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To assess the effects of two flooding events occurred in the years 2001 and 2002, fish distribution, diversity and abundance in Madarsoo River of the Golestan National Park were studied and compared to that of the years before the events. A total of five fish species from four sites were collected. Assemblage of fish population showed changes in their overall relative abundance and distribution across sampling sites and times. We estimated the abundance of Capoeta capoeta gracilis at 2.331:2.6, Alburnoides bipunctatus at 0.17±0.15, Paracobitis malapterura at 0.0474±0.031 and Leuciscus cephalus at 0.005±0.01 fish per square meter of the stream. The abundance of Cc.gracilis showed significant increase while that of the A. bipunctatus did not undergo such significant change in comparison with the data from the years before flooding events (P

In the years 2001 and 2002, two great floods occurred in the Madarsoo River in Golestan National Park, north-east of Iran. To study the effects of these floods, we assessed the age, growth and reproduction of Capoeta capoeta gracilis (Keyserlhng, 1861) in the river from November 2003 to December 2004. Fishes were caught by electroshocker and 1025 specimens were randomly selected from the catch. Growth in length was expressed for both sexes with Von Bertalanffy equation. Growth parameters were estimated as L infinity =249mm, K=0.22 per year and t _ (0) = -0.30 year for males and L infinity =306mm, K=0.21 per year and t _ (0) = -0.38 year for females. The length-weight relationships were described for males as lnW= -4.48 + 3.03 lnTL and as lnW= -4.59+3.0551 lnTL for females which shows a good feeding condition and a positive-isometric growth. Difference of length distribution between males and females is significant and males have lower lengths because this sex matures sooner so its growth is lower than females. Difference of fish length distribution between this study and previous studies on the C. capoeta gracilis of the river is significant and Lenkoran's length shows great decrease in this study as compared to previous studies. Age of the fishes ranged from 1 ^+ to 5 ^+ for males and 2 ^+ to 8 ^+ for females, the dominant age was 2 ^+ and the mean age was calculated as 2.009 ±0.034. We found that C. capoeta gracilis population had become younger in this study, so we concluded that the floods had made great changes in age and length structure of the fish.