Cortisol Response to a Crowding Stress: Heritability and Association with Disease Resistance to Yersinia ruckeri in Rainbow Trout

—The U.S. Department of Agriculture’s National Center for Cool and Cold Water Aquaculture is conducting a breeding program for rainbow trout Oncorhynchus mykiss to develop improved germplasm for the U.S. aquaculture industry. Current selection efforts are focused on growth and disease resistance, but stress response is also a concern. Using plasma cortisol concentrations after a 3-h crowding stress as a measure of stress responsiveness, we have previously shown family variation in cortisol responsiveness and a positive phenotypic correlation between poststressor cortisol level and growth performance within our broodstock. Selection for improved growth performance may therefore increase cortisol responsiveness to stress. In the present studies, narrow-sense heritability (h) for cortisol responsiveness was estimated to be moderately high by means of midparent–offspring regression and nested family design analyses (h 0.40 and 0.56, respectively). In addition, fish from the same families were used to determine associations between cortisol responsiveness to crowding and resistance to Yersinia ruckeri, the pathogenic agent of enteric redmouth disease, in an immersion challenge. Phenotypic and breeding values for cortisol responsiveness were not correlated with survivability with respect to Y. ruckeri whether or not the fish experienced repeated daily exposure to a crowding stress before the disease challenge. Furthermore, phenotypic and breeding values for poststressor plasma glucose and chloride levels and lysozyme activity did not correlate with disease survivability in stressed and unstressed fish. Finally, there was no correlation between the survival values of families when they were stressed before the disease challenge and when they were not, suggesting that there is variability among families in the effect of stress on resistance to Y. ruckeri. The U.S. Department of Agriculture (USDA) National Center for Cool and Cold Water Aquaculture (NCCCWA) is conducting a selective breeding program for rainbow trout Oncorhynchus mykiss. A family-based selection scheme is being used to breed two lines of rainbow trout that spawn in alternate years: an even-year line selected for improved growth performance and an odd-year line selected for improved disease resistance. Improved resistance to stress has also been identified as a target for genetic improvement because stress has been reported to have negative effects on many production traits, including our targeted production traits of growth and disease resistance (see reviews, Pottinger and Pickering 1997; Barton 2002; Portz et al. 2006). Stress resistance is a complex trait that is difficult to characterize. Many physiological measures of stress response have been evaluated as markers of stress resistance in rainbow trout, including plasma cortisol, glucose, chloride, and lysozyme activity, after a crowding stress (Fevolden et al. 1992, 1999, 2002, 2003; Pottinger and Carrick 1999a, 1999b, 2000, 2001; Weil et al. 2001; Røed et al. 2002; Trenzado et al. 2003; Øverli et al. 2005; Lankford and Weber 2006; Weber and Silverstein 2007). Plasma cortisol after a crowding stress has been found to be associated with both growth performance and disease resistance in rainbow trout (Fevolden et al. 1992, 2002; Pottinger and Carrick 1999a, 1999b, 2001; Weil et al. 2001; Øverli et al. 2005; Lankford and Weber 2006; Weber and Silverstein 2007). Although associations between these markers of stress response and traits of interest, such as disease resistance and growth, have been identified, the associations have not been strong * Corresponding author: [email protected] Received June 26, 2007; accepted October 27, 2007 Published online October 30, 2008 425 North American Journal of Aquaculture 70:425–433, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/A07-059.1 [Article]