bidimeter of the type used primarily in fresh-water 

 studies; more accurate results are obtained with an 

 electrophotometer and spectrophotometer. The 

 turbidity of the collected samples is compared with 

 standards made of known dilutions of a suspension 

 of 1 g. of kaolin (or silica clay) in 1 1. of water. 



Evaluation of the effect of sedimentation on 

 oyster bottoms can be made by considering the 

 location of oyster bottom in the estuary; the 

 amount and character of sediment in suspension; 

 the type of estuarine circulation; and the rate of 

 accumulation of sediment on oysters. A score 

 of zero is assigned to the ideal conditions under 

 which no deposits settle on live oysters. The 

 opposite extreme, valued at 10, is found in the 

 areas of heavy sedimentation, not suitable to 

 oysters. All intermediate conditions scored from 

 1 to 9 can be evaluated on the basis of field 

 observations. 



DISEASE 



Oysters suffer from both noncontagious and 

 infectious diseases. The first category is associated 

 with the malfunction of physiological systems of 

 organs and deficiencies in the environment, such 

 as lack of food, unsuitable salinity and water 

 temperature, and pollution by domestic sewage 

 and various trade wastes. The second category, 

 infectious diseases, is caused by pathogens and 

 parasites. Clear distinction between the two 

 types of pathological conditions is not always 

 possible because resistance to infection is lowered 

 by an unfavorable environment, and an oyster 

 weakened by adverse conditions more easily 

 succumbs to infection. 



With few exceptions the outward symptoms of 

 a disease are nonspecific. The more common 

 symptoms are slow growth, failure to fatten and 

 develop gonads, recession of the mantle, and 

 valves that remain slightly agape. There is often 

 a corresponding abnormal deposition of shell 

 material that in a chronic condition causes the 

 formation of short and thick shells ("huitre 

 boudeuse" of French biologists). The valves do 

 not close tightly because the adductor muscle is 

 weakened. The body of a sick oyster is watery, 

 often discolored (dirty green and brown), and 

 bloody with blood cells accumulating on the 

 mantle and on the surface of the gills. 



The etiology of oyster diseases is not well 

 known. A few microorganisms infecting tlie 

 oyster have been definitely identified as pathogens; 

 the taxonomic position of others is not known. 



and some are called by code numbers. Oyster 

 populations throughout the world suffer from 

 periodic widespread mortalities which may be 

 associated with infections, but since the life 

 cycles of some of the pathogens have not been 

 described, the evidence remains circumstantial. 



The widespread mortality of oysters rarely can 

 be attributed to a single factor of the environ- 

 ment; in most cases it occiu's as a result of the 

 combination of several adverse conditions in- 

 cluding infection. 



Malpeque Bay disease 



One of the most persistent and mysterious ail- 

 ments of oysters is the Malpeque Bay disease, 

 which in 1915 and 1916 struck the populations of 

 C. virginica in the bays of Prince Edward Island, 

 Canada, causing the death of 90 percent of the 

 oysters, and in later years appeared along the 

 Canadian mainland. The most distinctive symp- 

 tom associated with the disease was the occurrence 

 of yellow-green pustules, up to 0.5 cm. in diame- 

 ter, on the surface of the visceral mass, along the 

 edges of the mantle, and on the adductor muscle 

 and the heart. Despite lengthy field and experi- 

 mental studies conducted from the epidemic year 

 to the present, the causative agent has not been 

 found, although there is no doubt that the mortal- 

 ity of Malpeque Bay oysters was due to an in- 

 fection (Needier and I^ogie, 1947). With the 

 expectation that the survived oysters were of 

 disease resistant stock, the Department of Fish- 

 eries of Canada and the Fisheries Research Board 

 organized ua 1957 a rehabilitation project and 

 transferred oysters from Prince Edward Island 

 to devastated mainland areas. Unfortunately the 

 hopes did not materialize fully since a high propor- 

 tion of the spat that settled on the rehabilitated 

 area did not show the expected level of resistance. 

 It is hoped, however, that a resistant stock will 

 develop from a small number of survivors over a 

 period of several years (Drinnan and Medcof, 

 1961). 



Dermocystidium marinutn 



Dermocystidium marinum Mackin, Owen, and 

 Collier, a fungus of uncertain taxonomic position 

 infecting C. fdrginica, is probably the most danger- 

 ous pathogen associated with periodic mortalities 

 of oysters in the waters of southern States. The 

 microorganism infects oj^ster tissues producing 

 single, spherical, vacuolate cells which reproduce 

 by endogenous free cell formation and subse- 



PACTORS AFFECTING OYSTER POPULATIONS 



415 



