Genetic (co)variation between harvest weight and resistance to both Streptococcus iniae and S. agalactiae capsular type Ib in Nile tilapia (Oreochromis niloticus)

Two bacterial pathogens that impact worldwide tilapia production through direct mortalities and treatment costs are Streptococcus iniae and S. agalactiae. An economically important trait is rapid growth; however, it is not known if this trait is correlated to Streptococcus disease resistance. The objectives of this study were to: 1) determine the relationship between growth of Nile tilapia (Oreochromis niloticus) and survival to both S. iniae and S. agalactiae capsular type Ib; 2) verify previous results demonstrating heritability of resistance to S. agalactiae Ib and S. iniae through positive assortative mating; and 3) determine the response to selection obtained from genetic trend analysis and estimates of heritability. Data from eight generations of tilapia were used in this study including growth data for all generations, S. iniae mortality data from three generations, and S. agalactiae Ib mortality data from five generations. Additive genetic variation was found for all three traits: harvest weight heritability was 0.23, S. iniae heritability was 0.43 and S. agalactiae heritability was 0.21. Significant effects common to full-sibs were found for all traits and were of higher magnitude for harvest weight (c(2) = 0.11) than for both S. iniae (c(2) = 0.04) and S. agalactiae (c(2) = 0.05). The genetic correlation between harvest weight and S. iniae survival was low (r(g) = 0.15 +/- 0.12) and not significantly different from zero. The genetic correlation between harvest weight and S. agalactiae Ib survival was intermediate and negative, but not significantly different from zero (r(g) = -0.21 +/- 0.11), suggesting there is no relationship between growth and resistance to Streptococcus sp. These results demonstrate that multi-trait selection is needed to balance growth and resistance to both Streptococcus sp. to provide commercial tilapia farmers with a fast-growing fish with reduced susceptibilities to these two important pathogens.