The River's Last Cetacean
The Yangtze River, Asia's longest, is a waterway of superlatives, but also of absence. It once supported two species of cetacean, but the Baiji dolphin was declared functionally extinct in 2006. This leaves its smaller cousin, the Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis), as the river's only remaining mammal. This porpoise is a unique creature and of only a handful of freshwater cetacean species in the world. Adults reach about 1.8 meters in length and weigh around 45 kilograms. Instead of a dorsal fin, it has a subtle ridge along its back covered in small, wart-like tubercles.
The porpoise navigates the turbid, sediment-heavy waters of the Yangtze not by sight, but by sound. It produces high-frequency, narrow-band ultrasonic pulses for echolocation. These clicks, with a peak frequency centered around 125-135 kHz, allow it to find food and create a detailed acoustic map of its dark, underwater world. The population of this critically endangered animal had dropped to an estimated 1,012 individuals in 2017, a sharp decline from around 2,550 in the early 1990s. More recent surveys in 2022 show a hopeful increase to 1,249 porpoises, suggesting that conservation efforts may be having an effect.
An Acoustic Dragnet
Monitoring a small, shy animal in a vast, murky river presents an enormous challenge. Visual surveys are difficult and often miss animals. In response, scientists from institutions like the Institute of Hydrobiology of the Chinese Academy of Sciences have turned to passive acoustic monitoring (PAM). This technique involves deploying networks of hydrophones—underwater microphones—to continuously listen for the porpoises' distinctive echolocation clicks.
These acoustic arrays can be stationary or even mounted on moving cargo ships, creating a cost-effective way to survey long stretches of the river. One study recorded an average of 6,059 clicks per survey trip along an 1,100-kilometer stretch of the river. The data gathered allows researchers to map the porpoises' distribution, identify critical habitats, understand seasonal migration patterns, and assess how the animals react to human activity. Studies of their vocalizations show that the porpoises are primarily active at night and twilight, using short-range "buzz" sounds during prey capture. This acoustic surveillance helps researchers understand the main threats to their survival: heavy shipping traffic, sand mining, and illegal fishing, all of which create intense underwater noise that can interfere with the porpoises' ability to navigate and hunt.