Three generations of selective breeding improved rainbow trout (Oncorhynchus mykiss) disease resistance against natural challenge with Flavobacterium psychrophilum during early life-stage rearing

Bacterial cold water disease (BCWD) is a frequent cause of mortality in freshwater rainbow trout (Oncorhynchus mykiss) and the implementation of a family-based genetic selection program to improve disease resistance has been previously described. To quantify and understand the consequences of selection, three lines of outbred rainbow trout were created by divergent selection: ARS-Fp-R (resistant), ARS-Fp-C (control) and ARS-Fp-S (susceptible). Herein, results from both field and laboratory studies are described comparing survival of third-generation ARS-Fp-R line eggs/fish stocked at equal proportions to control and susceptible lines and reared together in a common environment. The survival of the pooled-line cohort was compared to the survival of an age-matched hatchery line, Fish Lake-DeSmet (FLDS) during early life-stage rearing. A typical BCWD outbreak occurred in the FLDS line while a more limited mortality event occurred in tanks containing the pooled lines. Mortalities from days 42-70 of the trial were saved for pedigree assignment and pathogen load measurement. After accounting for differences in early post-hatch survival, ARS-Fp-R line fish demonstrated two-fold lower BCWD-related mortality while ARS-Fp-S line fish demonstrated two-fold higher mortality; however, there was not a survival advantage of the ARS-Fp-R line fish over ARS-Fp-C line fish. At the end of the trial period, F. psychrophilum was not detected in spleen tissue of live-sampled ARS-Fp-R line fish while 10.3% of the ARS-Fp-C line and 36.4% of the ARS-Fp-S line fish exhibited detectable loads. Experimental challenge of the genetic lines with two different F. psychrophilum strains confirmed significantly higher survival of the ARS-Fp-R line fish as compared to either the ARS-Fp-S or FLDS genetic lines, albeit the susceptibility of the FLDS line was confounded by slower growth rate. These data demonstrate that three generations of artificial selection translated into improved survival following either natural or experimental exposure to F. psychrophilum. These data also suggest that hatchery lines, such as Fish Lake-DeSmet, may have higher relative susceptibility to BCWD thereby contributing to location outbreak severity.