Rainbow trout ŽOncorhynchus mykiss., average weight 32.2 g, were obtained from
the Finnish Game and Fisheries Research Institute ŽFGFRI., Laukaa, Finland. The fish
were fed with a standard commercial feed ŽEwos Vextra Mini 2.5 mm. and had not been
vaccinated. The fish were held in one tank before the start of the trial. After 2 weeks
Žday 15., they were randomly divided into three experimental group, each containing
100 fish and were placed into 225-l tanks each. A fourth group with approximately 80
fish was held as a control, to determine any mortality not related to the study during the
experimental period. Two groups recieved their respective Lactobacillus feeds: 109 and1012 CFUrg from day 15 to the end of the trial. On day 30, the challenge was started by
cohabitation Žsee below. in the two Lactobacillus-fed groups and one control group. The
fourth group, with 80 fish, was not challenged nor fed lactobacilli.
The flow rate of the water was about 4 l miny1 in each tank. The water temperature
was 158C pre-challenge, and during the challenge trial, the water temperature was raised
to 17.5–198C and kept at this temperature for the rest of the study time.