Set between irrigated farmland and rocky desert southwest of Cairo, once pristine Qarun Lake used to teem with tilapia, bass and bream and shrimp, offering generous catches to fishermen from surrounding villages.

But pollution from agricultural runoff and industrial and domestic waste has in recent years sullied its beauty and sent those stocks plunging, destroying an industry that had provided jobs for generations, fishermen say.

"The lake was so generous, all kinds of fish were abundant. The taste of the fish was the best and the prices were good," said one, Farag Abdel Sattar Awad.

"But the lake became polluted, so people started to find alternatives to fishing," he said. Only tiny fish of little value can still be found there, said his brother Ramadan, also a fisherman, seated on his boat on the lake's rubbish-strewn shores.

Some former fishermen have adapted their small wooden boats to offer day trips for weekend visitors from Cairo, while others migrated to cities in eastern or southern Egypt.

The 42km-long (26-mile) lake lies below sea level and was fed in the time of the pharaohs by Nile floods.

Now it is replenished by drainage water and has become increasingly saline, with marine species introduced to replace freshwater fish.

Egypt's scarce farmland along the Nile is farmed intensively, and agricultural run-off has damaged water quality in Qarun Lake, a problem documented in a 2017 government study that also noted increased salinity due to evaporation.

A 2020 study published in Environmental Science and Pollution Research, an international journal, also showed higher-than-normal contamination with metals including copper, zinc, cadmium and lead.

Magdy Allam, an environmental expert at Egypt's Climate Information Center, an official research group, said work was under way to purify local sewage water and cut Lake Qarun off from drainage from nearby farms.

But its waters have deteriorated rapidly in part because it is an inland lake. "Closed lakes ... have a more fragile eco-system than other lakes that have open access to seas and oceans," he said.

Some fishermen have tried relocating to Wadi El Rayan, a nearby oasis, but that was also found to be polluted in the 2020 study.

Mohsen Mofreh, who owns a local seafood restaurant, said the area used to produce enough fish to export to major cities.

"The number of fish back then was uncountable, about 20 or 30 tonnes per day. Nowadays, not even one kilogram," he said.