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Journal of Shellfish Research








Densities of Crassostrea virginica remain high enough to support substantial fisheries throughout the Gulf of Mexico despite high mortality rates produced by the endoparasite Perkinsus marinus. The infrequency of epizootics in these populations suggests that controls exist on the disease intensification process. The progression of epizootics in oyster populations, the factors that trigger epizootics, and the factors that terminate epizootics once started were investigated with a coupled oyster population—P. marinus model.

The time development of a simulated epizootic was triggered by environmental conditions that occurred and disappeared as much as t8 months prior to the onset of mortality in the oyster population. Initiation of epizootic conditions was detected as an increase in infection intensities in the submarket-size adult and juvenile portions of the oyster population. Infection intensity of the market-size adults is maintained at a relatively stable level by the death of heavily infected individuals and the slow rate of P. marinus division at high infection intensities. Once started, most of the simulated epizootics resulted in population extinction in 2 to 4 years. Stopping an epizootic required reducing the infection intensity in the submarket-size adults and juveniles. The infection intensity of market-size adults does not need to be reduced to stop an epizootic nor must it be raised to start one.

The simulated oyster populations show that a reduction in ingestion rate (by reduced food supply or increased turbidity) can trigger an epizootic, especially if the reduction occurs during the summer. lncreasing food supply or decreasing turbidity in the following year does not necessarily prevent the occurrence of an epizootic. Rather, the onset or the event is simply delayed. Additional simulations show that the relative combination of variations in salinity and temperature is important in determining the occurrence of an epizootic. A dry (high-salinity) summer followed by a warm winter produces conditions that favor the development of an epizootic. Conversely, a warm dry year followed by a cool wet year fails to produce an epizootic. Simulations that consider variations in the biological characteristics of oyster populations, such as changes in recruitment rate or disease resistance, show that these are important in regulating the occurrence of an epizootic as well as in terminating the event. In particular, increased recruitment rate dilutes the infected population sufficiently to terminate an epizootic.

One primary conclusion that can be obtained from these simulations is that epizootics of P. marinus in oyster populations are difficult to generate simply with changes in either temperature or salinity. Rather, the epizootics are triggered by some other factor, such as reduced food supply or reduced recruitment rate, that occurs prior to or coincident with high salinity or temperature conditions.


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Original Publication Citation

Powell, E. N., Klinck, J. M., & Hofmann, E. E. (1996). Modeling diseased oyster populations. II. Triggering mechanisms for Perkinsus marinus epizootics. Journal of Shellfish Research, 15(1), 141-165.