A genetic island
In the murky waters of the Ayeyarwady River lives a critically endangered subpopulation of the Irrawaddy dolphin, Orcaella brevirostris. While the species exists in other freshwater rivers and coastal areas of Southeast Asia, the Ayeyarwady dolphins are a demographic island. Genetic studies focusing on mitochondrial DNA reveal a significant level of differentiation between riverine and coastal populations. This shows a long-standing isolation from their marine relatives, with no evidence of interbreeding. This genetic separation makes them particularly vulnerable. With a population estimated at fewer than 80 individuals, their low genetic diversity increases the risk of inbreeding and reduces their capacity to adapt to environmental changes.
The physical appearance of the Irrawaddy dolphin is distinctive. It has a blunt, rounded head and lacks the prominent beak common to most other dolphins, giving it a look sometimes compared to the beluga whale. Adults can reach up to 2.75 meters in length and weigh between 90 and 200 kilograms. Their coloration is typically a uniform slate-gray, slightly paler on the underside. Unlike many other dolphins, they possess a flexible neck, allowing them to turn their head to scan their surroundings, a useful adaptation for navigating the complex river environment. They navigate and hunt using echolocation, producing a series of clicks and buzzes to interpret their environment.
An alliance under threat
For at least 150 years, these dolphins have engaged in a remarkable form of cooperative fishing with local fishermen. The fishermen summon the dolphins by tapping on the sides of their boats. In response, the dolphins herd schools of fish towards the fishermen's cast nets, often signaling with a tail flick when it is time to throw the net. This partnership is mutually beneficial; the fishermen can increase their catch size significantly, and the dolphins feed on the fish that become disoriented or escape the nets.
This unique cultural and ecological phenomenon is now in jeopardy. The most severe threat to the dolphins is illegal electrofishing, where car batteries are used to send a current through the water to stun fish. This practice indiscriminately harms all aquatic life and can directly kill the dolphins. Entanglement is another danger in gillnets, which are vast nets that hang in the water and can drown the air-breathing mammals. Habitat degradation from gold mining, which releases mercury into the river, and increased sediment loads also pose serious risks. In 2005, the Myanmar government, with support from the Wildlife Conservation Society, established the Ayeyarwady Dolphin Protected Area (ADPA), a 74-kilometer stretch of the river, to mitigate these threats through patrols and community engagement.