Prolonged Freshwater Exposure and Its Immediate and Long-Term Effects on Eastern Oyster (Crassostrea virginica) Populations in Breton Sound, Louisiana

With several large-scale freshwater diversion structures across coastal Louisiana, it is important to know the immediate and long-term effects of prolonged freshwater exposure on economically and ecologically valuable oyster populations. Beginning in early 2010, seed and market sized oysters in cages resting on the bottom, as well as recruitment plates suspended in the water column, were monitored at four sites in Breton Sound, LA, along what is typically a salinity gradient ranging from ~5 to ~20.  In April 2010, the Caernarvon Freshwater Diversion was fully opened with the goal of minimizing the impacts of the Deepwater Horizon oil spill on wetlands, resulting in extreme low salinity (< 2) at all sites through August 2010. High seed and market-sized mortality and reduced condition were observed in oysters at all sites. Only the site that normally has the highest salinity showed any recruitment in 2010.  Perkinsus marinus infection prevalence in surviving market oysters was low at all sites and all infection intensities were light. While low salinity may be beneficial to oyster populations by reducing P. marinus infection levels, prolonged extreme low salinity through spring and summer appears to cause heavy mortality and negatively impact recruitment in the short term. In August 2010, the expected salinity gradient was re-established and this study continues to monitor oyster populations for any lasting impacts of this low salinity event.