Both age and size influence susceptibility of Pacific oysters (Crassostrea gigas) to disease caused by Ostreid herpesvirus-1 (OsHV-1) in replicated field and laboratory experiments

New farm management practices are required to reduce the impact of disease caused by Ostreid herpesvirus-1 (OsHV-1) on commercial production of Crassostrea gigas. Both the age and size of oysters are thought to be correlated with survival after exposure to OsHV-1. It is important to distinguish between the protective effects of these factors because the phenotypic plasticity of C. gigas enables a wide range of oyster sizes at any given age. The purpose of this study was to: (1) determine if the age and size of C. gigas have independent effects on susceptibility to disease caused by OsHV-1; (2) determine whether deliberate manipulation of the size of oyster spat (defined as oysters ≤ 12 months old) or adults (oysters > 12 months old) would alter their susceptibility to disease; and (3) assess whether size manipulation could be used as a feasible management strategy to protect C. gigas from mortality due to OsHV-1. Two experiments were conducted, with replicated field and laboratory challenges. In Experiment 1, a protective effect of larger size, independent of age (spat v. adults), was shown for oysters that had grown to be larger under standard commercial farming conditions. After adjusting for clustering due to site and basket, the hazard of death in small oysters was 1.9 times that of large spat and adult oysters. In a second experiment, the size of oysters was deliberately manipulated by restricting the growth of a half of the oysters (independent of starting size); immersion time was limited using a higher longline height compared to oysters held at the optimal intertidal growing height for 6 months, in a region free of OsHV-1. After controlling for the effects of variation in exposure due to location in the field, size group did not impact mortality for either age group but the hazard of death for oyster spat (8 months old) was 5.5 times that of adult oysters (17 months old). Laboratory trials for these prospectively differentiated oysters indicated an interaction between age and size whereby smaller spat (growth restricted) were relatively protected (hazard ratio 0.6) compared to those grown to a larger size. For adults the hazard of death was higher (HR = 2.3) for smaller oysters. Further investigation is required before size manipulation of oysters can be effectively utilised as a management strategy to protect C. gigas from OsHV-1 associated mortality. A thorough understanding of the physiological and metabolic condition of oysters, produced under different grow-out conditions when challenged with OsHV-1 is required.