Response to five generations of selection for growth performance traits in rainbow trout (Oncorhynchus mykiss)

A pedigreed rainbow trout population (similar to 100 families per generation) was selected for five generations to improve growth performance to the standard similar to 500-gram US market weight and beyond (>1 kg). Body weights (BW) were recorded each generation at 5, 8, 10, and 13 months post-hatch. Selection was applied to a two-trait index of estimated breeding values for 10-month body weight (i.e., approximate market weight) and thermal growth coefficient between 10 and 13 months, and both traits were given equal weighting in the index. Objectives of this study were to estimate direct selection responses for BW traits and thermal growth coefficient, and correlated response for condition factor traits. Heritability estimates were 0.28 +/- 0.08 to 0.43 +/- 0.14 for BW between 5 and 13 months post-hach, 0.12 +/- 0.03 for thermal growth coefficient, and 0.40 +/- 0.04 and 0.36 +/- 0.04 for condition factor at 10 and 13 months post-hatch, respectively. Genetic correlations among the four BW measures were highest between adjacent BW measures (0.54 +/- 0.19 to 0.82 +/- 0.05) and decreased as time between measures increased. Genetic correlation between thermal growth coefficient and 10-month BW was low (0.18 +/- 0.13), and genetic correlations between condition factor and BW measured at the same age were moderate (0.39 +/- 0.09 to 0.45 +/- 0.08). Compared to randomly-mated control lines, selection response in the selected line averaged 10.6% (5-month BW), 9.8% (8-month BW), 12.7% (10-month BW), 11.9% (13-month BW), 2.1% (thermal growth coefficient), 3.4% (10-month condition factor), and 3.3% (13-month condition factor) per generation. The rate of inbreeding averaged 0.86% per generation in the selection line, with a cumulative inbreeding estimate of 4.5% in the last generation. The low heritability of thermal growth coefficient, and low genetic correlation with 10-month BW, resulted in only modest selection response for thermal growth coefficient. Larger heritabilities of BW measured at fixed ages, and large genetic correlations between serial BW measures, suggest that direct selection on BW will be more favorable than selection on TGC. This study demonstrates the large and sustained potential for genetic improvement of growth performance in domesticated rainbowtroutwith onlymodest accumulation of inbreeding, and providesempirical selection response estimates that have direct inference to commercial breeding programs aiming to select for growth to larger market BW.Statement of relevance: Quantitative genetics data from this relatively long-term and large-scale selective breeding program have direct inference to commercial breeding programs selecting for improved growth, but for which maintenance of control lines is not practical. Published by Elsevier B. V.