Prevalence and Bacteriological Identification of Bacterial Isolates Associated with Disease in Farmed Rainbow Trout (Oncorhynchus mykiss) in Kashmir, India

Abstract

Rainbow trout (Oncorhynchus mykiss) is an important cold-water aquaculture species in the Himalayan region, where bacterial diseases can adversely affect fish health and production. This study investigated the occurrence and bacteriological characteristics of bacterial isolates associated with clinically diseased rainbow trout at Kokernag Trout Fish Farm, Jammu and Kashmir, India. A total of 96 diseased rainbow trout were examined, and samples from the liver, kidney, gills, and external lesions were subjected to conventional bacteriological examination. Isolates were characterised based on colony morphology, Gram staining, and biochemical tests, including catalase, oxidase, urease, citrate utilisation, hydrogen sulphide production, indole, methyl red, Voges–Proskauer, hydrolysis reactions, and carbohydrate utilisation. One or more bacterial isolates were recovered from 77 fish (80.2%). The presumptively identified bacterial isolates included Streptococcus iniae, Streptococcus parauberis, Lactococcus garvieae, Lactococcus lactis, Flavobacterium branchiophilum, Flavobacterium psychrophilum, and Yersinia ruckeri. Lactococcus garvieae was the most frequently detected bacterial species, being recovered from 46 fish (47.9%), followed by Streptococcus spp. (19.8%), Flavobacterium spp. (14.6%), and Yersinia spp. (9.4%). The liver yielded the highest number of bacterial recoveries (47), followed by the kidney (37), gills (15), and external lesions (12). Mixed bacterial recoveries were also observed. These findings provide baseline information on bacterial isolates associated with diseased rainbow trout at Kokernag and support the need for continued bacteriological surveillance. Molecular confirmation and broader seasonal and multi-farm investigations are recommended to determine the distribution and broader epidemiological significance of these bacterial isolates in Kashmir trout aquaculture.