This investigation was conducted in order to find the etiological factors of fry mortality syndrome that causes notifiable economical lost in rainbow trout fish farms. During 43 months, from Sep. 1999 until May 2002, 52 fish farms and hatcheries which were located in Tehran, Mazandaran, Guilan, Fras, Markazi, Kerman and kohgiloyeh & boyerahmad provinces, were visited and samples from suspected fishes were obtained. The samples were consist of ovarian fluids and ovule, milt, egg, eyed-egg, larvae, fry less than 1 g and at 1-3 g as well as adult fish consist of internal organs. From these collected samples, 410 tissue sample, 899 sample, 55 Tissue sample, 200 larvae sample, 56 blood serum sample, 50 whole blood sample, 104 Tissue sample and 55 samples were used for Virological, Bacteriological, Electron Microscope assay, DFAT) , ELISA, Hematological, Histopathological and PCR assay, respectively. The clinical signs of suspected fishes were color darkness, exophthalmia, aschitis, whirling and abnormal swimming. No cytopathic effects (CPE) were observed after inoculation of suspected samples on the cell lines of BF-2, FHM, and EPC. A number of some samples of 56 blood serum samples were identified as positive sample for IHN, VHS and IPN virus. Optical Density (OD) of positive samples was 7-9 times more than blank samples. Regarding to the result of this study, it could be indicated that the causative agents for fry mortality syndrome may be IHN, VHS and IPN viruses. Further investigation using PCR for definitive diagnosis and confirmation of the causative agents would be recommended.

Iranian Fisheries Science Research Institute

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