Occurence of Gas-bubble disease in Rainbow Trout

High mortality rate of rainbow trout was observed in one of the Kermanshah province rainbow trout farms in July, 2000. This mortality mostly occured infishes with 100-200g weight. Some symptoms such as inappetition, skin darkening, aggregation offishes in inlets and outlets of ponds and swimming near the surface were observed amongst most of fishes. 20 samples were collected from different ponds randomly and transferred to laboratory for further examinations. Gas bubble were observed on skin, fins and operculum of all studied fishes. This symptom was observed in mouth of only 10% of them. Also, in or studied samples, the unilateral or bilateral exophthalmia was Observed. Also, strong plants growing in water from spring to the farm was observed in which these plants caused a high fluctuations in amount of O2 and CO2 of pond's waters during day and night time