Abstract
The effects of extreme freshwater events on Perkinsus marinus-Crassostrea virginica interactions remain unexplored. The effects of freshwater events on P. marinus infection in C. virginica and oyster survival were therefore examined in controlled laboratory experiments and a field study. For the laboratory experiments, oysters were collected in spring, summer and winter from an area in Louisiana where P. marinus is endemic. Oysters were placed in 2 recirculating water systems at a salinity and temperature similar to their collection site. They were subjected to 2 salinity treatments (freshet and control). Freshet events were simulated by reducing the water to salinities of 0 to 1 ppt over a 48 h period, and maintained for a 21 d period. Control oysters were maintained at the initial salinity. Thirty oysters were sampled prior to the freshet event, and 30 oysters per treatment group (freshet, control) were sampled on Days 7, 14 and 21 after initiation of the freshet event. Oyster mortality, P. marinus infection intensities, oyster condition index and oyster plasma osmolality were measured in weekly samples. All 3 simulated freshet events (i.e. spring, summer, winter) resulted in a significant reduction in P. marinus infection intensity, but failed to eliminate infection. The failure of the oyster plasma to reach very low osmolality (<50 mOsm kg-1) provides a likely explanation for the lack of complete P. marinus elimination. The field study involved sampling oysters monthly in the Caloosahatchee estuary, Florida, from September 2000 to February 2002, and determining P. marinus weighted prevalence and condition index of wild oysters, and growth and survival of caged juvenile oysters. The data strongly support the contention that the numerous freshwater releases to the Caloosahatchee River kept P. marinus infection intensities in oysters at low levels, resulting in an overall low weighted prevalence, low oyster mortality and good growth. Data from our field study appear to support the hypothesis that repetitive and well-timed freshet events can prevent infection of oysters with P. marinus or at least maintain P. marinus infections at non-lethal intensities (e.g. <106 parasites g-1 wet tissue) in oyster populations. The use of an adaptive management approach involving control of freshwater inflows could be invaluable to the oyster industry in areas close to freshwater diversion projects.