THE RELATIONSHIP BETWEEN INCREASING SEA-SURFACE TEMPERATURE AND THE NORTHWARD SPREAD OF PERKINSUS-MARINUS (DERMO) DISEASE EPIZOOTICS IN OYSTERS

From its initial discovery in the Gulf of Mexico in the late 1940s unt il 1990, Perkinsus marinus, the parasite responsible for Dermo disease in the eastern oyster, Crassostrea virginica was rarely found north o f Chesapeake Bay. In 1990-92, an apparent range extension of the paras ite led to epizootic outbreaks of the disease over a 500 km range nort h of Chesapeake Bay. One of the hypotheses for the range extension arg ues that small, undetected numbers of parasites were already present i n northern oysters as the result of repeated historical introductions, and that a sharp warming trend in 1990-92 stimulated the disease outb reak. This argument was based on trends in air temperature. The presen t study examined this hypothesis by analysing water temperatures, rath er than air temperatures, for five stations located in areas affected by the recent epizootics. At all five stations, there was a strong inc reasing trend in winter sea-surface temperature (SST) between 1986 and 1991. At four of the five stations, there was a smaller increasing tr end in winter temperatures after 1960. There were no consistent or obv ious trends in summer (August) temperatures. In Delaware Bay, which ha s a 40 year history of monitoring for oyster diseases, occasional find ings of P. marinus in oysters were correlated with warming episodes th at were especially notable in the winter (February) record. Empirical orthogonal function (EOF) analysis showed that winter temperatures var ied consistently at the stations examined and were associated with var iations in P. marinus prevalence. Associations using EOF analysis with August temperatures were much weaker. The SST record is consistent wi th the hypothesis that increasing winter water temperatures have been important in the recent outbreak of P. marinus epizootics in the north -eastern U.S.A. (C) 1998 Academic Press Limited.