Population Thinning to Control Furunculosis in Steelhead

Furunculosis, which is caused by Aeromonas salmonicida, is a major source of mortalities at hatcheries rearing coldwater fish species, such as salmonids. Thinning and density reduction during an outbreak may be a viable method to control mortality. In this study, we tested whether a significant reduction in mortality would occur if an infected stock were split into smaller populations. A subpopulation of infected steelhead Oncorhynchus mykiss was removed from a raceway and split among 12 troughs to test for mortality differences between high‐density (3.99 fish/L) and low‐density (1.59 fish/L) populations and between fish treated with Terramycin‐medicated feed and fish given nonmedicated feed. Fish placed in the high‐density troughs were 1.6 times more likely to die than those in the low‐density troughs. Fish fed nonmedicated feed were 5.8 times more likely to die than those fed medicated feed. Additional observations of mortality differences between high (4.69 and 4.45 fish/L) and low densities (1.88 and 1.79 fish/L) within a raceway during furunculosis outbreaks in two separate years were also made. In both years, differences in mortality were apparent between density groups. The study showed that mortality reduction can occur when infected stocks are split into less dense populations.