Yangtze finless porpoise

Introduction

IUCN Red List Conservation status: Critically Endangered

The Yangtze finless porpoise is the only freshwater porpoise in the world. It can only be found in China’s Yangtze River, the longest river in Asia. At one point, this porpoise shared the waters with the Yangtze river dolphin (Baiji)— a species last seen in 2002, and declared functionally extinct in 2007¹. A range of developments along the Yangtze River have put finless porpoises at risk of extinction as well. In an effort to prevent this, China enacted the Yangtze River Conservation Law in 2021, which bans fishing in the Yangtze River and its tributaries for 10 years so as to allow recovery of the species.

Distribution and habitat

The Yangtze finless porpoise is found only in the Yangtze River, where its range has declined significantly since the early 1980s^{2, 3}. The subspecies is now fragmented and confined to a few sections of the river’s main stem (with one concentration between Ezhou and Nanjing, and low densities from Yichang to Jingzhour, Xintan to Taunfeng and Jiangyin to Shanghai), as well as to Poyang and Dongting Lakes².

Abundance

The species has been in a continuous decline since the early 1980s. The population reduced from over 2,500 in 1991, to an estimated 1,800 in 2006⁴. The population was then halved between 2006 and 2012, with only 505 individuals recorded in the river’s main-stem⁵. However, in 2020, population estimates showed a possible halt in the rate of decline, with 445 animals estimated to remain in the river’s main-steam, and a total estimate of 1,012².

One of the conservation measures believed to have contributed to the halting of population decline is the establishment of seven natural and two “semi-natural” reserves. Since 1996, a small group of finless porpoises has been successfully cared for at a facility at the Institute of Hydrobiology of the Chinese Academy of Sciences. Several groups of these porpoises, caught or rescued in the mainstream of the Yangtze River, have been introduced into the Tian’e-Zhou semi-natural reserve since 1996, and porpoises are successfully breeding in the reserve⁶.

Threats

The distribution of the Yangtze finless porpoise is highly fragmented with at least three gaps in the main stem of the river⁵. The rapid decline of the Yangtze finless porpoise cannot be attributed to a single threat. Continued threats to the Yangtze finless porpoise include bycatch in unregulated and unselective fishing, habitat degradation through dredging, pollution and noise, vessel strikes and water development^{4, 7, 8}. The rapid deterioration and degradation of the porpoise habitat followed industrial growth, increased shipping and urbanisation since the early 1990s all play a significant part in the decline⁵. The extinction of the baiji was strongly linked to the entanglements in gear used in unregulated and unselective fishing including gillnets, electric fishing¹, and this is certainly also the case with the Yangtze finless porpoise⁹. The Three Gorges Dam is the largest dam on the Yangtze River. Its construction has caused changes in the flow and sediments downstream, which have caused a 90% decline in the total number of juveniles (known as ’fry’) of the four economically-important Chinese carp species¹⁰. In 2011, a programme to release environmental flows from the dam was initiated, and the China Three Gorges Corporation released a flood pulse from the dam in May or June every year. The primary purpose of these environmental flow releases has been to promote carp spawning¹⁰.

Biology and ecology

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 September^{13, 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.

Diet

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.

Behaviour

(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.

External links

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References

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