Differences in Bacterial Population in Rainbow Trout (Oncorhynchus mykiss Walbum) Fry after Transfer from Incubator to Pools

The microflora of rainbow trout (Oncorhynchus mykiss Walbaum) fry from a commercial freshwater hatchery, along with important water quality parameters such as temperature, dissolved oxygen and pH, was analysed. Samples for bacteriological analysis were taken from gill, heart and kidney, from the third to the eighth week after hatching. Pure bacterial colonies were examined macroscopically, with Gram staining and biochemical tests. For final identification, the APILAB Plus programme (bioMérieux, France) was used. The bacterial populations of rainbow trout fry changed depending on age. Most of the bacterial colonies were cultured from the gills (64.4 %), rather than the heart (21.8 %) and kidney (13.8 %). The bacterial community of fry gills from an incubator was composed mostly of Gram-positive bacteria such as Renibacterium salmoninarum, Lactobacillus spp., Staphylococcus spp. and Corynebacterium aquaticum. After the transfer of fry from incubator into the pools the Gram-negative bacteria increased in number and became the dominant microflora of rainbow trout fry and comprised more than 95 % of its bacterial flora. Flavibacterium, Acinetobacter and Yersinia were the predominant Gram-negative genera in fry in the incubator, whereas Aeromonas and Pseudomonas were the main isolates from rainbow trout fry until the end of the experiment