Acipenser ruthenus

In the Acipenser, the search characteristics of the sterlet are: the whole body is covered with 5 rows of bone plates, the snout length accounts for less than 70% of the head length, and there are 4 snout barbels; the snout tip is tapered, the edges on both sides are rounded, and there is a water blowhole on the head; the mouth is horizontal, the opening is downward, and the snout barbels are round; the highest point of the body is not at the first dorsal plate, and the first dorsal plate is not the largest bone plate; there is no post-dorsal fin plate and post-hip plate; the side bone plates are lighter in color than the trunk, and the gill rakers have no tubercles; the number of side bone plates is more than 55, and there are 2-4 protrusions under the snout.
The sterlet is small in size, except for a few that can reach 125 cm in length and 16 kg in weight, but generally does not exceed 100 cm in length and 6-6.5 kg in weight. The height of the sterlet is 5.9-16.6% of the total len

The waters are distributed in freshwater waters such as the Black Sea, Caspian Sea, Ob River, and Yenisei River in Russia. China introduced it from Russia for trial breeding, and it is only distributed in the water system of the Irtysh River upstream of the Ob River in Xinjiang (Habahe, Burqin, and Altay Salt Pond Fishery).
National distribution: Austria, Azerbaijan, Belarus, Bosnia and Herzegovina, Bulgaria, China, Croatia, Germany, Hungary, Kazakhstan, Moldova, Romania, Russian Federation, Serbia, Slovenia, Ukraine. The sterlet inhabits lowlands and hilly areas of rivers, usually staying in the low-lying areas of the riverbed, where the bottom is stone, gravel or sandy. In reservoirs, the sterlet inhabits the upstream end of the reservoir where the water flows.

The Latin name of the small-bodied sturgeon is Acipenser ruthenus, and its foreign name is Sterlet. There is no subspecies.

The small-bodied sturgeon is a freshwater sedentary fish that usually does not migrate long distances. The migratory species of sterlet start to migrate upstream to spawn during the spring flood season. The larger the water volume during the flood season, the more individuals will participate in the migration and the farther they will migrate upstream. After 4-5 weeks of migration, sterlet reaches the spawning ground to spawn. Some individuals that are not yet fully mature also migrate with the parent sturgeon. After spawning, the parent fish slowly descend to the river bay, beach or muddy river channel to find food. The sterlet sometimes descends to the salt water bodies at the mouth of the Volga River and other rivers (within 8 meters of water depth) to feed.

The sterlet mainly feeds on the larvae of various insects such as midges, and also feeds on small mollusks, oligochaetes, polychaetes, leeches and other invertebrates. During the spawning period of other fish, the sterlet also feeds on the eggs of these fish, including other sturgeons, and more than half of the contents in the stomach are fish eggs. When insects such as mayflies occur in large numbers, sterlet sturgeons can sometimes jump out of the water to prey on insects.

The sterlet sturgeon is a freshwater fish that often lives in rivers and mainly feeds on aquatic insects. There is little difference in the growth of male and female fish. In the spring of the Chulym River, female fish grow faster than male fish; from a regional perspective, the growth rate of the sterlet sturgeon in the Chulym River is close to that of the sterlet sturgeon in the Irtysh River, but exceeds that of the Yenisei River. In natural waters, sterlet sturgeons can hybridize with Russian sturgeons, glitter sturgeons, naked belly sturgeons and Siberian sturgeons. Under artificial conditions, sterlet sturgeons can achieve inter-genus hybridization with European sturgeons.

The age composition of sterlet sturgeon populations in various waters is mainly 4-7 years old. After the sterlet sturgeon moved to the reservoir, the environmental changes affected the reproduction of the sterlet sturgeon, resulting in an increase in the number of old fish in the group, and these old fish did not participate in reproduction. The overall sex ratio of the sterlet sturgeon is 1:1, of which male sturgeons account for about 60-70% in the spawning grounds; in the fattening grounds, the sexes are roughly equal; in the deep-water wintering sites, there are more female sturgeons than male sturgeons.

The sterlet sturgeon is a sturgeon species that matures sexually earlier. Most populations of females first mature at the age of less than 8 years. Male fish mature at 4-5 years old, and female fish mature at 5-9 years old. Some people believe that the reproduction cycle of the sterlet sturgeon is annual spawning, while others believe that it is spawning at intervals of 1 year or even longer. Some people have found that in the Danube River, male sturgeons and younger female sturgeons spawn every year, while older female sturgeons spawn every two years. The egg carrying capacity of sterlet is relatively small. In the middle reaches of the Danube River, the Kama River, the middle reaches of the Volga River, the Northern Dvana River, the Tsimlin Reservoir, and the Irtysh River, the absolute egg carrying capacity of sterlet is 7,000-108,000, 3,900-137,600, 4,200-76,400, 6,800-149,600, 7,100-30,700, and 5,900-16,300. The eggs of sterlet are dark gray; the egg diameter is small, 2.01-2.86 mm; the egg weight is 8-9 mg.

There are two types of spawning grounds for sterlet, one is the floodplain formed by spring water, and the other is the riverbed. In riverbed spawning grounds, sterlet sturgeons spawn at a depth of 7-15 meters. The eggs are mainly laid on pebbles with a diameter of 1-7 cm, and rarely on gravel and sand. The water flow speed at the spawning site is 1.5-5 meters per second. The riverbed pits where sterlet sturgeons overwinter are generally located near the spawning grounds. In reservoirs, sterlet sturgeons generally spawn at the upstream end of the reservoir. Sturgeons spawn in the spring flood season. The suitable water temperature for spawning is 12-17℃. When the water temperature rises to 20℃ or 21℃ or is lower than 9.4℃, spawning stops.

Male sturgeons arrive at the spawning grounds before female sturgeons. At this time, the water temperature is 9-11℃. When female sturgeons arrive at the spawning grounds, the water temperature is 12-13℃. After spawning, female sturgeons generally leave the spawning grounds immediately and migrate downstream, while male sturgeons continue to stay in the spawning grounds for a period of time to participate in the reproduction of other female sturgeons. During the breeding period, the sterlet hardly eats. In the Volga River, when the water temperature is 10℃, the hatching time of the sterlet embryo is more than 320 hours, and at 20℃, it only takes 80 hours at most. The newly hatched fry is 6-7 mm in length. The yolk of the sterlet fry disappears 6-10 days after hatching. It reaches 3-4 cm after 1 month, and can reach 8-18 cm in the same year, and some reach 25 cm.

Historically, the catch of sterlet was large, among which the sterlet catch in the Volga River system was the largest. From 1935 to 1939, the annual catch of sterlet around the world totaled 750-800 tons, of which Russia accounted for about 700 tons. Later, the sterlet fishery in most waters in Russia was closed, and only a small number of male sterlet were retained for hybridization with female sturgeons of European sturgeons. In the Danube River system, the average annual catch of sterlet was 63.5 tons from 1958 to 1981, of which 57.5% was in the former Yugoslavia and 28.1% in Bulgaria. From 1984 to 1989, the average catch of sterlet was 4.7 tons in the Danube River system in the former Yugoslavia. Later, the sterlet resources in almost all waters have been significantly reduced. The sterlet is confined to the middle and upper reaches of the Danube River, and the population in the lower reaches has basically disappeared.

The main threats to the sterlet species are the impact of river damming and increasingly serious environmental pollution. Since the 1930s, dams have been built on almost all rivers in the distribution area of sterlet, resulting in the loss of the main habitat and spawning grounds of sterlet. The regulation of water flow by hydropower projects directly affects the spawning of sterlet sturgeons, especially the low water level in spring, which causes a large number of deaths of sterlet sturgeon eggs and juveniles along the river. Changes in hydrological rhythms, especially the reduction in flow velocity, also cause the degradation of the sterlet sturgeon breeding system.

As of the early 1990s, sterlet sturgeons were artificially cultivated in Russia, Ukraine, Hungary and Germany, and sterlet sturgeons were also introduced to Belgium, Italy and other countries for breeding. In 1993, the breeding output of sterlet sturgeons in Germany was estimated to be 5 tons. The hybrid of sterlet sturgeons and European sturgeons grows rapidly and has fertile offspring. It has been cultivated in many European countries.

Listed in the "Red List of Threatened Species of the World Conservation Union" (IUCN 2019 ver 3.1) - Endangered (EN).

Listed in the "Red List of Threatened Species of the World Conservation Union" (IUCN 2009 ver 3.1) - Vulnerable (VU).

Listed in the "Convention on International Trade in Endangered Species of Wild Fauna and Flora" (CITES) - Appendix II.

Listed in the second level of China's 2021 edition of the "National Key Protected Wildlife List" (only for wild populations).

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