SPRING FORWARD WITH IMPROVED NILE TILAPIA Oreochromis niloticus RESISTANT TO Streptococcus iniae AND Streptococcus agalactiae Ib

Tilapia aquaculture worldwide is valued around US $ 7 billion.  Tilapia are an important source of protein for domestic (top 5 most consumed seafoods) and global food security.  Two gram postitive bacteria, Streptococcus iniae and S. agalactiae, are responsible for billion dollar losses annually.  Genetic gains in performance traits (e.g., growth) have been realized in Nile tilapia and interest in breeding for disease resistance has received attention.  The objectives of this study were three fold: 1) to verify previous results demonstrating heritability of S. iniae resistance in Nile tilapia; 2) to determine if realized genetic gain in resistance and/or susceptibility to S. iniae is possible following positive assortative mating between parents with high or low estimated breeding values; and 3) to determine if resistance to S. iniae and S. agalactiae Ib is genetically correlated. A total of 144 and 130 full sib families were challenged intraperitoneally with S. iniae and intramuscularly with S. agalactiae Ib, respectively.  Cumulative mortality at test end was 46 % for S. iniae and 68 % for S. agalactiae.  There was a high additive genetic component found for survival in fish injected with S. iniae (estimated heritability 0.52 ± 0.12) similar to 0.42 ± 0.07 determined the prior year.  The estimated heritability for S. agalactiae was 0.38 ± 0.11 based on the univariate linear animal model.  Positive assortative mating further demonstrated resistance to S. iniae is heritable and thus resistance can be improved upon through selective breeding (Table 1).  No genetic correlation was noted between resistance to S. iniae and S. agalactiae Ib.   The lack of correlation suggests if resistance to both Streptococcus sp. is desired, selection for both traits must be simultaneous.  Selection of fish to improve survival to Streptococcus sp. may require a thorough understanding of the type of pathogen prevalent in the region so that custom genetic material may be tailored to meet the needs of the individual farm and/or region.