Risk factor studies of bacterial gill disease outbreaks in Ontario government salmonid hatcheries
This thesis is an investigation of risk factors associated with bacterial gill disease (BGD) outbreaks in early-rearing salmonids in Ontario government (Ontario Ministry of Natural Resources (OMNR) and Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA)) fish culture stations. Two retrospective studies were conducted utilizing (i) centralized OMNR records (1994-2001), and (ii) on-site records (1999) from three OMNR hatcheries. Subsequently, a 14-month (July 2002--August 2003) prospective matched nested case-control study was carried out to identify factors associated with BGD outbreaks in five OMNR and one OMAFRA hatcheries. Using multivariable proportional hazards survival analysis, the retrospective study using centralized records revealed that the species brook trout (' Salvelinus fontinalis') and the Spring season (March--May) were significant (p<0.10) risk factors for early-rearing lot treatment for BGD. Data from three hatcheries were analyzed separately using multivariable repeated measures in the second retrospective study. Significant risk factors for BGD were (i) the species brook trout ('S. fontinalis') and lower biomass (tank-lot level); (ii) lower individual fish weights, lower water exchange rates, and higher feeding rates (tank-lot level); and (iii) early-rearing, higher biomass, and a history of BGD treatments (lot-level). Multivariable logistic regression analysis of the prospective case-control data indicated tank-lots with confirmed BGD outbreaks were significantly more likely to have lower fish numbers, lower individual fish weights, higher mortality levels and higher feeding rates during the week preceding BGD outbreaks than were asymptomatic control tanks matched on hatchery, species, and time. The predictive (as opposed to causal) nature of these identified risk factors indicate the need for further research to understand the relationships between these factors and BGD.