Collapsed oyster populations in large Florida estuaries appear resistant to restoration using traditional cultching methods—Insights from ongoing efforts in multiple systems

Abstract

Objective: Depressed eastern oyster Crassostrea virginica populations in the northern Gulf of Mexico have been the target of numerous post-Deepwater Horizon restoration projects. These projects primarily have focused on replacing oyster cultch (substrate) to promote spat settlement, increase recruitment, and bolster adult oyster populations. This study assessed the outcomes of six such efforts, which used different cultch types and densities between 2015 and 2022 in three estuaries on the Florida panhandle (Pensacola, St. Andrew, and Apalachicola bays). Total restoration costs for these projects were more than US$14 million. Methods: Using generalized linear models, we analyzed oyster count data collected from diver surveys in three size-classes (spat, seed, and adult). We tested whether oyster population responses in the six restoration efforts varied over time, location, or study design. Result: Oyster counts did not persistently increase after restoration, regardless of cultch type or density. Positive responses to restoration efforts were irregular and short-lived and seemed only to occur for spat-size oysters immediately after restoration. None of the restoration efforts significantly improved the abundance of oysters of any size-class in any of the study estuaries. Factors contributing to these results likely include design and implementation elements, such as the materials used and the height of the restored reefs. However, monitoring programs have not been able to deliver a clear picture of what is hindering restoration success. Conclusion: For oyster restoration efforts to succeed, changes are needed—both in their implementation and in the way they are monitored—in order to promote continuous learning and improvement in restoration outcomes.