Date of Award

Spring 5-10-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Marine Biology

First Committee Advisor

Paul Rawson

Second Committee Member

Damian C. Brady

Third Committee Member

Chris Davis

Abstract

Aquaculture of the Eastern oyster (Crassostrea virginica) occurs across a broad range of environmental gradients along the East and Gulf Coasts of the United States. Multiple breeding programs have been in pursuit of stocks with improved growth, disease resistance, and yield across these variable conditions. Field trials assessing the performance of genetic lines developed in these programs typically measure size at discrete time points, on oysters haphazardly sampled from replicate grow-out cages. Estimates of growth are then derived from the average sizes measured at each time point for each cage, potentially masking the tremendous variability in the growth of individuals over time. Additionally, in a market that demands uniformity in size, farmers cull “slow growing” individuals rather than keep all size classes, and the genetic and evolutionary consequences of this process is unknown. This highlights the need to better understand within- and between-individual variation in oysters, and whether they result from genetic variation, environmental differences, or both. I monitored the individual growth of oysters cultured at two farms, Pemaquid Oyster Co. (POC) and Ferda Farms, located in two different Maine estuaries across a full growing season (May–November, 2024). Oysters were tagged and measured bi-weekly for shell height and whole weight, and paired with high-resolution environmental monitoring of temperature, chlorophyll-a, salinity, and turbidity. Using Generalized Additive Models (GAMs), we modeled growth rates as a function of shell height, chlorophyll, and temperature, accounting for both site and genotype (fast vs. slow growing) effects. To assess genotype-by-environment (G×E) interactions, I controlled for genetic background by deploying oysters from a single hatchery cohort derived from a selectively bred line. This approach ensured that all individuals shared a similar genetic origin, minimizing broad-scale genetic variation unrelated to growth classification. In addition, my analyses compared the performance of fast and slow growing oysters (phenotypes) at each site. Determining the underlying causes, mechanisms, and consequences of the capacity of a genotype to produce different phenotypes in response to environmental variation (phenotypic plasticity) is crucial to the continued success of Maine’s aquaculture industry as it continues to expand. My results revealed strong site-specific differences in growth performance, with higher overall growth at POC despite oysters at Ferda starting the season larger. Growth rate declined with increasing shell size, particularly among fast growers, though the change was not statistically significant. This observation was consistent with metabolic scaling theory. Both chlorophyll and temperature were significant drivers of growth, but their effects varied by site, providing clear evidence for G×E interactions. Fast growers exhibited greater plasticity in response to favorable environmental conditions, while slow growers showed more constrained growth across gradients. These findings underscore the importance of incorporating individual-level data and G×E effects into breeding strategies, especially as the industry moves toward greater environmental diversification and selective culling practices.

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