Effects of Freshwater Inflows on Oysters in the Caloosahatchee River, Florida

To examine the impacts of freshwater inflows on oysters, physical parameters such as salinity, temperature, dissolved oxygen and freshwater flows as well as oyster responses (condition index, disease prevalence and intensity of Perkinsus marinus), spat recruitment, juvenile growth and gonadal condition were determined in the Caloosahatchee River, Florida. Significant relationship exists between freshwater inflows and salinities at various points in the Caloosahatchee estuary. Flows below 4000 CFS into the estuary from S-79 will result in salinity regime that will enable oysters to survive and grow. Disease prevalence was lower at upstream locations and increased with distance downstream, suggesting that higher salinities result in increased disease incidence. Although disease prevalence of P. marinus is higher in oysters, the intensity is relatively low (< 1.5) due to a combination of freshwater releases resulting in lower salinities and the antagonistic effect of higher temperatures and salinities in summer and winter, respectively. Limited freshwater releases for durations of less than two weeks will result in lower disease prevalence and intensity of oysters and higher survival. Condition index of oysters appears to be related to spawning activity. Oysters at the upstream location, Iona Cove had higher condition index compared to other sampled locations. Oysters in the Caloosahatchee appear to spawn actively between May and October, a period that coincides with freshwater releases and or watershed runoff. While downstream locations attract higher spat recruitment due to higher substrate availability and estuarine conditions during high flow summer-fall months, growth and survival of juveniles is poor. Limiting freshwater releases to < 4000 CFS during these months will limit flushing of oyster larvae to downstream locations and create favorable salinity regime for spat recruitment and survival. Gonadal index of oysters at Iona Cove was higher and oysters appear to spawn early at this location compared to other sites. Growth rate of juvenile oysters also appear to be higher at Iona Cove compared to other locations, although low salinities resulting from high lake releases affect their survival. Low disease incidence, high condition index, sufficient spat recruitment and high growth rate at the upstream locations (e.g. Iona Cove) suggest that with the provision of suitable substrate and limiting freshwater flows during the spawning season, oyster reefs will survive and grow at the upstream locations. With the implementation of CERP projects and reduction in the freshwater flows, it is anticipated that focus of oyster reef development will be shifted upstream compared to current locations.