PEOGRESS IN BIOLOGICAL INQUIRIES, 19 30 615 



In response to this request a cooperative investigation was begun 

 by H. F. Prytherch and W. H. Dumont of the Federal Government 

 on May 12, 1930, for the purpose of determining : First, the cause of 

 the mortality of oysters during the previous winter; and second, 

 the most suitable means for restoring and increasing their production. 

 In carrying out its program, the bureau has received excellent coop- 

 eration from the department and officials of the Commission of Fish- 

 eries of Virginia, from the department of biology, William and Mary 

 College, and from the State health department. The investigators 

 are particularly indebted to Capt. John B. Bush of the Katie for 

 information regarding local conditions and assistance in the experi- 

 mental work. 



Though the investigation is still in progress, it was deemed ad- 

 visable to issue a brief summary of the work performed and the 

 results which have been obtained up to the present time (January 

 15, 1931). 



Since the greatest loss of oysters occurred in Mob jack Bay and 

 vicinity, this phase of the investigation has been concentrated chiefly 

 in that region. A temporary field laboratory was established at 

 Yorktown and cruises made at regular intervals with the State boat 

 Katie to various stations in Mobjack Bay and York River. 



Experiments were made also in the James and York Rivers for the 

 purpose of determining a practical policy for rehabilitation of the 

 oyster resources. 



During this phase of the investigation particular attention Avas 

 paid to the oxygen content of the water in Mobjack Bay as a defi- 

 ciency of oxygen has been responsible for the destruction of oysters 

 in other regions. In the lower layers of water in this bay the oxy- 

 gen content varied during the summer from 1.1 parts per million to 

 4.9 parts per million, while at the surface it was always from 2 to 5 

 parts higher. The soft mud bottom was found to contain from 50 

 to 120 parts per million of hydrogen sulphide, which reduced the 

 oxygen content of the water to zero whenever stirring up of the 

 bottom occurred. Large numbers of oysters were not only partly 

 buried in this mud, but previous to their death were subjected to the 

 continual settling of clouds of mud upon them as extensive dredging 

 operations were in progress. 



Precipitation and river discharge in tidewater Virginia were 

 higher than usual in October and November, 1929, so that the oyster 

 beds received a considerable deposit of sediment, eelgrass, and debris, 

 Avhich would further reduce the oxygen in the lower layers of water. 

 The high mortality of oysters in Mobjack Bay maj easily have been 

 brought about by the combined effect of the following circumstances ; 

 namely, (1) unusually heavy planting of oysters in this region, (2J 

 deficency of oxygen supply because of overcrowding of oysters and 

 of sedimentation, stirring, and resettling of mud bottom by dredging 

 operations, (3) direct poisonous effect of hydrogen sulphide and 

 other toxic products of decomposition, (4) previous weakened and 

 diseased condition of the muscles of the oysters from causes at pres- 

 ent under investigation. 



This latter factor is perhaps of greatest fundamental importance 

 in the recent mortality since the studies show that there existed a 



