Oyster Mortalities, with Particular Reference 

 to Chesapeake Bay and 

 the Atlantic Coast of North America 1 



By 



CARL J. SINDERMANN, Fishery Biologist 



Bureau of Commercial Fisheries Biological Laboratory- 

 Oxford, Maryland 21654 



ABSTRACT 



A number of recent mass mortalities of oysters of the Middle Atlantic States 

 and elsewhere in the world have been attributed to the effects of disease. Oyster 

 production in Delaware Bay and lower Cheaspeake Bay has been seriously reduced 

 during the past decade by an epizootic of a protozoan pathogen, Minchinia nelsoni . 

 Other recent disease-associated mortalities of oysters have occurred in the Gulf 

 of Saint Lawrence and the Gulf of Mexico. Man may have aided spread of diseases by 

 transfers and overcrowding of beds. Reduction of this threat to oyster production 

 could be effected by quarantines, development of disease- resistant strains of oysters, 

 and use of environmental barriers (such as low salinity) to the pathogens involved. 



INTRODUCTION 



Many of the great fisheries of the world are 

 characterized by fluctuations in supply. The 

 causes of such fluctuations have been much 

 discussed but rarely determined. Accused as 

 causes of reduction in abundance of commercial 

 marine species have been overfishing, failure 

 of spawning, sudden and drastic changes in the 

 environment, and a host of other factors. 

 Disease has received limited attention as a 

 critical factor in population control. The fact 

 that marine animals get sick and die, often in 

 vast numbers, has been accepted but then often 

 ignored. Events have occurred in the oyster 

 industry in recent decades, however, that 

 force us to examine disease as a cause of 

 mass mortalities of epic proportions. This 

 paper summarizes recent information about 

 mass mortalities and their effects on the 

 American oyster ( Crassostrea virginica ) in- 

 dustry. 



' Based on material presented at the 22d meeting of the 

 American Fisheries Advisory Committee, Irvington, Va., 

 October 24, 1966. 



OYSTER MORTALITIES EST 

 CHESAPEAKE BAY 



The history and present status of the Chesa- 

 peake Bay oyster fishery can provide impor- 

 tant background information on a declining 

 resource. For most of this century, about 50 

 percent of the total national harvest of oysters 

 has come from the complex estuarine system 

 that we call Chesapeake Bay. The somewhat 

 dismal picture of national oyster production is 

 shown in figure 1. Landings descended errat- 

 ically from a peak of 170 million pounds of 

 meats in the late 1890's to the present level 

 of about 60 million pounds--which includes 

 about 10 million pounds of Pacific oysters, 

 Crassostrea gigas (Engle, 1966). Of this total, 

 Chesapeake Bay production once exceeded 

 100 million pounds but is now only about 20 

 million pounds. The reasons for this downward 

 trend for the past 70 years are complex. Some 

 reasons given are hotly debated, but they 

 certainly include such factors as loss of grow- 

 ing areas because of pollution, intensive fishing 

 on a natural stock without adequate manage- 

 ment, and the lack of development of anything 

 but the most rudimentary cultivation. 



