Reductions in Susceptibility of Channel Catfish, Ictalurus punctatus , to Enteric Septicemia of Catfish through Two Generations of Selection

Selective breeding for disease resistance has hadvaried success in agricultural animals. In aquaculture, selection for disease resistance is ongoing in salmonids, carp, Japanese flounder, and catfish as well as in other fish and shellfish (Haskin and Ford 1979; Gjedrem 2000; Vandeputte 2003; Fuji et al. 2006). Improving disease resistance through selective breeding can be problematic because disease resistance is typically a complex trait and selection based solely on phenotypic response to challenge can be less than ideal (Gjedrem 1983; Beacham and Evelyn 1992). The nature of the challenge model may also affect heritability. Heritabilities are higher in experimental challenges in salmonids than in natural challengeswith the same pathogens, with the exception of furunculosis (see review in Gjedrem 2000). Enteric septicemia of catfish (ESC) is the most prevalent disease affecting commercial catfish operations in the southeastern USA (USDA 1997) and is caused by the bacterium Edwardsiella ictaluri (Hawke et al. 1981). The USDA–ARS Catfish Genetics Research Unit (Stoneville, MS, USA) has focused on genetic improvement of channel catfish, Ictalurus punctatus, through selective breeding since 1991. The early stages of the channel catfish breeding program at the USDA–ARS Catfish Genetics Research Unit focused on improvement of growth through strain comparison and within strain selection. This resulted in a joint release of germplasm to commercial catfish producers by U.S. Department of Agriculture–Agricultural Research Service and Mississippi State University. This strain of channel catfish was designated the NWAC103. Among the NWAC103 strain of channel catfish, there is considerable phenotypic variation among full-sib families and consistent family rankingswithin a year class for ESCsusceptibility across multiple challenges (Wolters and Johnson 1994; Bilodeau et al. 2003, 2005). There are also differences in lysozyme activity and pathogen loads carried during challenge among families with large differences in susceptibility to ESC. Strain and species comparisons of catfish have also demonstrated a large range in susceptibility to ESC challenge (Bosworth et al. 1998, 2004). Since the release of the NWAC103 strain, the selection program has been expanded to incorporate multiple traits, including growth, carcass yield, and ESC resistance. Each trait is assigned an equal weight. The current generation of fish (USDA303) is the result of two generations of family selection using multitrait selection. The purpose of this study was to assess the effectiveness of the multitrait selection index for reducing susceptibility to ESC by comparing the second-generation selected strain (USDA303) of channel catfish to the unselected base population (NWAC103) during experimental challenge with E. ictaluri.