Despite Tanzanian government initiatives promoting cage fish farming to offset income losses from declining capture fisheries, disease-related mortalities have emerged as a significant challenge, resulting in substantial economic setbacks for farmers. Since the pathogens responsible for these mortalities have not been adequately screened, farmers have resorted to guesswork, employing treatments that are often incorrect or insufficient. Therefore, this study aimed to identify and characterize the etiological agents responsible for infectious diseases in cage-farmed fish in Lake Victoria. A total of eighty-one swab samples were collected from morbid fish at twenty-six cage fish farms on the Tanzanian side of Lake Victoria, across six districts: Nyamagana, Sengerema, Musoma Rural, Musoma Urban, Rorya, and Busega. Microbiological and biochemical analyses revealed three bacterial genera: Aeromonas spp. (43.75%), Streptococcus spp. (31.25%), and Pseudomonas spp. (25%). A pathogenicity challenge trial was conducted at the Blue Economy Research Center, Sokoine University of Agriculture, involving 180 healthy Nile tilapia weighing 50-200 g. These fish were inoculated intraperitoneally with 0.5 ml of overnight cultures of the virulent bacterial isolates at a concentration of 3×107 cfu ml−1. The mortality rates observed were 86.7% for Pseudomonas spp. within six days, followed by 66.7% for Aeromonas spp. and 40% for Streptococcus spp. for ten days. Fish exposed to Pseudomonas spp. exhibited ragged fins, gill erosion, skin hemorrhage, white patches, detached scales, and scattered hemorrhagic spots. Streptococcus spp. caused fin rot, scale loss, eye hemorrhage, bulged eyes, eye opaque and finally ayes loss. Aeromonas spp. led to fin rot, skin hemorrhage, skin ulcers, and finally open sore. The findings provide crucial insights into the specific pathogens responsible for fish diseases, establishing a baseline for developing more effective disease management strategies in cage fish farms across the country.
Key words: Cage Fish Farming, Lake Victoria Basin (LVB), Fish Diseases, Mortality Rates, Clinical Signs