- Two reported mudslides overflowed the Serayu River on the Indonesian island of Java in the space of a week earlier this year, dirtying its waters and killing fish.
- The incidents have prompted calls from conservationists and fishermen for restoration and fish restocking efforts in the river.
- Fish restocking has been carried out for several decades in Indonesian rivers, mainly with the aim of increasing fishery 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 evaluation.
BANYUMAS/JAKARTA, Indonesia – A muddy flood polluting a river on the Indonesian island of Java earlier this year depleted its fish stock, prompting calls for restoration and restocking efforts in the body of water that is flows into the Indian Ocean.
Two reported mudslides submerged the Serayu River in a week between March 30 and April 6, fouling its waters that flow through five districts in Central Java Province. Both incidents were attributed to the activities of a nearby hydroelectric plant operated by PT Indonesia Power Mrica Banjarnegara. The company’s hydroelectric dam opened to flush out agricultural waste that had accumulated in the reservoir.
Local fishermen blamed the subsequent sedimentation and low oxygen levels in the river for polluting the water and killing many fish. The Serayu River is home to at least a dozen endemic species of freshwater fish.
“I estimated that hundreds of thousands of endemic fish and millions of small fish died…due to the extremely high concentration of mud,” said Eddy Wahono, chairman of the local Serayu Downstream Water Resource Management Society. Forum (FMPSDA), in Mongabay Indonesia. in April.
Indonesia has nearly 20 million hectares (49 million acres) of inland waters, mostly swamps and wetlands, but also 1.7 million hectares (4.2 million acres ) of rivers, according to the Ministry of Fisheries.
Pollution of rivers due to human activity is rampant in Indonesia, hammering freshwater fish populations across the country. A biodiversity hotspot, Indonesia is home to over 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 in Java, where more than half of Indonesia’s 270 million people live, according to environmental group Ecoton.
“Before the mudslide polluted the Serayu, I used to stock up on catfish daily,” Aris, 54, owner of a food stall near the river, told Mongabay Indonesia a few weeks after the last mudslide incident. “But after the Serayu River pollution incident, they completely disappeared.”
Fisheries experts have called on the government to restore the river’s waters and restock its fish, especially endemic species, to prevent further damage to the local ecosystem.
“It is quite difficult and time-consuming to bring the river back to its original state,” Windiariani Lestari, an ecologist at Soedirman General University (UNSOED), told Mongabay Indonesia.
Windiariani said the rivers can self-purify, but it will take years without any additional restoration efforts. “With the mud polluting the river and killing fish and other biota, the natural food chain of the Serayu River has been greatly affected,” she said.
Isdy Sulistyo, head of UNSOED’s Department of Fisheries and Marine Sciences, suggested restocking fish as part of recovery efforts in the Serayu River, located about 300 kilometers (190 miles) southeast of the capital, Jakarta.
Fish restocking has been done here since before Indonesia’s independence in 1945. The former Dutch colonial rulers did it mainly for the purpose of increasing fishing productivity and fish populations in inland waters , such as rivers, reservoirs and lakes, according to the Department of Fisheries. . Baby fish born and raised in aquaculture are collected and moved to the designated water body for the restocking effort. In addition to boosting fishing, it can also serve as a way to address threats to inland waters, such as the impacts of climate change, overfishing, invasive species, habitat degradation and pollution, according to the experts.
“Restocking is an effort to conserve endemic fish species,” I Nyoman Radiarta, head of the research department at the Ministry of Fisheries, told Mongabay in an interview. “Inland waters are very important for people’s livelihoods as a source of fresh water, biodiversity, food security and a source of income.”
Radiarta said restocking efforts need to be carefully studied and conducted to ensure they are successful in rebuilding fish populations and preventing unwanted damage. He said fish releases had to meet several criteria, including that the fish were the right size and weight, that they were not hybrid variants, and that the season was right for their release.
“Fish restocking activities are often carried out by environmentally conscious communities, academics and government agencies to commemorate certain events,” Radiarta said. “However, in its development, the fish species is often not adapted to the aquatic ecosystem, which can instead pose a threat to the endemic fish population.”
Other experts have also criticized the emphasis on ceremony during fish release efforts at the expense of measures to ensure success, such as post-monitoring evaluation.
“There are various reasons why post-monitoring is lacking, but the main causes are due to lack of awareness and knowledge, and limited operational budget,” said Rajendra Aryal, representative for Indonesia and Timor- Leste at the Food and Agriculture Organization of the United Nations. United Nations (FAO), Mongabay said in an interview.
“As a result, not much is known about the results of restocking activities that have been carried out in Indonesia in the past,” Aryal added.
FAO is involved in some fish restocking efforts in Indonesian rivers, such as the Sangolan River in Riau Province on the island of Sumatra. Ten thousand mahsir (Tor tambroides) and 80,000 baung (Hemibagrus nemurus) fish were released there last March.
Aryal said restocking is a case-by-case intervention that must be combined with other efforts, such as sustainable regulation, habitat rehabilitation and community empowerment, to be effective and meaningful.
“Restocking can balance the rate of mortality and recruitment of target fish species in certain areas over short periods of time. It can also help recolonize certain water bodies with extinct species,” Aryal said.
“However, it is important to note that restocking by itself cannot solve inland water issues. Nor can restocking solve the problem of declining fish population sizes. It is also impossible to release and replenish all aquatic species in our water,” Aryal added.
For the Serayu River, experts asked PT Indonesia Power to better manage its dam flushing schedule to avoid dangerous levels of mudslides, and upstream farmers to limit their use of chemical pesticides and fertilizers . The company has publicly apologized for the flushes that caused the mudslides in the river.
“It is important so that the Serayu River is not affected again,” Isdy said.
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