Predicted Response to Selection for Early Growth in Tilapia nilotica

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Abstract
Heritability estimates and genetic and phenotypic correlations were determined for length and weight at 45 and 90 days in the Auburn strain of T. nilotica. Heritability estimates were determined by half-sib analysis from an experiment that contained 16 paternal half-sib families or 32 full-sib families, i.e., 2 full-sib families nested in each of 16 paternal half-sib families. Average length and weight were 63 mm and 4.9 g at 45 days, and 99 mm and 18.6 g at 90 days. Sire heritability estimates ranged from 0.04-0.10, and dam heritability estimates ranged from -0.02-0.54. Dam heritability for length at 45 days was the only heritability estimate that was significantly different from zero (P = 0.05). At 45 days, environmental and dominance genetic variance were the major contributors to phenotypic variance, while at 90 days environmental variance accounted for virtually all of the phenotypic variance. Genetic and phenotypic correlations between length and weight at 45 and 90 days ranged from 0.94-1.12. Predicted gains from mass selection were small because of low heritabilities and small standard deviations. Low genetic variation may have been caused by drastic reductions in the effective breeding number during transfer of the fish stocks from the Ivory Coast to Brazil and from Brazil to Auburn [Alabama, USA]. Problems caused by reduction of genetic variance as a result of reductions in effective breeding number are discussed.