POPULATION GENETICS OF EASTERN OYSTER Crassostrea virginica RESTORATION IN THE CHESPEAKE BAY
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The strategic release of captive-bred organisms is one of the most popular methods to restore species, but concerns exist regarding genetic impacts on natural populations over the long-term. Slow recovery of depleted eastern oyster C. virginica populations in the Chesapeake Bay prompted a large-scale hatchery-based restoration program consisting of the mass-release of hatchery-produced juveniles from local, wild broodstock. This dissertation characterized the genetic impact of this program, with the overall goal of understanding how characteristics of species life-history interact with hatchery practices to shape genetic variation in populations over short and long-time scales. In Chapter 2, analysis of genetic diversity changes resulting from hatchery production under two spawning designs (mass- and controlled-spawns) revealed substantial reductions in diversity and the number of breeders from parents to offspring, due primarily to high variance in reproductive success among adults in hatchery culture. In Chapter 3, high-resolution genomic data was used in a population genetic analysis comparing diversity of restored reefs in Harris Creek with variable planting histories and husbandry practices to ‘wild’ Chesapeake Bay oyster reefs. While restored reefs showed similar levels of diversity as wild reefs, strong positive relationships between planting frequency or broodstock numbers and genetic diversity were found, suggesting that hatchery practices can significantly impact diversity in natural populations. These genomic data also permitted the investigation of local adaptation and genotype by environment associations which revealed that salinity was correlated with loci putatively under selection, suggesting potential fitness tradeoffs for sourcing non-local broodstock. In Chapter 4, an individual-based model was created using biological and demographic data from Chesapeake Bay oysters to simultaneously evaluate the impact of multiple hatchery practices on natural population genetic diversity over time scales not possible with empirical methods. Overall, hatchery practices had a large effect on genetic diversity in most scenarios, but spawning practices (mass or controlled) and broodstock rotation were more important than broodstock number, suggesting that broodstock-limited programs may have other options to maintain diversity. In summary, these studies advance our understanding of how marine supplementation impacts both neutral and adaptive variation and will provide critical information for future oyster restoration efforts.