Figure 5. — Plasmodium of Minchinia nelsoni in oyster tissues, X 1,000. 



has been sustained by beds in low salinities, 

 and much of the Maryland section of the Bay, 

 because of lower salinities, has been free of 

 the disease — until recently. Extreme drought 

 conditions during 1963 to 1966 pushed the "sa- 

 linity barrier" of 15 parts per thousand further 

 up Chesapeake Bay into Maryland waters 

 (Rosenfield and Sindermann, 1966). Oyster 

 stocks previously unaffected acquired the dis- 

 ease (fig 6) and suffered mortalities --thus act- 

 ing to offset beneficial effects of the State's 

 shell planting and seed transfer program. 



Another critical aspect of the disease is the 

 nature of the oyster's resistance to infection. 

 It seems from studies carried on at the Bureau 

 of Commercial Fisheries Biological Labora- 

 tory, Oxford, Md., and elsewhere, that survi- 

 vors of the epizootic are more resistant to the 

 disease than are unexposed populations. Pos- 



sibly offspring of survivors will be more 

 resistant than those of unexposed populations 

 as selection pressure favoring resistance 

 continues over a number of generations. Inten- 

 sive work by the States of Maryland, Virginia, 

 Delaware, and New Jersey, financed partly by 

 Federal funds, is designed to develop resistant 

 stocks of oysters and by this method to bring 

 beds back into production sooner than might 

 occur naturally. 



Here then is a major producing segment of 

 the American fishing industry that is seriously 

 crippled by the single factor of disease, which 

 is still present in epizootic form in Delaware 

 Bay and the lower half of Chesapeake Bay. 

 Oyster stocks have not recovered and, if left 

 to nature, may take a number of years to do so 

 unless steps can be taken to hasten propagation 

 of resistant populations. 



