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In a selective breeding program for the Portuguese oyster, Crassostrea angulata, oysters are cultured in several environments including the broodstock for breeding (Cat Ba) and production (Van Don) locations. This raises the potential that a genotype by environment interaction may occur and oyster genotypes may re-rank across environments. Whole weight and uniformity in weight at the time of harvest are two of the most important economic traits for oyster production in northern Vietnam. Limited information, particularly with respect to uniformity, exists for C. angulata. Our primary aim was to quantify the genetic variance in harvest whole weight and its uniformity and the genetic correlation between them using a double hierarchical generalized linear model. We investigated the degree of genotype by environment interaction on the uniformity of harvest whole weight in the two environments. Since the weights had a skewed distribution, we also investigated the affect of log-transformation on the estimates of the genetic variance in harvest whole weight, its uniformity, the genetic correlation between them and on the genotype by environment interactions. Results showed that heritability estimates for uniformity of harvest whole weight were low for Cat Ba (0.07) and Van Don (0.06) environments using standardized data, while these estimates were lower using log transformation for both environments. There was no re-ranking of oysters with regard to harvest whole weight and uniformity across environments; the genetic correlations were effectively one, with and without log transformation. Therefore, there is no scope to improve uniformity in the Portuguese oyster population while increasing whole weight at harvest.

Production of the Portuguese oyster, Crassostrea angulata, is increasing rapidly in Vietnam, and consumer preferences for oyster appearance and meat quality have become more economically important. However, no studies have been conducted to estimate genetic parameters for meat colour, edible traits and the occurrence of disfiguring parasite infestation in any oyster species. In this study, we report genetic parameters for parasite disease traits, objective measurements of shell and mantle colours (L*, a* and b* colour) and edible traits (water holding capacity, taste and tenderness) for a selectively bred Portuguese oyster breeding population. We applied a restricted maximum likelihood method to estimate genetic parameters for the traits studied. Heritability estimates for the colour measurements of shell and mantle were low to high (h2 = 0.13–0.57), whereas those obtained for the parasite diseases on shells and tissues were close to zero (h2 = 0.011). Interestingly, there was genetic variation in edible traits (h2 = 0.04–0.17). The genetic correlations among whole weight and parasitic shell and tissue disease traits was significant but unfavourable, whereas the genetic correlations between whole weight and water holding capacity was considerable and favourable (rg = 0.87 ± 0.05). These suggest that a multi-trait restricted selection index should be used to improve productivity and economic returns for the oyster aquaculture sector.

The Portuguese oyster Crassostrea angulata is one of the economically important mollusc species but to date there has been no genetic improvement program conducted for this species. The principal aim of this study was to assess genetic gains achieved from a selection program for improved whole weight at harvest over three generations from 2015 to 2017. A total of 30,212 oysters had performance data recorded; they were the offspring of 323 sires and 461 dams. Linear mixed models were used to estimate genetic parameters of the traits studied. Selection for improved harvest whole weight achieved significant direct response by 17.4% after three generations. There were also positively correlated changes in soft tissue weight and shell shape traits (3.67% and 3.98%, respectively). The genetic gains in harvest whole weight and favourably correlated changes in meat yield and shell shape traits were consistent with the positive genetic correlations of whole weight with these traits (0.08–0.50). There is abundant genetic variation in the selection criterion (heritability for whole weight = 0.24), suggesting that the present population of Portuguese oysters will continue showing responses to future selection. The estimates of heritability for meat yield and shell shape traits were low (0.10–0.16). Hence, a multi-trait selection program can be used to enhance overall productivity and economic return for oyster producers.