Mounds of slimey carcasses pile up on a riverbank in Jakarta where authorities are fighting an uphill battle against a fast-breeding invasive fish flourishing in Java island's heavily polluted waterways.
The sapu-sapu, a sucking fish introduced from South America decades ago to keep Southeast Asian aquariums algae-free, has made a home in Javan rivers, many after being thrown out for outgrowing their tanks.
Unlike in the Amazon River where it has natural predators, the sapu-sapu quickly took over in Java, outbreeding indigenous freshwater fish, eating their eggs and outcompeting them for food.
Residents and campaigners have complained about the stinky, unsightly pileups of fishy remains, and the potential health risk for those who consume snacks made from polluted sapu-sapu stock.
Invasive species are spreading ever faster across the globe and cost well over $400 billion a year in damages and lost income, according to a 2023 United Nations assessment.
Surprisingly adept at living in polluted water, the sapu-sapu -- also known as suckermouth catfish, janitor fish or pleco -- can grow to half a meter (20 inches) in length and a female can lay thousands of eggs every year.
"That there are thousands of (sapu-sapu) fish in some of these river bodies, where, you know, the rivers are like dark black, almost smelling like rotten eggs... is completely crazy," clean river campaigner Gary Bencheghib told AFP.
Killing sapu-sapu is not addressing the real problem, he added in a phone interview, nearly halfway through a 1,200-kilometer (746-mile) run from Bali to Jakarta to raise money for river cleanups and having witnessed the crisis at several stops along the way.
The real solution is cleaning up "the waste it lives off... that you find in these polluted waters," said Bencheghib, co-founder of the Sungai Watch NGO.
- 'A dangerous snack' -
More than half of Indonesia's rivers are heavily polluted, and two of the country's major river systems are among the dirtiest in the world, according to data from the UN Environment Program.
Nationwide, only about 7.4 percent of municipal wastewater is safely collected and treated.
Greater Jakarta, a water-stressed metropolis of 42 million people, is particularly affected.
But sapu-sapu are able to live in oxygen-poor and polluted waters, digging holes into riverbanks to lay their eggs and weakening walls that have been known to collapse as a result.
In recent weeks, Jakarta has embarked on a killing campaign involving residents, sanitation workers, fisheries ministry workers and even soldiers.
In two weeks, about 5.3 tons of the fish have been removed from rivers in South Jakarta alone, mayor Muhammad Anwar told AFP while overseeing an operation at Babakan Lake this week.
The sapu-sapu are caught in nets, separated from indigenous fish that are thrown back, then decapitated to make sure they are dead, and buried in mass graves.
Some activists have criticized the procedure as wasteful, arguing the skin could be used for fish leather products and the rest processed into fertilizer or animal feed.
But others say the fish are too contaminated to be useful, and definitely not fit for human consumption.
"It contains... heavy metals and is dangerous to humans," Anwar said.
Tests conducted for scientific studies have found traces of lead and mercury as well as E.coli bacteria beyond safe levels in sapu-sapu.
"In some places, it is consumed as a dangerous snack called siomay (steamed fish dumpling) and pempek (fish cakes)," the mayor said.
"Please be careful when buying fish cakes. Don't be led by the (cheap) price."
