LAUSR

Abstract Several Pacific oysters breeding programs selecting for improved aquaculture traits have been described in the literature; however, there are differences in the approaches and traits under selection. Here we discuss the approach and results of the Molluscan Broodstock Program (MBP) to improve traits of interest for US West Coast oyster farmers. In particular, we compare selection methods based on individual versus group traits measured per grow-out container. A total of 442 group records (per container) and 2197 individual records for field traits of up to 55 families were used for statistical analyses. Heritability estimates for group traits at harvest for survival, yield and individual average harvest weight were moderate-to-high and the latter estimate was not statistically significantly different from harvest weight based on individual measurements. Family-by-site (G × E) correlations were moderately negative between shell length and shell shape and strongly positive between yield and individual average weight. Genetic correlations were all positive among group traits, ranging from moderately positive between individual average weight and survival to highly positive between individual average weight and yield. Genetic correlations between individual average weight (a group trait) and individual weight (an individual trait) ranged from 0.95 to 0.99. In contrast, genetic correlations between shell shape and group traits (yield, individual average weight and survival) ranged from −0.63 to −0.22. The potential genetic gains were higher for group traits (12.1%, 8.4% and 6.9% for yield, individual average weight and survival, respectively) than for individual weight (5.6%). The BLUP accuracies were similar between group traits and individual weight. High genetic correlations between individual average weight and individual weight at harvest suggest that gains in individual weight can be achieved by selection for either individual average weight or individual weight; however, our results indicated that selecting families for shell shape will cause an unfavorable decrease of other commercially important traits. Statement of relevance The Pacific oyster, Crassostrea gigas, is a globally cultivated species that is grown under a wide range of environmental and husbandry conditions. In this study, we describe genetic parameters for desirable traits of selected Pacific oysters planted at commercial farms on the West Coast, US, as well as a comparison of selection methods based on individual versus group traits measured per grow-out container. Our results indicate that group traits and individual traits are genetically associated and have moderate-to-high heritabilities. Furthermore, we found a very high genetic correlation between individual average weight per container (a group trait) and weights of individual oysters sampled per container (an individual trait) at harvest, suggesting that individual average weight is a valid and less costly alternative to measurements of sampled individual oysters in breeding programs designed to improve oyster harvest weight. The results will be of interest to other breeding programs for Pacific oysters.