Submitted to: Preventive Veterinary Medicine
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/1/2009
Publication Date: 10/1/2009
Publication URL: http://handle.nal.usda.gov/10113/55565
Citation: Good, C.M., Thorburn, M.A., Ribble, C.S., Stevenson, R. 2009. Rearing unit-level factors associated with bacterial gill disease treatment in two Ontario, Canada government salmonid hatcheries. Preventive Veterinary Medicine. 91:254-260. Interpretive Summary: Bacterial gill disease (BGD) is an important disease in cultured salmonids worldwide, and can cause high mortalities in young fish populations during conditions optimal for the causative agent, Flavobacterium branchiophilum. As this agent is ubiquitous in fresh water, outbreaks of BGD are considered related to environmental stressors such as poor water quality, increased rearing densities, and fish movements. Despite the importance of this disease to the freshwater salmonid aquaculture industry worldwide, there has been very little observational epidemiological research carried out to identify, and quantify the importance of, the various environmental variables associated with outbreaks of BGD. This study presents findings from a rearing unit-level retrospective epidemiological risk-factor study of BGD treatments in two Ontario Ministry of Natural Resources (OMNR) fish culture stations during the 1999 production year. Important findings made through this study were the identification of elevated feeding rate as an associate variable with BGD treatment, as well as reduced tank water exchange, specific species and life-stages, and prior mortalities (evidence of an ineffectual treatment regime or the persistence of BGD in affected populations despite treatments to control BGD-related mortalities). The findings of this study were used to guide more in-depth, prospective epidemiological investigations with refined validity for identifying important BGD risk factors.
Technical Abstract: Early-rearing salmonids in Ontario Ministry of Natural Resources (OMNR)fish hatcheries have been consistently affected by bacterial gill disease (BGD)(causative agent: Flavobacterium branchiophilum) for many years. Separate retrospective epidemiological investigations of BGD treatments at two OMNR fish hatcheries (Hatcheries A and B) for the 1999 production year were conducted using on-site hatchery records. Both investigations were carried out at the rearing unit-level, with early-rearing (<9 months of age) "tank-lot" as the unit of analysis to identify unique fish populations over time. Multivariable repeated measures logistic regression models were created for both hatchery datasets, controlling for lot-level and species effects. For Hatchery A, the species brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) were significantly associated with BGD treatment, as well as lower water exchange rate, and higher feeding and mortality percentages during the two weeks previous to BGD treatment. At Hatchery B, the species brook trout (S. fontinalis) and splake (Salvelinus namaycush X S. fontinalis) were significantly associated with BGD treatment, as well as lower individual fish weights and treatment for BGD during the previous week. These results emphasize the importance of water quality, feeding rate, fish size and prior mortality on the development of BGD. Significant hatchery and species effects were evident, and future observational research on BGD must account for these factors in their design and analysis.