Jakarta, April 23 -- As the Ciliwung River winds through Kalibata in South Jakarta, stories unfold of people striving to see another day by catching suckermouth catfish, a species widely recognized for its invasive nature.

Belittled by many, the species-better known as janitor fish-has long served as a source of livelihood for some, including Ajum, 39, who pursues it day in and day out, earning him the moniker of fisherman among locals.

Ajum and his peers see their earnings hinge on the river's condition. A day of calm waters often lets them haul sizeable catches, while poor conditions quickly choke their cash flow.

"When the water is decent, we can catch 20-30 kilograms of fish. During floods, our haul shrinks to 10 kilograms at best,...