Stark differences in spatial gradients of Eastern oyster (Crassostrea virginica) productivity in two Florida, USA, estuaries

The eastern oyster (Crassostrea virginica) is a foundation species in U.S. Atlantic and Gulf coast estuaries because of its numerous ecosystem services and importance to local fisheries. Concerns about the persistence and sustainable harvesting of oysters in the state of Florida is growing due to environmental factors (e.g., increased predation, salinity, and disease) and increased harvesting pressure. Thus, we need to understand which estuary locations are best suited for conservation or restoration efforts. Here, we use a combination of field experiments and modeling to assess the potential oyster productivity at sites throughout two estuaries at the same latitude but in different ocean basins: Apalachicola Bay (Gulf of Mexico coast) and the Guana Tolomato Matanzas National Estuarine Research Reserve (GTM, Atlantic coast). Our population model used the per-recruit approach from fisheries science, allowing us to combine field data on growth and mortality to estimate productivity (measured as lifetime spawning output) in an elegant way, without requiring assumptions about unknown larval supply processes. Subtidal Apalachicola Bay oysters exhibit an expected pattern, with greater productivity in the inner estuary nearer the river mouth and predation reducing productivity in the seaward direction. In contrast, intertidal GTM oysters do not exhibit strong spatial gradients, with little effect of predators and slightly higher productivity near the river mouth; intertidal oysters in Apalachicola exhibited similar productivity and lack of predator effect. We attribute these differences in oyster productivity to differences in tidal exposure, the predator community, and hydrology of oyster reefs at the two study sites. Thus, we would give diverging recommendations for spatial prioritization of restoration and conservation efforts. Additionally, these results lead us to caution against generalized statements about spatial patterns of oyster productivity within estuaries without consideration of the predation context.