Deep in East Kalimantan, the Mahakam River flows through rainforests and floating villages, carrying a local myth of two siblings reborn as dolphins. These mythical beings — the Pesut Mahakam — Indonesia’s only freshwater river dolphins, now critically endangered.
Once abundant, their numbers have dwindled to around 60, according to Rare Aquatic Species of Indonesia's (RASI) 2024 report. With roughly five births and five deaths each year, the population remains stagnant and at risk of collapse.
What Does the Mahakam River Dolphin Look Like?
Unlike sleek oceanic dolphins, the Pesut Mahakam has a rounded head, small eyes, and a smooth gray body with a pale underside. Its short dorsal fin and broad pectoral fins aid graceful movement through narrow river bends. Without a long beak, its face appears soft — an adaptation to muddy, shallow freshwater.
This dolphin is modest in size but sturdy in build. Adult individuals typically measure between 1.5 and 2.8 meters.
Their weight ranges from 114 to 133 kilograms. Newborns are much smaller, measuring around 96 centimeters and weighing about 12 kilograms.
In their first few months, they grow rapidly. Within seven months, a calf can gain up to 59% in length and 266% in weight — a testament to the care and nourishment provided by its mother.
As a carnivore, the Mahakam River Dolphin feeds on fish, mollusks, and crustaceans, playing a vital role in maintaining the river’s ecological balance. With a lifespan reaching up to 30 years, they are long-lived residents of these waters — yet their slow reproduction makes them fragile.
Breeding occurs from December to June, with males competing in gentle displays. After a 9–14 month pregnancy, females give birth to one calf every three years. Sexual maturity takes three to six years, making recovery slow and uncertain.
Social by nature, Pesut Mahakam dolphins live in small pods of three to six individuals. They are often seen swimming in harmony or interacting with neighboring groups. Like other dolphins, the Pesut Mahakam is also an intelligent animal.
Why Is the Mahakam River Dolphin in Decline?
Once thriving in the calm bends of the Mahakam River, the Pesut Mahakam now lives a quieter, more fragile life. Its survival is threatened by pollution, dwindling food, and ongoing human disturbance.
Researchers stress that water quality is vital to the survival of Pesut Mahakam dolphins. Aisyah et al. highlight that poor pH, low oxygen, and chemical pollution disrupt fish populations, triggering a dangerous ripple effect across the river ecosystem.
1. Declining Water Quality
Every drop of fertilizer from farms, every trace of industrial waste, and every stream of domestic sewage eventually finds its way into the Mahakam.
Over time, this nutrient buildup triggers eutrophication, an excessive growth of algae that depletes oxygen and suffocates aquatic life. What was once a rich feeding ground becomes a stagnant, oxygen-starved river, forcing the dolphins to travel farther for food.
2. Noise Pollution
But it’s not just the water that’s changing — it’s the sound. The Mahakam, once a quiet habitat, now buzzes with boats, ferries, and tourist vessels. For dolphins that depend on echolocation — using high-frequency clicks between 60–80 kHz to navigate and find prey — this constant noise is devastating.
Imagine trying to talk in a room full of roaring engines. That’s daily life for these dolphins. Studies by Kreb and Rahadi, as well as Beasley et al., show that boat noise disrupts their communication, often forcing them to pause their calls. Over time, this leads to stress, confusion, and abandonment of once-safe habitats.
3. Dangerous to Hunt for Food
Food scarcity pushes the Pesut Mahakam into perilous zones. Many dolphins are drawn toward fishing nets, especially rengge nets, to snatch trapped fish — a fatal mistake for some. According to RASI, 70 percent of recorded deaths occur because dolphins become entangled and can’t reach the surface to breathe.
At the same time, overfishing and illegal fishing methods such as poisoning and electric shocks worsen the crisis. These destructive practices not only wipe out fish stocks but also endanger the dolphins directly.
In their search for food, some dolphins venture upstream — areas bustling with mining operations and heavy transport traffic. The sharp sounds of engines, the risk of collisions, and the murky waters polluted by runoff turn these regions into dangerous territory.
Can the Mahakam River Dolphin Be Saved?
While the threats seem overwhelming, hope for the Pesut Mahakam is far from gone. Across East Kalimantan, villages, conservationists, and local authorities are quietly building a movement to protect what remains of this rare freshwater dolphin.
In Pela village near Semayang Lake, change began with grief over repeated dolphin deaths. In 2019, residents turned to ecotourism, guiding visitors to watch dolphins responsibly. Under new village regulations, harmful methods like electrocution, poisoning, and explosives are now also fully banned.
Further down the river, innovation meets tradition. With help from RASI, Pertamina Hulu Mahakam, and local officials, communities introduced acoustic “pingers”- small devices that send out ultrasonic signals to keep dolphins away from fishing nets. This simple yet effective tool has drastically reduced fatal entanglements in high-risk areas.
Meanwhile, education has become a powerful ally. Programs like Kampung Dongeng Etam (Etam Storytelling Village) weave environmental messages into folklore and puppet shows.
Through a project called Domain — short for dongeng dan permainan, meaning storytelling and play — children learn the ancient legends of the Pesut Mahakam not just as tales of the past, but as calls to protect their living heritage.
Policy is also catching up. In 2020, the Kutai Kartanegara Regency issued Decree No. 75/2020, designating a formal Mahakam River conservation zone and restricting coal barge traffic through dolphin habitats. Though enforcement remains uneven, it signals the start of a new awareness that river management must go hand in hand with species protection.
But is it all enough? The Mahakam River Dolphin’s survival hinges on whether local efforts can grow into a broader movement that balances human progress with the rhythm of the river.
Sources:
https://indonesia.go.id/kategori/keanekaragaman-hayati/7210/mengenal-lumba-lumba-di-sungai-mahakam?lang=1
https://fpk.unair.ac.id/mengulik-faktor-terancamnya-sang-hewan-legenda-pesut-mahakam/
https://greennetwork.id/gna-knowledge-hub/bagaimana-desa-wisata-pela-lindungi-keberadaan-pesut-mahakam/
http://ejournal-balitbang.kkp.go.id/index.php/aureliajournal/article/view/11011
https://www.instagram.com/p/DMjlnzJtetr/
