Biological study on Gorganrood River was performed from November 1990 until October 1991. Fourteen sampling Stations along the 200km distance from Chai Qushan to estuary. Fish distribution was not similar in all parts and was more dense in lower parts. Density Of common carp, roach and white fish were more considerable particularly in spawning season. Density of benthic organism was very low and blood worms (Chironomidae) and shells of dead mollusca were obtained. Frequency of phytoplankton maximum in September and minimum in January. Bacilliarophyceae, Chlorophyceae and Mynophyceae had more biomass during studying period. Zooplankton had maximum biomass in September and minimum in February and March. Among zooplankton, Protozoa and Rotatoria had maximum biomass.
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