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A captive population of Paci¢c white shrimp (Penaeus vannamei) replicated in two environments was evaluated for genetic variability and covariability of size traits. A total of 37 full-sib families, each with an average of 16^20 individuals within family, were used for the analysis. There was no family by environment interaction for any of the traits. However, both ¢xed effects, sex and environment, were signi¢cant. The shrimp grown in environment 'B' (Sinaloa) grew larger and heavier than those grown in environment ' A' (La Paz), possibly because of the higher temperatures and lower densities in the ¢rst one during grow-out. The females were signi¢cantly larger than the males for all traits, except for abdominal (tail) length and weight. The heritabilities, estimated through MTDFREML (multiple-trait derivative-free restricted maximum likelihood) after introducing environment and sex as ¢xed e¡ects, were total length 0.2270.07, abdominal length 0.2370.07, cephalothorax length 0.1770.06, total weight 0.1770.06, abdominal weight 0.1870.06, cephalothorax weight 0.157 0.06, and width of the ¢rst abdominal segment (0.1470.05). Genetic correlations (r g ) were high between the length and weight (total and abdominal), but lower for cephalothorax weight or length with the other traits. The width of the ¢rst abdominal segment correlations with other traits was intermediate.

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Heritabilities of size traits and growth rate traits, as well as genetic, phenotypic and environmental correlations were estimated at three ages for a captive population of Pacific white shrimp (Penaeus vannamei) grown indoors. A covariate, mean size or mean growth rate during early growth in individual tanks before tagging and mixing, was introduced in the analyses to reduce the error caused by unique previous growth conditions on variance components. Heritabilities of size traits increased with age, with the h2 for TL, AL, TWt and Wi1AS being 0.15, 0.20, 0.20 and 0.22, respectively, at 17 weeks, increasing to 0.28, 0.33, 0.34 and 0.35 at 29 weeks of age. Heritabilities of growth rate traits estimated between consecutive growth periods decreased from the first (h2 for ΔTL 0.65, ΔAL 0.71, ΔTWt 0.63 and ΔWi1AS 0.84) to the second period (h2 for ΔTL 0.34, ΔAL 0.50, ΔTWt 0.54 and ΔWi1AS 0.52). Phenotypic correlations were always larger than genetic correlations for both, size and growth rate traits. Genetic correlations between size traits within age were high (rG >0.95), but those between the same size trait at different ages decreased as the age difference increased in spite of a consistently high environmental correlation (rE 0.80–0.85) between the same trait at different ages. Phenotypic and genetic correlation's between the same growth rate trait at the two different growth periods evaluated were negative or zero (rG TL –0.26, AL –0.24, Wi1AS 0.00) with the exception of total weight (rG TW 0.35) and the environmental correlations between growth periods were also low (rE 0.13–0.32).