The study of the biological condition of kutum (Rutilus frisii Nordman 1804), Common carp (Cyprinus carpio) and Persian sturgeon (Acipenser persicus) released from artificial reproduction in the Tajn River was carried out in line with a research project entitled “The study of habitat suitability in the Tajn River during the years 2021-2023 . Sampling was done in 7 study stations, including 1) Sari diversion dam, 2) Mahfroz Mehaleh 3) Panbeh Chole 4) Soteh 5) Farah Abad 6) after dam outlet 7) river estuary. The purpose of this research is to investigate the identification, distribution and diversity of fish species and their abundance in different sampling stations. A total of 2,454 fish were examined, belonging to 15 families, 25 genera and 28 species were identified, which include the families Acipenseridae, Atherinidae, Clupeidae, Cobitida, Cyprinidae, Gastrosteidae, Gobiidae, Leuciscidae, Mugilidae, Poecillidae, Percidae, Petromyzonida, Xenocyprididae, Acheilognathidae and Syngnathidae. The largest diversity of species belongs to the Leuciscidae. In this research, the species diversity increased from the upstream to the downstream of the river. According to the Brillion,s index (BI), the lowest BI was in station 3 with 1.415 and the highest in station 7 with 2.989. There was a significant difference in terms of BI between different stations (p<0.05). Examination of the diet of Kutum (R. frisii), C. carpio and A. persicus showed that different types of food items were detected in their digestive system. The average length and weight of R. frisii caught were 2.5 cm and 1.25 g, respectively. The study of the most important species index (ISI) in the digestive tract of Kutum showed that the Bacillariophyta was recognized as the most important food in the intestinal contents with 43%. The average length and weight of the caught C. carpio fry were 4.6 cm and 2.2 g, respectively. The ISI of the Cyanophyta was recognized as the most important food in the gut contents with 43.7%. The average length and weight of the caught fishes were 5.8 cm and 4.3 g, respectively.. The study of ISI in the digestive system of A. persicus fry showed that Copepoda with 46.8% as the most important food items. The study of the biological condition and the release of R. frisii, C. carpio, and A. persicus from artificial propagation in the Tajen River showed that the release time of each species was different, so that the A. persicus from the 20th of May, the R. frisii from the beginning of July and C. carpio were released into Tajen river from the end of August, Of course, the release of fry depends on the conditions of Shahid Rajaei Semeskandeh center in terms of weather conditions, water input, cost of maintenance and fish feed in different years, which may be done a little earlier or later. However, one of the most important conditions at the time of fries fish release is the water inlet of the Tajen River, which unfortunately caused many problems due to the construction of a rubber dam near the estuary of the river, and at the time of release, the flow of the river at the point where it meets the sea was zero and the only water returned from the Sea has been to the river. It seems that the fishes migrate to the sea after being released, because the visit and sampling ten days after the release of the fry showed that the fish had a short shelf life in the river, because no losses were observed from the released fry during this period. Therefore, it is suggested that a tagging method is needed in order to accurately check the return coefficient of juvenile fishes.
Key words: Fish, Juvenile fish obtained from artificial propagation, (Rutilus frisii, Cyprinus carpio, Acipenser persicus), Diversity index, Feeding regime, Tajen River, Mazandaran
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