Age, Growth and Reproduction of the Female Kutum, Rutilus kutum (Kamensky, 1901) (Teleostei: Cyprinidae), in Gorgan-Rud Estuary, Northern Iran

Fishes in the eastern part of the Caspian Sea experience a higher temperature and salinity, compared to those in the western part. This study was conducted on 66 specimens of female kutum, Rutilus kutum, from a migratory population to Gorgan-Rud River during March-May to compare it with the most studied western populations and to study the effects of these ecological factors on biology of kutum. The population comprised six age groups. The 5+ and 6+ age groups were dominant in the studied population. The sex ratio was 1:1.5 (M:F) that was significantly different from 1:1 ratio (p<0.05). The slope (b) of the fork length-weight regression line indicated an isometric growth (p>0.05). Relative and instantaneous growth rates decreased with age. The mean value of condition factor was 1.50 which was not correlated with age. Absolute fecundity varied from 13494 (7 years old) to 115177 eggs (8 years old). The average absolute and relative fecundity were 70300 and 51, respectively. The relationships of fecundity (F) with fork length (FL) was better stated as a power and with total weight (W), ovary weight (Wo) and age (t) as linear equations. Fecundity with FL, W, Wo and age had positive and high correlations. The relationships of ovary weight with total weight and fork length were as a linear function. Egg diameter varied from 158 μm to 251 μm (mean=194±17). Gonadosomatic index (GSI) varied from 4.92 to 29.05 (19.42±4.53) during the reproduction period and its peak determined to be in the last week of April. It is concluded that these ecological factors could highly affect the kutum populations and compared to the western populations, the eastern population differed in a lower b value, a lower ratio of male:female, a lower absolute and relative fecundity and a higher egg diameter. This extends our knowledge on the biology of kutum populations in different areas of the Caspian Sea.