Effects of hatchery rearing density, handling, and nutrition on Renibacterium salmoninarum infection prevalence in juvenile Chinook salmon (Oncorhynchus tshawytscha).

Bacterial kidney disease (BKD), caused by Renibacterium salmoninarum (RS) is a chronic, often fatal disease of salmonid species, and can be particularly harmful to hatchery reared Chinook salmon (Oncorhynchus tshawytscha). Considerable research has focused on prevention of vertical and horizontal transmission; however, comparatively little research has investigated factors that increase prevalence of RS infection in captive environments. We evaluated the effects of three common hatchery conditions (handling, nutrition level, and rearing density), on RS infection prevalence. Fish were sampled at 30- and 60-days post-exposure to RS. Of 577 juveniles examined, 65 (11.27%) had anterior kidneys infected with RS. Using a logistic mixed model analysis, we found effects of nutrition level (P=0.018), handling (P=0.010) and sampling period (P=0.003) on the prevalence of RS. The interactions of nutrition and handling (P=0.008), and nutrition and time (P<0.001) were also significant. When fed a standard nutrition diet, proportionately fewer fish were infected with RS when not handled (7.16% vs 0.04%; P=0.003). Fish in the standard nutrition group also had a lower prevalence of RS during the second sampling period (4.08% vs 0.08%, respectively; P<0.001). When not handled, rearing with standard nutrition (11.50% vs 0.04%; P=0.004) resulted in a reduction in prevalence of RS infection. Additionally, non-handled fish had a much lower prevalence of RS infection during the second sampling period (2.66% vs 0.21%; P=0.009). While density did not affect prevalence of RS infection (P=0.145), fish reared at higher density had lower RS infection when not handled (16.48% vs 0.84%, p=0.004). For higher density fish, RS prevalence was lower during the second sampling period (10.57% vs 1.40%; P=0.002). Our results suggest that hatchery managers can reduce RS infection prevalence by maintaining an adequate nutritional regime as recommended by the manufacturer. Additionally, prevalence of RS may be reduced if managers decrease handling of hatchery-reared Chinook salmon if exposed to RS.