The Chinese paddlefish (Psephurus gladius; : literal translation: "white sturgeon"), also known as the Chinese swordfish, is an extinct species of fish that was formerly native to the Yangtze and Yellow River basins in China. With records of specimens over and possibly in length, it was one of the largest species of primarily freshwater fish. It was the only species in the genus Psephurus and one of two recent species of paddlefish (Polyodontidae), the other being the still-living American paddlefish (Polyodon spathula). The species was not a strictly freshwater fish, with individuals having migrated down the Yangtze into the sea as juveniles, where they spent time in coastal waters, before returning into the river by adulthood, and migrating upriver to spawn. Unlike the American paddlefish, which is a filter feeder on plankton, the Chinese paddlefish was a piscivorous predator that primarily consumed small to medium-sized fish.
Since the 1990s, the species was officially listed by the International Union for Conservation of Nature (IUCN) as critically endangered, and was last seen alive in 2003. A 2019 paper including scientists from the Yangtze River Fisheries Research Institute found the species to be extinct based on its absence from extensive capture surveys, with the extinction suggested to have occurred by 2005, and no later than 2010, although it had become functionally extinct by 1993. It was unanimously agreed to be extinct by the Species Survival Commission Sturgeon Specialist Group of the IUCN on 15 September 2019, with its conservation status being formally updated by the IUCN Red List in July 2022.
The main cause of its extinction was the construction of the Gezhouba Dam in 1981, fragmenting the population and blocking the migration of the fish to its upriver spawning grounds required to sustain its population. Overfishing also played a significant role in its decline. Fishing of the Chinese paddlefish dates back centuries, with annual harvests reaching 25 tons by the 1970s.
Description
left|thumb|A specimen at Museum of Hydrobiological Sciences, [[Wuhan Institute of Hydrobiology]]
The Chinese paddlefish had a white underbelly, and its back and head were grey. Its eyes were small and round. The tail fin was heterocercal (spine extending into the upper lobe), with the lower lobe being well developed. The teeth were small, sharp, canine shaped and inward curling, and became proportionally smaller relative to the jaw during growth, and in mature adults were completely fused into the bone. Compared to Polyodon, the jaws were shorter, and had a proportionately narrower gape, and unlike the American paddlefish, but similar to fossil paddlefish, the upper jaw was not firmly attached to the braincase. The body lacked scales,thumb|Size of the 2003 female specimen, (red, top), compared to the American and fossil paddlefishThe maximum length of the Chinese paddlefish is often quoted as , with this estimate apparently being given by C. Ping (1931) as quoted by John Treadwell Nichols in 1943, though according to Lance Grande and William Bemis in 1991, the Chinese paddlefish was only definitively known to have body lengths in excess of . However, Grande and Bemis (1991) suggested that this enormous length may have been the result of a translation error.
The lifespan has been estimated at 29–38 years, though the theoretical maximum lifespan is likely to have been significantly higher, as the estimate reflects anthropogenic impacts on the population. It was placed into a separate, monotypic genus by Albert Günther in 1873. The species was also given a different name, Spatularia angustifolium by Johann Jakob Kaup also in 1862, but this is considered a junior synonym of P. gladius. The oldest representatives of the genus containing the American paddlefish, Polyodon date to around 65 million years ago, from the beginning of the Paleocene. Various molecular clock estimates have been given for the age of the divergence between the American and Chinese paddlefish, including 68 million years ago 72 million years ago, and 100 million years ago, all dating to the middle to Upper Cretaceous.
Relationships of recent and fossil paddlefish genera, after Grande et al. (2002). They were also historically found in coastal waters of the East China Sea and the Yellow Sea; occasionally spring tides would bring individuals into the lower reaches of the Qiantang and Yong rivers of Zhejiang province. The ovaries of the female fish contained over 100,000 eggs, each approximately across, thumb|Closeup of the tip of the rostrum, showing electrorecepting [[Ampullae of Lorenzini|ampullae |alt=]]
The fish was largely solitary, and occupied the lower-mid layers of the water column. Chinese paddlefish were noted for being strong swimmers. Unlike its relative the American paddlefish, which is a planktivorous filter feeder, the Chinese paddlefish was primarily piscivorous, mainly feeding on small to medium-sized fishes like anchovies (Coilia), cyprinids (Coreius, Rhinogobio), gobies (Gobius) as well as bagrid catfish and bothid flounders. It also consumed crustaceans like shrimp and crabs. Paddlefish, like other Acipenseriformes and several other groups of vertebrates, engage in passive electroreception (the sensing of external electric fields) using structures called ampullae that form an extension of the lateral line system of sensory organs. Passive electroreception (where electric fields are sensed but not generated, as in electric fish) is primarily used for detecting the weak electric fields generated by prey. The head and rostrum of Chinese paddlefish, like those of other paddlefish, was densely packed with ampullae, indicating that enhancing electroreception was one of the rostrum's primary functions.<!-- see "Supporting Information" in Xie2018 --> Declines were significant throughout its primary range in the Yangtze basin, but annual captures of 25 tonnes continued into the 1970s. The species was still being found in small numbers in the 1980s (for example, 32 were caught in 1985), and young were seen as recently as 1995. the former died despite attempts to save it and the latter was radio-tagged and released, but the tag stopped working after only 12 hours.
During a search conducted in the Yangtze basin from 2006 to 2008, a research team from the Chinese Academy of Fisheries Science in Jingzhou failed to catch any paddlefish, A comprehensive study published in 2019, including scientists from the Yangtze River Fisheries Research Institute, found that the species was certainly extinct, based on its absence from extensive capture surveys of the Yangtze between 2017 and 2018. The paper estimated that the species went extinct between 2005 and 2010, but became functionally extinct by 1993. The paper thus recommended the reclassification of the species as Extinct by the IUCN. Sichuanese fishermen had a saying regarding the fish, referencing its large size. Some common names for the fish in Chinese compare the rostrum of the fish to an elephant's trunk.
Like the related sturgeons and American paddlefish, the species was fished for its caviar, though it made up only a small proportion (less than 1%) of the total number of fish caught from the Yangtze in the late 20th century by weight.