''Hucho bleekeri'', the Sichuan taimen, is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

freshwater fish Freshwater fish are fish species that spend some or all of their lives in bodies of fresh water such as rivers, lakes, ponds and inland wetlands, where the salinity is less than 1.05%. These environments differ from marine habitats in many wa ...

in the

salmon Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

(

Salmonidae Salmonidae (, ) is a family (biology), family of ray-finned fish, the only extant member of the suborder Salmonoidei, consisting of 11 extant genera and over 200 species collectively known as "salmonids" or "salmonoids". The family includes salmo ...

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

to the

Yangtze The Yangtze or Yangzi ( or ) is the longest river in Eurasia and the third-longest in the world. It rises at Jari Hill in the Tanggula Mountains of the Tibetan Plateau and flows including Dam Qu River the longest source of the Yangtze, i ...

basin in

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. Their typical habitat includes mountain streams and small rivers. They are found in rivers and tributaries in the

Sichuan Sichuan is a province in Southwestern China, occupying the Sichuan Basin and Tibetan Plateau—between the Jinsha River to the west, the Daba Mountains to the north, and the Yunnan–Guizhou Plateau to the south. Its capital city is Cheng ...

Shaanxi Shaanxi is a Provinces of China, province in north Northwestern China. It borders the province-level divisions of Inner Mongolia to the north; Shanxi and Henan to the east; Hubei, Chongqing, and Sichuan to the south; and Gansu and Ningxia to t ...

, and

Qinghai Qinghai is an inland Provinces of China, province in Northwestern China. It is the largest provinces of China, province of China (excluding autonomous regions) by area and has the third smallest population. Its capital and largest city is Xin ...

provinces. The Sichuan taimen is valued in various academic fields, and may prove helpful in studying fish evolution, and the impacts of

climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

on inland, cold-water fish species. They are listed as "

critically endangered An IUCN Red List critically endangered (CR or sometimes CE) species is one that has been categorized by the International Union for Conservation of Nature as facing an extremely high risk of extinction in the wild. As of December 2023, of t ...

" by the

International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the stat ...

Species description

The Sichuan taimen typically has a dark black, dorsal, and

adipose fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only b ...

; a silvery white underside; and small, irregular dark spots across the body, head, and

gill cover The operculum is a series of bones found in bony fish and Chimaera, chimaeras that serves as a facial support structure and a protective covering for the gills; it is also used for respiration and feeding. Anatomy The opercular series contain ...

. Their coloring can range from a darker orange/red, to a lighter tan/yellow depending on stage of life. They typically have between 125 and 152 scales, 14

gill rakers Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch (gill arch) and are involved with suspension feeding tiny prey. They are not to be confused with the gill filaments that compose the fleshy part of the ...

, and can grow to 72 cm (2 ft 3in) in length.

Biology

The Sichuan taimen is a carnivorous species that feeds on aquatic insects, worms, and smaller fish. As juveniles, they feed on

zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...

and aquatic insects. Females mature at four years of age; males earlier. The estimated generational time of Sichuan aimen is 17 years. Sichuan breeding season is from March to May. Fish seek out low current areas with dense vegetation on both sides of the bank, and temperatures between 4 and 9 °C. Adults dig shallow, oval nests in the soft bottom of the river, then lay their eggs inside. Once laid, the eggs are buried in the sand, where they develop. Their eggs are heavy, yellow, and non-sticky, approximately 3–4 mm in diameter. A 2020 study by Zhang et al. showed low levels of inbreeding in the Sichuan taimen in the Taibai River, indicating that there is a relatively high number of sexually mature individuals in this population.

Distribution and habitats

Geographic distribution of Hucho bleekeri

Sichuan taimen are endemic to the

Yangtze basin The Yangtze or Yangzi ( or ) is the longest river in Eurasia and the third-longest in the world. It rises at Jari Hill in the Tanggula Mountains of the Tibetan Plateau and flows including Dam Qu River the longest source of the Yangtze, in ...

in China. They are found in the upper tributaries of the Min and

Yangtze River The Yangtze or Yangzi ( or ) is the longest river in Eurasia and the third-longest in the world. It rises at Jari Hill in the Tanggula Mountains of the Tibetan Plateau and flows including Dam Qu River the longest source of the Yangtze, i ...

province, the upper and middle reaches of the

Dadu River The Dadu River (, ), known in Tibetan as the Gyelmo Ngul Chu (), is a major river located primarily in Sichuan province, southwestern China. The Dadu flows from the eastern Tibetan Plateau into the Sichuan Basin where it joins with the Min Rive ...

in Sichuan and

provinces, and the upper reaches of the Taibai and

Xushui Xushui District () is a District of the People's Republic of China, district of the city of Baoding, Hebei province, China. Administrative divisions Towns: *Ansu, Hebei, Ansu (), Cuizhuang (), Dayin, Xushui County, Dayin (), Suicheng, Xushui Cou ...

as well as the tributaries of the Hanjiang River in

province. The Sichuan taimen predominantly inhabits fast-flowing streams with sandy or gravel bottoms. The species prefers mountain brooks at 700–3,300 m (2,300– 10,800 ft) meters above sea level with high dissolved oxygen (>5 mg/L), and a low water temperature (less than 15 °C or 59 °F). Juvenile Sichuan taimen distribution is affected by elevation, river sinuosity, and vegetation. They tend to prefer shallow waters (2 feet deep), with a

flow velocity In continuum mechanics the flow velocity in fluid dynamics, also macroscopic velocity in statistical mechanics, or drift velocity in electromagnetism, is a vector field used to mathematically describe the motion of a continuum. The length of the f ...

of 0.5 ± 0.24 m/s, and an offshore distance of roughly 8 m. Smaller juvenile fish prefer shallower habitats that are nearer to shore.

Threats and conservation

The Sichuan taimen is a glacial relic that expanded long ago, but has since experienced a great loss in

genetic diversity Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species. It ranges widely, from the number of species to differences within species, and can be correlated to the span of survival for a species. It is d ...

. This is reflected in the low number of closely-related mtDNA

haplotypes A haplotype (haploid genotype) is a group of alleles in an organism that are inherited together from a single parent. Many organisms contain genetic material (DNA) which is inherited from two parents. Normally these organisms have their DNA orga ...

, the values of which are similar to those reported in other endangered taimen species. A 2014 study sequenced the entire

mitochondrial genome Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the DNA contained in ...

of Sichuan taimen. The genome is 16,837 bp in length and contains 13 protein-coding genes, 2 ribosomal RNAs, 22 transfer RNAs. According to the IUCN, the Sichuan taimen is threatened mostly by habitat loss from the construction of hydropower stations, erosion of soil due to

deforestation Deforestation or forest clearance is the removal and destruction of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. Ab ...

road construction A road is a thoroughfare used primarily for movement of traffic. Roads differ from streets, whose primary use is local access. They also differ from stroads, which combine the features of streets and roads. Most modern roads are paved. The ...

, sand excavation, and

illegal fishing Illegal, unreported and unregulated fishing (IUU) is an issue around the world. Fishing industry observers believe IUU occurs in most fisheries, and accounts for up to 30% of total catches in some important fisheries. Illegal fishing takes pl ...

. Recent studies have estimated the species has endured a 50–80% decline in population over the past three generations, and the decline is expected to continue at a rate of at least 20% over the next two generations (~34 years). An estimated 2,000 to 2,500 mature individuals remain in the wild. The Sichuan taimen was listed as a first-class protected species in China in 2021. A major driver of Sichuan population decline is

habitat loss Habitat destruction (also termed habitat loss or habitat reduction) occurs when a natural habitat is no longer able to support its native species. The organisms once living there have either moved elsewhere, or are dead, leading to a decrease ...

. Since the 1960s, this species has lost an estimated 98% of its historic range. A 2023 study by Zhao et al. used creel netting and environmental DNA monitoring to detect the Sichuan population in the

Qinling The Qinling () or Qin Mountains, formerly known as the Nanshan ("Southern Mountains"), are a major east–west mountain range in southern Shaanxi Province, China. The mountains mark the divide between the drainage basins of the Yangtze and Ye ...

Mountain region. They found a population present in the Taibai River, and the Sujiagou Stream. The Taibai River population was originally re-discovered in 2012. In the upper region of the

River basin, they did not find any evidence of the Sichuan taimen, indicating that they may have gone extinct in that area. This species may also be extinct in the Qingyi, Minjiang, and Dadu rivers, as no taimens have been reported caught from these regions in recent years. In 2022, China invested an equivalent sum 63.4 billion US dollars in a hydropower dam along the Yangtze River. This station is expected to destroy four of the remaining Sichuan taiment spawning sites. A 2023 study by Leng et al. achieved successful artificial propagation of this species. Not only does this study offer important insights to the reproduction of this species, but may prove vital for the long-term conservation of the Sichuan taimen.

Cultural significance

Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...

, taimen are called the children of the river god, providing a connection between humans and spirits.

References