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Title: Factors associated with the incidence of bacterial gill disease in salmonid lots reared in Ontario, Canada government hatcheries

Author
item GOOD, CHRISTOPHER - FRESHWATER INSTITUTE
item THORBURN, MARGARET - UNIVERSITY OF GUELPH
item STEVENSON, ROSELYNN - UNIVERSITY OF GUELPH

Submitted to: Preventive Veterinary Medicine
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2007
Publication Date: 1/20/2008
Citation: Good, C.M., Thorburn, M.A., Stevenson, R.M. 2008. Factors associated with the incidence of bacterial gill disease in salmonid lots reared in Ontario, Canada government hatcheries. Preventive Veterinary Medicine. 83:297-307.

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 retrospective epidemiological risk-factor study of BGD treatments using laboratory and centralized data from the Ontario Ministry of Natural Resources (OMNR) fish culture program, 1991-2001. From the results of this study, there appear to be important hatchery-level factors that influence the occurrence of BGD in the OMNR fish hatchery system, and hence the design and analysis of future research should account for the hatchery effect. The species (and strains within species) reared by a given OMNR hatchery appear to strongly influence the level of BGD experienced at that site. Among all hatcheries observed, BGD risk percentages for individual species were inconsistent from year-to-year, and it is difficult to determine the contribution that hatchery-level factors made toward these calculated values. Detailed within-hatchery studies would avoid the potential problems of hatchery being confounded with species (and hatchery-level environmental or management factors). Brook trout were consistently associated with BGD compared to other species, and a seasonal trend in BGD diagnosis and treatment was noted, with peak occurrence in the spring season. However, the variable size of the population at risk throughout the year needs to be taken into account when considering this apparent seasonal pattern. 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: Bacterial gill disease (BGD) (causative agent: Flavobacterium branchiophilum) has been a persistent problem in early-rearing salmonids in the Ontario Ministry of Natural Resources (OMNR) fish hatchery system. Retrospective epidemiological investigations of BGD diagnoses and treatments in OMNR fish hatcheries during the period 1991 -2001 were conducted using University of Guelph Fish Health Laboratory and OMNR central office data. All investigations were conducted at the lot-level, which is the major within-hatchery-level of population aggregation. Survivorship of BGD diagnosis in earlyrearing lots within seven individual hatcheries ranged from 84.2 to 100%; within individual species groups, survivorship was lowest (84.6%) in brook trout (Salvelinus fontinalis) lots. Annual risk percentages (cumulative incidence) for BGD diagnosis within hatchery and species groups varied considerably among years. Multivariable proportional hazards survival analysis indicated that the species brook trout, and the Spring (March -May) season, were significantly associated with treatment for BGD. Combined, these results emphasize the importance of hatchery, species, and time on the development of BGD. Future observational research on this disease must consider these factors in their design and analysis.