Previously unexplained and recurring 

 mortalities of oysters in the Gulf of Mexico 

 were examined during the late 1 940' s and were 

 found by Mackin, Owen, and Collier (1950) to 

 be caused by a fungus, Dermocystidium 

 marinum . Exerting its effects in higher salin- 

 ities and temperatures among dense aggrega- 

 tions of oysters, the pathogen has been reported 

 to cause annual mortalities in excess of 50 

 percent (Ray, 1954; Ray and Chandler, 1955; 

 Mackin, 1962). It has been identified in oysters 

 throughout the Gulf of Mexico and northward 

 along the Atlantic Coast as far as Connecticut 

 (fig. 7D). Although prevalences of the fungus 

 may at times reach epizootic proportions in 

 particular areas, its most significant effect 

 is probably that of continuing attrition year 

 after year during periods of high sea-water 

 temperatures. Effects of the disease on com- 

 mercial beds are now controlled to some 

 extent by planting and harvesting at prescribed 

 times of the year and by planting oysters 

 thinly on beds. 



In the Central Atlantic States oyster produc- 

 tion has been affected by "delaware bay 

 disease" causedby Minchinia nelsoni discussed 

 earlier (fig. 7C). Even within this area, a 

 second disease, called "seaside disease," 

 caused by a closely related pathogen, M. 

 costalis , has been found to kill oysters in 

 seaside bays of Maryland and Virginia (fig. 

 7B) and seems to be increasing in intensity. 



These mortality areas encompass a major 

 part of the oyster producing waters of the 

 Atlantic coast of North America--few areas 

 are exempt. In addition, other mortalities-- 

 some of them probably disease-related--have 

 occurred during the past decade in oyster 

 populations on the Pacific coast of the United 

 States and in Japan, France, and Australia. 



CONCLUSIONS AND RECOMMENDATIONS 



Obvious questions are: "Are these unusual 

 occurrences? Is the 20th century a time of 

 severe stress for oyster populations because 

 of these great epizootics? Could this period 

 be equated with the great human epidemics of 

 the Middle Ages, which reduced the abundance 

 of the human species significantly?" Although 

 it is true that disease is always with us, and 

 always with the oyster, and that mortalities 

 have undoubtedly occurred in the past, new 

 factors have been introduced by man to set 

 the stage for the spread of epizootic disease. 

 Oysters are transferred promiscuously from 

 one geographic area to another; populations 

 are often crowded in dense beds, sometimes 

 in areas where natural populations did not 

 exist previously; and drastic physical and 

 chemical changes have been made in oyster 



habitats. We may have helped to spread 

 diseases, and now we must control them 

 before we can move on to higher levels of 

 oyster culture. 



Another obvious question is "What can be 

 done when we are faced with a mortality 

 problem as great as the one that now exists 

 in the Middle Atlantic States?" The methods 

 of choice include: 



(1) Prevention of planting of susceptible 

 oysters in epizootic areas and a quar- 

 antine to prevent transfer of infected 

 individuals from such areas; 



(2) development of disease-resistant strains 

 of oysters by using hatchery techniques 

 and also by concentrating survivors of 

 epizootics on natural beds to serve as 

 spawning stocks for production of re- 

 sistant offspring; 



(3) use of the salinity barrier that was 

 mentioned earlier--the intolerance of 

 the disease agent to low salinities--to 

 prevent or possibly to eliminate infec- 

 tions; 



(4) protection of resistant populations from 

 fishing until adequate stocks are rebuilt; 



(5) use of artificial environments, such as 

 ponds, where disease control measures 

 can be effected; and 



(6) rapid development of basic information 

 about the life history and ecology of the 

 pathogen. 



Some progress in the development of know- 

 ledge about diseases of oysters--and in the 

 application of this knowledge to industry prob- 

 lems--has been made. Research and manage- 

 ment agencies --Federal, State, and univer- 

 sity—have been and are studying oyster 

 diseases and mortalities, so progress should 

 be rapid during the next several years. 



As a final note of perspective: It is of course 

 true that many different things--physical, 

 chemical, and biological--cankill oysters. Any 

 single factor, however, may become overriding 

 at any particular time in the life of a species; 

 here we have a clear instance in oysters of 

 how disease can reduce the abundance of a 

 marine species. It is likely that mass mortal- 

 ities are and have always been a natural method 

 of population control, but until recently they 

 would have been accepted with the same dazed 

 bewilderment and inaction that characterized 

 the behavior of our ancestors during the 

 plagues of the Dark Ages. We can now look 

 to methods of environmental control and stock 

 manipulation, particularly f o r sedentary 

 shallow- water species such as the oyster, as 

 part of the methodology in an increasingly 

 complex system of cultivation for coastal 

 waters. 



