ABSTRACT: Outbreaks of parasitic salmon lice Lepeoptheirus salmonis in sea-cage salmon farming regions of coastal seas have challenged the productivity of salmon farming industries and the conservation of wild salmon. We used a simple mathematical model to evaluate the population ecology of louse outbreaks, parasiticide treatment, and louse population decline for 2 farms in the Broughton Archipelago region of British Columbia, Canada. Results suggest that exponential population growth of lice within a farm, rather than sustained louse immigration from wild sources, drive outbreaks on farms. Model analysis indicates that louse infection pressure from farms to wild juvenile salmon may be minimized by parasiticide application 2 to 3 mo preceding the juvenile salmon outmigration. The observed timing of parasiticide use and population decline of lice on farms is consistent with reported declines of lice on wild juvenile salmon. If parasiticides do not have adverse environmental effects and lice do not evolve resistance, optimized parasiticide use on salmon farms may help reduce the spread of lice to wild salmon populations.