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Fishery and population dynamics of Caspian kutum, Rutilus frisii kutum (Cyprinidae), in the Caspian SeaThe Caspian Sea is the largest enclosed body of water in the world. The water is slightly brackish. The aim of project was to determine length at maturity (Lm_50%), evaluate of feeding items, investigation some of biological aspects included in sex ratio, growth and age structure, instantaneous mortality coefficient, maximum sustainable yield (MSY) and break-even point of Rutilus frisii kutum in southern part of the Caspian Sea. Specimens of Kutum were collected from the catches obtained from 131 sample sites along the Iranian coast of the Caspian Sea. The minimum and maximum fork length and weight was 21cm and 58cm with mean 38.4±6.4 cm; 104 and 2450 g with mean 775.2±382.5 g, respectively. The sex ratio of males to females was 0.65: 1 (males: females). The condition factor in Guilan was more than Mazandaran and Golestan provinces. The length at maturity (Lm_50%) of Kutum obtained was 39.1cm. The minimum and maximum absolute fecundity varied from 15,713 to 130,737 eggs with mean 69,961.7±3,836.4. The reproduction phase was extended from February to April, peak in March, with the highest average values 5.52 for males and the highest value was 17.00 for the females in April. Due to males‟ migration from sea to river occurred sooner than that of the females. The diet consisted of different prey items, bivalves representing the most important prey group (59%), and followed by Cerastoderma lamarki (57%). The instantaneous natural mortality rate was estimated M= 0.46 y^-1. Based on total instantaneous mortality coefficient (Z) from the seasonalized length-converted catch xxv curve, F (the instantaneous fishing mortality coefficient) as 0.82 y^-1 through the relationship: F=Z-M, and giving a current exploitation rate E (=F/Z) as 0.64 y^-1. The length-at-first capture and age-at-first capture was obtained L_c =36.8 cm and 3.92 years, respectively. The Maximum sustainable yield (MSY) based on cohort analysis for Kutum resulted MSY=20,004 tonnes with biomass B of 51,937 tonnes. The selection ogive procedure gave the following summary: E_max=0.76, E_0.1=0.65 and E_0.5=0.39. Yield per recruit analysis suggests that the stock of this species has a moderately exploitated at E=0.64. Kutum contributed about 76% of total sell bony fish in south of the Caspian Sea and total income of Kutum sell were estimated US$30,415,998. The income of the fishermen through selling of Rutilus frisii kutum from total bony fish income was 74% for Guilan, 85.8% for Mazandaran, and 26% for Golestan provinces. Therefore, R. f. kutum plays a significant role in fishermen‟s income in southern coasts of the Caspian Sea. The break-even point of Kutum obtained with mean 19,220±5,364.5 for Guilan, 18,843±4,989.2 for Mazandaran, and 10,677±1,358.2 for Golestan provinces, which coincided with the fishing effort, catch ratio, number of fishing cooperatives, and number of labour.