Officials in Indonesia have released a predictive calendar they hope will give fish farmers in the country’s largest lake a heads-up on water conditions that have previously killed off fish by the millions.
By M Ambari
The issuance of the calendar on Sept. 13, online and in print, comes in the wake of the death of millions of fish last month in Toba, a lake inside a volcanic crater in northern Sumatra. The lake experienced a similar phenomenon in 2016.
Researchers have attributed the mass die-offs to a sudden depletion of oxygen in the water due to a buildup of pollutants from aquaculture, agricultural runoff, and sewage from hotels and houses. Unfavorable weather conditions and unsustainable practices by local fish farmers have also been cited as factors.
The new calendar is meant to notify fish farmers about the lake’s condition using a progressive scale running from safe to alert to dangerous, based on previous field assessments of the area.
Workers haul in thousands of dead fish on the shore of Lake Toba. Image by Ayat S. Karokaro for Mongabay.
A “safe” rating means fish farming can proceed as normal, while an “alert” rating means the farmers have to reduce their use of fish feed. Uneaten fish feed and feces from the fish pens fuel a process called eutrophication, in which the water becomes excessively rich in nutrients. This nutrient loading can induce oxygen-devouring algal blooms that suffocate fish, as is thought to have happened in 2016. Under “alert” conditions, fish farmers are asked to reduce stocking density in the floating cages and pay attention to the changes in the water. They are also advised to harvest earlier.
A “dangerous” rating indicates the lake’s water temperature is low and the dissolved oxygen is less than 3 milligrams per liter, so fish may suffocate. The weather during this period will mostly be intense rainy and windy. Under such conditions, the farmers are advised to harvest fish that are ready for harvesting, pause their farming activities, and attend to fish that can survive low water quality. They are also expected to carry out aeration — a practice to bring water and air in close contact in order to remove dissolved gasses — and relocate some of the floating cages to deeper parts of the lake.
“This predictive calendar and management scheme can increase the awareness of fish farmers and policymakers so that they won’t take mass fish die-offs lightly,” Sjarief Widjaja, head of capture fisheries at the national fisheries ministry, told reporters in Jakarta.
Spanning 1,130 square kilometers (440 square miles), Lake Toba produces an estimated 76,000 metric tons of aquaculture products a year, much of it from local farmers. Two companies — Swiss-owned PT Aquafarm Nusantara and PT Suri Tani Pemuka, an arm of Singapore-listed Japfa Group — also cultivate tilapia in the lake and export fillets internationally, including to the United States and Europe. PT Aquafarm Nusantara, responsible for 40,000 tons of production in 2015, has received backlash for dumping waste into the lake.
Haranggaol Bay, the lake’s second-largest source of farmed fish, has long been a pollution concern because of the high concentration of fish farms there. Where in 2005 the village had 854 floating pens, today there are more than 6,000. And that explosion, while a boon to the local economy, has come at a cost, particularly to the lake’s health.
The government has long struggled to curb the rise of unregulated community fish farms. The campaign against fish farming also ramped up after President Joko Widodo announced a plan to turn Toba into a major tourist destination along the lines of a “Monaco of Asia.” In 2014, he signed an order stipulating that aquaculture would only be allowed in one district, Toba Samosir.
Local governments have since attempted to carry out the mandate. In July 2016, Simalungun district, which covers Haranggaol, dispatched military and police officers to dismantle cages in Sualan village, about 70 kilometers south of Haranggaol.
But for many landless residents, fish farming is their only dependable source of income. Many fund their businesses with bank loans. Losing or severely reducing aquaculture would not only disrupt their income, but also leave them with massive debts. The recent mass fish die-offs have also cost the fish farmers billions of rupiah.
After pleas from local fish farmers, the government agreed to postpone the eviction. Instead, the seven districts on the lake and the provincial government agreed on new zoning regulations, while asking the farmers to adhere to stricter environmental standards.
A carpet of dead fish in one of the floating net cages in Lake Toba. Image by Ayat S. Karokaro for Mongabay.
In addition to releasing the predictive calendar, the government has recommended the planting of water hyacinths (Eichhornia crassipes), a free-floating aquatic plant known to reduce heavy metals and pesticide residues in waters where it grows.
“Water hyacinth roots are natural filters for polluted water from various industry-manufactured chemicals,” said Toni Ruchimat, the fisheries ministry’s head of research.
Officials have also advised the fish farmers to adopt other tools developed by the ministry, including Buoy Pluto, an early-warning pollution-monitoring system, and KJA SMART, a series of floating net cages with built-in systems to handle organic pollutants.
In a bid to achieve environmental sustainability in Lake Toba, the Coordinating Ministry for Maritime Affairs, which oversees the fisheries ministry, has launched a research initiative involving the World Bank, the state-funded Indonesian Institute of Sciences (LIPI), and state-owned water company Perum Jasa Tirta. The local governments with authority over the area are expected to apply the research findings.
“One of the recommendations from the researchers is to conserve the forests [around the lake] as there’s continuous tree cutting upstream,” said Luhut Binsar Pandjaitan, the coordinating minister of maritime affairs.
The coordinating ministry has also recommended that a reforestation program be undertaken in the area around Lake Toba.
Luhut said one of the key problems in Lake Toba was the high density of fish farmers. He cited research from LIPI that indicated the lake could sustainably accommodate at most 1,925 floating cages. Companies operating in the area have been asked to reduce their number of cages by 70 percent.
“LIPI has said that Lake Toba requires 75 years to clean itself, but that process will never be achieved if the lake continues to be contaminated,” Luhut said.
Fish farmers pile up dead fish on the shore of Lake Toba. Image by Ayat S. Karokaro for Mongabay.
The story was reported by Mongabay’s Indonesia team and was first published on our Indonesian site on Sept. 14, 2018.