2012 Annual Report
1a.Objectives (from AD-416):
Infectious disease is an important factor limiting trout production. Stakeholders have identified control of bacterial cold water disease (BCWD) as a high priority, and in response, the National Center for Cool Water Aquaculture initiated a breeding program to improve rainbow trout BCWD resistance. Rainbow trout have been selected for two generations for increased survival following experimental challenge with F. psychrophilum, the causative agent of BCWD. The objective of this research is to determine whether laboratory selection of trout for increased BCWD survival translates to improved performance following natural BCWD challenge in a production environment. Improved performance is defined as reduced mortality, clinical disease, and bacterial load as compared to control fish. The overall impact of this research is improved animal well-being, reduced antibiotic use and increased production efficiency.
1b.Approach (from AD-416):
Improved germplasm from ongoing ARS NCCCWA programs and breeding populations under investigation will be established in small and large scale field trials. In the small-scale trial, 25,000 eyed eggs will be produced from two NCCCWA fish lines: ARS-Fp-R (resistant) and ARS-Fp-S (reference) fish. Eyed eggs will be shipped to a Clear Springs Foods, Inc. and subsequently hatched and reared under standard production conditions. This experiment will evaluate three objectives:.
1)production and shipping of eggs;.
2)obtain baseline/comparative production and survival data; and.
3)investigate the incidence of BCWD infection. A large-scale field trial will include 70,000 fish from the ARS-Fp-R line (resistant); 35,000 fish from the ARS-Fp-C line (control line); and 35,000 fish from the ARS-Fp-S line (reference). All fish will be reared in a single production lot in order to ensure a uniform environment and equal pathogen exposure. Farm performance evaluation will consist of recording daily raceway mortality, monitoring clinical disease symptoms and kinetics including lesions and splenomegly, determining pathogen load in internal organs (by microbiological plating and quantitative PCR), and measurement of fish weight and length. Fish group will be determined by fin-clip with parental assignment made by PCR-based genotyping.
The NCCCWA has created three lines of rainbow trout with varying resistance to bacterial cold water disease: ARS-Fp-R (resistant), ARS-Fp-C (control) and ARS-Fp-S (susceptible). In collaboration with Clear Springs Foods (CSF), Inc, field trials were initiated in 2010 and 2011 to test the relative performance of these lines in Idaho. Analyses of results from field trials continued in 2012.
In 2010, the ARS-Fp-R line and ARS-Fp-S lines were reared in adjacent raceways and evaluated over an 80 day hatch-house period starting after first feeding. While several minor elevated mortality events occurred, no bacterial cold water disease was identified in either line. As a surrogate measure of relative performance, the phenotypes of the genetic lines were measured at CSF and also at the NCCCWA using laboratory injection challenge with F. psychrophilum. There was no difference in survival between resistant and susceptible lines measured at CSF while a significant difference was observed between the resistant and susceptible lines reared at the NCCCWA. These results suggested that environmental exposure modified the relative resistance phenotype. These findings as well as insufficient egg production at the NCCCWA led us to modify 2011 field trial design.
In 2011, ARS-Fp-R, C and S line fish were cultured within a single raceway split into three sections and received third-, second-, and first-use water, respectively. During this trial, clinical BCWD was diagnosed in the ARS-Fp-S line fish at 48 days post-hatch and elevated mortality followed, resulting in greater survival of the ARS-Fp-R line (95.7%) compared to the ARS-Fp-S line (91.2%). In conjunction with monitoring raceway mortality, fish were removed at three time points (31, 70 and 97 days post-hatch) and challenged with F. psychrophilum. The phenotypes of these fish were compared to an equivalent cohort, retained at the NCCCWA, which were challenged at the same time as the CSF cohort and with an equivalent dose of pathogen. Preliminary analyses confirm that the phenotype of the resistant, control and susceptible lines are sensitive to environmental influence, although the resistant line consistently had a greater, but not always statistically significant, survival rate compared to the other lines. The greater survival of the ARS-Fp-R line provides evidence of genetic improvement through selective breeding and warrants the release of the germplasm to industry and the continued evaluation of the ARS-Fp-R genetic line in large-scale production trials.