Elevated mortalities of triploid eastern oysters cultured off-bottom in northern Gulf of Mexico

© 2019 Elsevier B.V. After unusually high mortalities of eastern oysters (Crassostrea virginica) were reported at commercial farms in Alabama during the summer of 2016, this study was conducted to determine whether the mortalities were reproducible, site-specific, and linked to ploidy. Seed of triploid and diploid oysters were deployed in November 2016 at four sites adjacent to commercial oyster farms in Alabama: Mobile Bay, Navy Cove, Grand Bay, and Dauphin Island. Mortality, growth, condition index, gametogenesis, and Perkinsus marinus infection intensity were monitored until October 2017. Mortalities were significantly greater in triploids by the end of the study, but the magnitude and timing of mortalities differed among sites. Triploids and diploids at Mobile Bay experienced severe mortalities in summer (100% in about two months), and the start of this mortality event was delayed in diploids. Elevated triploid mortalities were observed at Navy Cove (77%) and Dauphin Island (60%) in late spring through fall, while oysters at Grand Bay suffered the lowest triploid mortalities (34%). Possible causes of the high mortalities included environmental conditions, such as prolonged periods of low salinity and high temperature in summer at Mobile Bay, as well as, heavy P. marinus infection intensities in fall at Grand Bay. Despite elevated triploid mortalities, both triploids and diploids showed faster shell height growth at Navy Cove and Dauphin Island. Conversely, the site with minimal triploid mortality had the slowest shell height growth overall, but the greatest difference in shell height between triploids and diploids. Analysis of gametogenic stages and gonad-to-body ratios over the summer revealed diploids primarily in the spawning stage and triploids showing delayed and diminished gametogenesis, with lower gonad-to-body ratios than diploids