This thesis presents an investigation into Ontario Ministry of Natural Resources Fish Health Laboratory data from 1981-1997, to determine risk factors associated with lot-level detection of 'Aeromonas salmonicida ' and 'Yersinia ruckeri' in hatchery fish and to simulate monitoring efficacy in the absence of routine screening for these pathogens. Using stepwise logistic regression modeling, brook trout and back-cross were significantly more likely to test 'A. salmonicida'-positive, and brook trout were significantly more likely to test 'Y. ruckeri '-positive. For both pathogens, the 1-5 month age group was significantly associated with detection. The impact of eliminating routine screening was evaluated using Monte Carlo simulations. The mean probabilities that a hatchery would be pathogen-free if no asymptomatic lots were screened and all diseased lots tested negative for 'A. salmonicida' and 'Y. ruckeri' were 0.987 and 0.986, respectively, indicating acceptable monitoring efficacy. However, limitations of the theoretical monitoring model must be considered.