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Summary In genomic selection (GS), genome‐wide SNP markers are used to generate genomic estimated breeding values for selection candidates. The application of GS in shellfish looks promising and has the potential to help in dealing with one of the main issues currently affecting Pacific oyster production worldwide, which is the ‘summer mortality syndrome’. This causes periodic mass mortality in farms worldwide and has mainly been attributed to a specific variant of the ostreid herpesvirus (OsHV‐1). In the current study, we evaluated the potential of genomic selection for host resistance to OsHV‐1 in Pacific oysters, and compared it with pedigree‐based approaches. An OsHV‐1 disease challenge was performed using an immersion‐based virus exposure treatment for oysters for 7 days. A total of 768 samples were genotyped using the medium‐density SNP array for oysters. A GWAS was performed for the survival trait using a GBLUP approach in blupf90 software. Heritability ranged from 0.25 ± 0.05 to 0.37 ± 0.05 (mean ± SE) based on pedigree and genomic information respectively. Genomic prediction was more accurate than pedigree prediction, and SNP density reduction had little impact on prediction accuracy until marker densities dropped below approximately 500 SNPs. This demonstrates the potential for GS in Pacific oyster breeding programmes, and importantly, demonstrates that a low number of SNPs might suffice to obtain accurate genomic estimated breeding values, thus potentially making the implementation of GS more cost effective.

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The robustness of Paci¢c oyster, Crassostrea gigas (Thunberg), sperm cryopreservation in the context of selective breeding based on family lines was investigated. Irrespective of egg density, high fertilization success was achieved with cryopreserved sperm when sperm:egg ratios of 1000:1 to 10 000:1 were used.Variation among replicate runs on the same oyster batches was minimal, indicating that cryopreservation and larval rearing procedures were repeatable. Twenty independent single male^female crosses were made to assess the utility of cryopreserved sperm in selective breeding. The fertility of unfrozen sperm was generally a poor predictor of cryopreserved sperm fertility. Based on D-larval yields, 17 of the 20 crosses were likely to yield adequate spat for selective breeding (410 5 D-larvae from 1million eggs), two were marginal (5 Â 10 4 D-larvae) and one was inadequate (4 Â 10 3 D-larvae). An alternative fertilization strategy to improve D-yield from a given number of sperm was then tested. Fertilizing10 million eggs at a sperm:egg ratio of 200:1 increased the total D-yield when compared with fertilizing 1 million eggs at a sperm:egg ratio of 2000:1 for the same male^female pair. We conclude that, despite wide variation in fertility, cryopreserved sperm is useful for family production.