Small and stream-lined, as its name would imply, the Yangtze finless porpoise lacks a dorsal fin. Instead, it has a narrow-ridge which runs the length of its back and is covered in wart-like ‘tubercles’. The have rather bulbous heads, do not have a beak. Fully grown, they can reach a length of up to 2 metres and weigh up to 100 kg11.
Unlike most dolphin species, finless porpoises do not communicate with whistles. However, they do use echolocation to navigate and locate food, and produce the high-frequency narrow-band ultrasonic pulses that are typical of other porpoise species12.
Yangtze finless porpoise Illustration by Uko Gorter.
Reproduction and growth
The Yangtze finless porpoise matures at the age of 4-6 years and is a seasonal breeder, with males apparently reaching peak hormone levels for breeding between March and September13, 14. Finless porpoises observed in the Tian-E-Zhou oxbow lake mated from May through June and gave birth towards the end of April of the following year. Gestation is estimated to last between 10 and 11 months. Calves began to be weaned at just over 5 months, when they were observed to start eating small fish15.
At least 20 different types of fish species have been identified from the stomach contents of the Yangtze finless porpoise16. The most frequently occurring species were a species of anchovy and two minnow species, and there was also significant overlap between the porpoises’ prey and fish species of commercial importance in the Yangtze river, putting the porpoises at risk of interacting with fishing gear. Previous studies have also identified crustaceans and molluscs16.
(c) WWF China
Yangtze finless porpoises can be quite active and have been observed darting about just under the water surface, changing direction quickly and often. A common pattern of behaviour is to take one long dive followed by two shorter ones. Dives lasting over four minutes have been recorded. Although groups of up to 50 individuals have been sighted, they are more commonly observed either alone or in small groups. They are generally shy around boats and humans. Porpoises swam preferentially with their right pectoral fin upward and their left pectoral fin downward with a clockwise swimming direction17.