looking to escape the soaring temperatures, take the shuttle train across the capital’s lake, past the industrial ports of Rades, and on to the beaches, where their scattershot encampments jockey for space over the crowded sands.

It isn’t just the waters off La Goulette that are causing concern. The entire Gulf of Tunis is drawing activists’ ire, as domestic and industrial waste from the capital’s 600,000 plus residents, in addition to that flowing from the ports and the industrial estates that line the Gulf, makes its way into the waters outside Tunis, impacting fish populations and presenting a clear hazard to human health.

Tunisia’s pollution issues aren’t new. Its heavy industries have been impacting water quality for years. However, since the revolution of 2011, conversation over the environmental impact of its industrial legacy has at least become possible, even if the kind of reversals activists are calling for remain some way off.

Officially around a quarter of Tunisia’s waste water is recycled, intended, among other things, to irrigate the country’s farmlands. The rest (around 247m cubic metres a year), is expelled from the country’s treatment plants directly into the sea and inland waterways. According to environmental regulations, industrial waste water should initially be treated at source, before being transferred for further treatment. However, campaigners question how rigorously this is being enforced.

There are three hulking water treatment plants servicing the population around the Gulf of Tunis, at Raoued, to the north-west of the Gulf, Rades, near La Goulette on the western side and Souliman, at the gulf’s industrialised southern reaches.

All are operated by Onas (L’Office National de l’Assainissement), a subdivision of the ministry of the environment and sustainable development and, according to activists, heavily subsidised by loans from international bodies.

“It’s crazy,” Morched Garbouj, president of the environmental pressure group SOS BIAA, tells the Guardian. “We tested the water going into these treatment plants and we tested it going out and, I can tell you, there’s very little difference.”

Across Tunisia, industrial and domestic waste water is channelled from broad areas to large treatment plants. Within the Gulf, the outcomes are clear. “We tested both input and output flows between 2016 and 2017 and the results were consistent,” Garbouj, an environmental engineer, says. “We found increased levels of nitrates, manganese particles, phosphate plus faecal coliforms and streptococci – both present within human waste – among other matters. All of these are harmful to health.

“The government has disputed these findings, but they haven’t shared their methodology with us, so it’s hard to say how credible those denials are.

“Waste water treatment in Tunisia is entirely centralised. Everything goes through Onas, including the development loans from, say, the World Bank, the EU and the German Development Bank. We’ve taken our findings to them. They’re aware of what’s happening. They know it’s not working. They’re just not interested.

“It seems no one really cares how well the treatment plants are performing. Onas, which runs the plants, is a subdivision of the ministry of the environment and, you know who’s responsible for testing their effectiveness? The ministry of the environment,” Garbouj says.

Wafa Hmadi, a programme coordinator with the environmental group RAJ Tunisie is equally damning, “It isn’t just the Gulf of Tunis,” she tells the Guardian. She says that around the industrial town of Sfax, and Gabes – near the phosphate mining basin of Gafsa – whole stretches of coastline were “unusable”.

The state-owned Tunisian Chemical Group’s phosphate processing plant, in front of a rare coastal oasis in Gabes. The authorities say the plant pumps 14,000 tonnes of phosphogypsum into the sea every day. Photograph: AFP/Getty Images

“Many of Tunisia’s inland waterways are also affected by heavy industry, such as paper manufacturing. Pollutants from industry find their way out into the local environment, impacting local populations, before heading out to the sea. Fish, particularly larger fish, are dying. Some areas are just totally dead.

“There really is hardly any monitoring. Industrial polluters can expel their waste largely untreated, as there are no inspections and no one is holding them accountable,” she says.

Little of this is news for Lobna and Samir. They and their families have been living with the outcome of rising water pollution for years. However, in a country struggling with ingrained unemployment and desperate to fight back against an ever increasing cost of living, both are equally aware of the desperate need for industry and jobs.

However, for a city built by the sea, change comes slowly. Outside the cafe, along the sun-drenched shoreline, children swim in the the cloudy waters, or dive off the walls of La Goulette’s ancient canals as they have done for years.

“People come here. They always will,” says Lobna.

The Tunisian ministry of the environment has not responded to requests for comment.