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With a number of oyster farmers in Alabama reporting high oyster mortalities across the summer of 2016, Extension personnel worked with oyster farmers to collect samples in late August and September 2016. Samples were collected from five distinct growing areas:
Where possible, oysters from both the 2015 and 2016 crop were collected. After collection, samples were processed at the Auburn University Shellfish Lab to assess Dermo (Perkinsus marinus, with more information here) and at Dr. Ryan Carnegie's shellfish pathology laboratory at Virginia Institute of Marine Sciences for a complete assessment of the oysters from a health perspective using histopathology. Notably these diseases do not affect people, but can cause oyster mortality.
In Dr. Carnegie's tests, he found no infection by MSX, providing further evidence that the Gulf of Mexico remains free of this major pathogen. In terms of Dermo, the disease was present in all the 2015 samples with the exception of that from Mobile Bay. Furthermore, within the 2016 oysters, only oysters from Grand Bay and Bon Secour Bay had any signs of infection and these were relatively light.
Considering Dermo in the 2015 oysters and based on the relative numbers of heavier infections, Dr. Carnegie indicated that 25-35% mortality could be attributed to Dermo in both the Grand Bay oysters and the Bon Secour triploids, but that it would account for a substantially lower amount of mortality at all the other sites. At these sites, he saw no reason to attribute high levels of observed mortalities to infectious diseases.
In the histopathology, erosion of oyster digestive tract lining and gills was observed in some cases. These are indications that the oysters were subject to some type of environmental stress, though the source of the stress cannot be identified. In general, external environmental factors, whatever they may have been, would be more plausible causes of elevated mortality that was observed.
For a copy of the report, with tabulated data, please see this report here.
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