Oyster Disease and Climate Change. Are Yearly Changes in Perkinsus marinus Parasitism in Oysters (Crassostrea virginica) Controlled by Climatic Cycles in the Gulf of Mexico

. The protozoan Perkinsus (=Dermocystidium) marinus is the most important pathogen of eastern oysters (Crassostrea virginica) in the Gulf of Mexico. Prevalence of P. marinus has been related to salinity and temperature, with low temperatures and salinities usually limiting infection. In 1986, a yearly monitoring program was begun to describe the regional distribution and yearly trends in P. marinus prevalence and infection intensity in the Gulf of Mexico in connection with NOAA's Mussel Watch program. Between 1986 and 1989, prevalence and infection intensity dropped in the southeastern and southwestern Gulf, infection intensity most strongly, and rose in the Florida panhandle, prevalence most strongly. Infection intensity fell but prevalence rose on both sides of the Mississippi delta; central/north Texas remained essentially unchanged. The regional and temporal distribution of P. marinus in the Gulf of Mexico can be considered a product of two spatio-temporal phenomena. (1) A relatively stable spatial pattern exists each year with centers of infection on the order of 300 km. (2) A concordance in yearly shifts in prevalence and infection intensity occurs among sites on a scale of > 1,000 km. The spatial scale of concordant yearly changes is much larger than the scale of centers of infection, and probably originates in broad shifts in weather patterns as they affect temperature and salinity (via rainfall and river runoff). Long-term climatic changes are most likely responsible for these spatio-temporal shifts and, as such, P. marinus prevalence and infection intensity may eventually be predictable from climatic models. Our data demonstrate the importance of multi-year cycles, not just seasonal cycles and occasional heavy rains, in determining P. marinus prevalence and implicate salinity as the primary mediating factor. Summary Perkinsus marinus plagues most oyster populations in the Gulf of Mexico including nearly every commercial one. Sixty to eighty percent of all Gulf oysters are infected and summer mortalities are frequently high. The regional and temporal distribution of P. marinus in the Gulf of Mexico can be considered a product of two spatio-temporal phenomena. (1) A relatively stable spatial pattern exists each year, with centers of infection on the order of 300 km. (2) A concordance in yearly shifts in prevalence and infection intensity occurs among sites on a scale of > 1,000 km. The spatial scale of concordant yearly changes is much larger than the scale of centers of infection, and probably originates in broad shifts in weather patterns as they affect temperature and salinity (via rainfall and river runoff). P. marinus is known to respond to climatic variables such as temperature and rainfall (salinity). The productivity of oyster populations has also been related to changes in climate (Allen & Turner, 1989; Ulanowiczet al., 1980; Chatryet al., 1983), salinity being the most important variable. Our data demonstrate the importance of multi-year cycles, not just seasonal cycles and occasional heavy rains, in determining P. marinus prevalence and also implicate salinity as the primary mediating factor. Only a shift in climate could explain the concordance in yearly shifts in prevalence on a 1,200 km scale. The El Nino/Southern Oscillation is a good candidate for the agent responsible, although 4 years of data are not adequate to conclusively prove this conjecture. Regardless of the source, long-term changes in climate clearly exert a significant impact on P. marinus prevalence and infection intensity, probably through salinity, and likely impact oyster population dynamics and fishing success in the Gulf of Mexico as well.