Investigating the Causes of Sudden Spatial Shifts in Oyster (Crassostrea Virginica) Abundance and Traits in Northeast Florida

Oyster reef habitats historically supported coastal economies and ecosystems; however, anthropogenic activities have caused a reduction in oyster populations to only 15% of historic levels. Estuaries along the Atlantic coast of Florida contain a portion of the remaining viable oyster reefs, but a recent sudden reduction in shellfish harvests suggests that oyster (Crassostrea virginica) populations may be experiencing a major decline. Habitat surveys throughout 15 km of the Matanzas River estuary evaluated the spatial component of this decline and found that southern reefs had less oyster biomass and more environmental and biological stressors than did northern reefs. Comparative field experiments revealed that the north-south gradient in the increasing loss of oysters was positively correlated with predatory crown conch (Melongena corona) abundance and water salinity, however, predation appeared to be the proximal stressor for oysters because oysters survived throughout the estuary as long as conchs were excluded. An additional comparative field experiment was conducted to further investigate the different effects of predation (consumptive vs. non-consumptive) on oysters and how these effects combine to influence oyster traits (shell length, shell thickness, tissue biomass). Tests across spatial gradients of environmental conditions evaluated the relative importance of oyster life stage and predator identity on oyster traits. Results revealed that simulated predation (consumptive effect) did not influence the growth of oyster traits in either life stage, however, traits varied considerably as a result of the interaction between study location and predator cue (non-consumptive effect). Oysters growing in the presence of crown conch cues had a reduced shell thickness compared to oysters growing in the absence of predator cues, and this pattern was consistent across both oyster life stages and each study location. Furthermore, the magnitude of the crown conch cue effect on reducing shell thickness was largely influenced by the site-specific tidal circulation patterns such as the proportion of time oysters had aerial exposure as well as the amount of water flow. Habitat surveys of natural oyster and crown conch populations illustrated that oyster reefs with the presence and abnormally high abundance of predatory conchs had smaller shell lengths and a higher abundance of newly deceased oysters in comparison to oyster reefs with the absence of conchs. The combination of the experiment and survey results suggest that the observed oyster trait modification (i.e. decrease in shell thickness) in response to crown conch cues has been unsuccessful as a mechanism for anti-predator defense and reducing mortality on oyster reefs throughout the Matanzas River estuary. The use of comparative field experiments accompanied by habitat surveys of natural conditions provides a better link between isolated predator-prey dynamics and the complex environmental and biological factors interacting within estuarine systems. Additional research that examines conch-oyster dynamics across spatial and temporal disturbance gradients is necessary to ensure the conservation as well as restoration of oysters throughout the NE Florida ecoregion.