Biological characteristics of nase fish ( Chondrostoma regium ) in Beheshtabad River in Chahar Mahal and Bakhtiari Province were investigated from April 2013 to March 2014. Some 323 specimens (139 females, 180 males and 4 immature) were caught, anesthetized in 1% clove oil solution and fixed in 10% formalin. In the laboratory, biometric measurements were performed on the specimens. Total length ranged from 8.4 - 22.10 cm in males, 7.84 - 23.15 cm in females and 5.95 – 7.08 cm in immature fish. Weight range for the males, females and immature fish were 5.20-105, 8.07-155 and 1.90-4.69 g, respectively. The length-weight relationship were calculated for males as W = 0.0089L 3.0451 (r² = 0.955), for females as W = 0.0082L 3.1095 (r² = 0.854) and for all fish as W = 0.0107L 3.0001 (r² = 0.916). Growth pattern in males and all fish were isometric while in females were positive allometric. Age groups of the males and females were 1 to 5 and 1 to 6 years, respectively. The age of immature fish was estimated as being under 1 year. The Von Bertalanfy length equation was calculated as L t = 31.89[1-e -0.148(t+2.067) ] and L t = 26.23[1-e -0.267(t+0.483) ] for males and females, respectively. The Von Bertalanfy weight equation was calculated as W t = 266.11[1-e -0.148(t+2.067) ] 3.1095 and W t = 26.23[1-e -0.267(t+0.483) ] 3.0451 , for males and females, respectively. The sex ratio was 1females:1.3males. Based on macroscopic gonad observation, maturation cycle consisted of five stages; immature, developing, spawning capable, regressing and regeneration phases. The minimum and maximum absolute fecundity were 1013 and 8177 ova with a mean (±SE) of 4210 ± 337 in females.The oocyte diameter increased significantly from April to June and decreased in July and August. The oocyte diameter range was estimated as 0.05 – 2.41 mm. The maximum value of gonadosomatic index of male fish was observed in April (5.19 ± 2.32) and of female fish in June (26.18 ± 1.81) both of which were significantly different from other months (p 0.05). The mean of this index in males and females were 1.41 ± 0.10 and 10.18 ± 0.62, respectively, and it was significantly higher than that in females. Mean condition factor was significantly higher in females (1.41 ± 0.02) than in males (1.02 ± 0.01). Mean relative length of the gut was 1.180 ± 0.050 for all specimens showing a herbivorous habit in this fish. Gastrosomatic Index in all fish, males and females showed no significant differences in different seasons (p 0.05). some 55% of digestive system of the fish, were empty and of 45% were full. The Vacuity index for all specimens was 48%, indicating a moderate feeding. Based on the observed macroscopic gonad, changes in oocyte diameter and gonadosomatic index, the spawning season of Chondrostoma regium was estimated from April to June. Keywords: nase, gonadosomatic index, age, Beheshtabad River
