• Two reported mudflows overflowed the Serayu River on the Indonesian island of Java within the space of a week earlier this year, mucking its waters and killing off fish.
• The incidents have raised calls from conservationists and fishers for restoration and fish restocking efforts in the river.
• Fish restocking has been carried out for several decades in Indonesian rivers, mainly as an effort to increase fisheries productivity and fish populations in inland waters such as rivers, reservoirs and lakes.
• But experts say these restocking efforts often emphasize ceremony at the expense of measures to ensure success, such as post-monitoring assessment.

A muddy flood polluting a river on the Indonesian island of Java earlier this year depleted its fish stock, raising calls for restoration and restocking efforts in the body of water that empties out into the Indian Ocean.

Two reported mudflows overwhelmed the Serayu River within a week between March 30 and April 6, mucking its waters that cross five districts in Central Java province. Both incidents were attributed to the activities of a nearby hydropower plant operated by PT Indonesia Power Mrica Banjarnegara. The company’s hydroelectric dam opened its gates to flush out agricultural waste runoff that had piled up in the reservoir.

Local fishers have blamed the subsequent sedimentation and low oxygen levels in the river for polluting the water and killing many of the fish. The Serayu River is home to at least a dozen endemic freshwater fish species.

“I’ve estimated that hundreds of thousands of endemic fish and millions of small fish died … because of the extremely high mud concentration,” Eddy Wahono, chairman of the local group Serayu Downstream Water Resource Management Society Forum (FMPSDA), told Mongabay Indonesia in April.

Indonesia has nearly 20 million hectares (49 million acres) of inland waters, most of it swamps and wetlands, but also 1.7 million hectares (4.2 million acres) of rivers, according to the fisheries ministry.

River pollution from human activity is rampant in Indonesia, hammering freshwater fish populations across the country. A biodiversity hotspot, Indonesia is home to more than 1,300 species of freshwater fish, the highest in Asia. Up to 80% of the country’s rivers are considered to be in poor condition, particularly those on Java, where more than half of Indonesia’s 270 million people live, according to the environmental group Ecoton.

Fisheries experts have called on the government to restore the river’s waters and restock its fish, especially the endemic species, to prevent further damage to the local ecosystem.

The full story from Mongabay can be read here.