in the samples were insignificant. Seasonal fluc- 

 tuations of plankton and nannoplankton follow a 

 typical pattern of maximum development from 

 about the middle of January to the end of April, 

 followed by a decrease during May to September 

 and a second maximum, smaller than the first, in 

 September to November. The latter period is of 

 greater significance for the northern oysters be- 

 cause they do not feed during the winter months of 

 January and February. 



Another method of determining plankton pro- 

 ductivity of water over an oyster bottom was used 

 in a study of the York River, Va. (Galtsoff, Chip- 

 man, Engle, and Calderwood, 1947). In this case 

 the water sample was obtained by means of a 

 Birge-Juday plankton trap. Two liters of the 

 sample were passed through the Foerst-Juday 

 centrifuge, and the centrifugate extracted for 15 

 minutes in 10 ml. of 80 percent acetone. A com- 

 parison of the color was then made with standards 

 of nickel sulfate and potassium dichromate (Har- 

 vey, 1934), and the results expressed in pigment 

 units per liter of original sample. Each unit was 

 found to be equivalent to approximately 10,000 

 diatoms and dinoflagellates present in York River 

 water. So far as seasonal changes in plankton are 

 concerned, the results were similar to those ob- 

 served in Long Island Sound. Although the total 

 energy requu'ements of certain filter-feeders are 

 known, no information is available about the 

 specific food that is needed for their growth and 

 reproduction. Certain types of phytoplankton, 

 such as Chlorella, have antibiotic properties and 

 are harmful to some bivalves. Too little is knowTi 

 about the specific food requirements of various 

 organisms. The total bulk of phytoplankton may 

 be made of the materials of low nutritive value or 

 may consist of such organisms as Rhizosolenia, 

 Chaetoceras, and others which cannot be ingested 

 by the oyster because of their size and shape. 



In evaluating the food factor the abundance of 

 plankton should be compared in different areas 

 during the period when the oyster is actively 

 feeding and accumulating glycogen. This period 

 usually occurs shortly after spawning (ch. XVII). 

 The amount of phytoplankton per unit of volume 

 of water on oyster bottoms of highest commercial 

 yield and consisting primarily of algae that can be 

 utilized bj^ the oyster represents the optimum 

 value of food factor (score 10); while the phyto- 

 plankton content of water from the marginal 

 areas is assigned the score 1. 



Another and more accurate method may be 

 used. It is based on the determination of meta- 

 bolic rate of oysters during the period of feeding. 

 The nutritive value (food energy) of the sample 

 of phytoplankton can be determined by using a 

 bomb calorimeter and measuring the heat of 

 combustion in calories. Knowing the rate of 

 water transport by the oyster at a given tempera- 

 ture and salinity, it is easy to calculate whether 

 the food supply on oyster grounds is adequate. 

 Unfortunately, the method widely used in nutri- 

 tional studies has not yet been applied in oyster 

 research. 



The high concentrations of phytoplankton 

 which occur during blooms are not desirable 

 features and can be harmful. Experimental work 

 has clearly shown that at a certain high concen- 

 tration of several forms {Nitzschia closterium, 

 Prorocentrum triangulatum, Euglena viridis, and 

 Chlorella sp.) the rate of water transport of oysters 

 is reduced and feeding ceases (Loosanoff and 

 Engle, 1947). The deleterious effect is caused by 

 the cells themselves and by their metabolites. 

 These laboratory findings are in accord with field 

 observations in Great South Bay, N.Y., where a 

 mass development of a Chlorella like organism 

 adversely affected valuable oyster beds. Another 

 example of a danger of excessive development of a 

 single microorganism is the so-called red tide 

 (Galtsoff, 1948, 1949) along the western coast of 

 Florida. Sudden development of the dinoflagel- 

 late, Gymnodiniuin breve, causes extensive mor- 

 tality of fishes and kills many oysters growing 

 along the shores of the affected area. 



Conditions are ideal for the feeding of oysters 

 when water free of pollution and containing a low 

 concentration of small diatoms and dinoflagellates 

 runs over a bottom in a nonturbuleut flow. 



NEGATIVE FACTORS OF ENVIRONMENT 



The environment itself may interfere with the 

 welfare of oyster populations. Negative factors 

 decrease or inhibit reproductive capabilities; 

 destroy the population by causing extreme adverse 

 conditions; increase the incidence of disease; 

 inhibit the fattening and the growth of oyster 

 body, thus decreasmg the productiveness of an 

 oyster bed; and interfere with the formation of 

 shell and so deprive the oysters of their principal 

 means of protection against adverse situations 

 and attacks of enemies. All negative factors are 

 evaluated by determining the degree of their 



FACTORS AFFECTING OYSTER POPULATIONS 



409 



