CHAPTER XVIIl 

 ENVIRONMENTAL FACTORS AFFECTING OYSTER POPULATIONS 



Page 



Positive factors of environment _ 399 



Character of bottom -. _. 399 



Water movements. 400 



Salinity -.- - --. 404 



Temperature 407 



Food - 408 



Negative factors of environment _ 409 



Sedimentation. 410 



Disease 415 



Malpeque Bay disease - - 415 



DeTmocystidium marinum 415 



Disease associated with HaplospoTidium 417 



Shell disease... 417 



Foot disease 418 



Hexamita. 419 



Nematopsis 419 



Trematodes and parasitic copepods 419 



Commensals and competitors... 420 



Boring sponges 420 



Boring clam 421 



Mud worms 421 



Oyster crab 425 



Splrochaetes 426 



Perforating algae 426 



Fouling organisms 426 



Predators 430 



Carnivorous gastropods. 430 



Starfish 437 



Flatworms 438 



Crabs 439 



Mud prawns and flsh. 439 



Birds 439 



Man 440 



Pollution 441 



Domestic sewage 442 



Industrial waste 443 



Radioactive waste-.. 445 



Combined effect of environmental factors 445 



Bibliography 446 



The various species of the family Ostreidae 

 inhabit the coastal waters within the broad belt 

 of the sea, limited by the latitudes 64° N. and 44° 

 S. Some large aggregations cover many square 

 miles of bottom of littoral and intertidal zones; 

 they also thrive above the bottom attached to 

 rocks and underwater structures, branches and 

 trunks of fallen trees, and miscellaneous objects. 

 These aggregations of live oysters and empty shells 

 are called oyster bottoms, oyster beds, oyster 

 banks, or oyster reefs. The expressions are not 

 well defuied either biologically or in the legal 

 sense and are used interchangeably. Only those 

 species of oysters which form large and dense 

 populations are important to man as a source of 



FISHERY bulletin: VOLUME 64, CHAPTER XVIII 

 T33-851 O — 64 26 



food. Those living singly and widely dispersed 

 are of no commercial value. 



Descriptions of oyster bottoms found in the 

 world literature combined with personal observa- 

 tions over the course of years in the United States, 

 France, Italy, the West Indies, Cuba, Venezuela, 

 Panama, Hawaii, and some of the South Pacific 

 islands have convinced me that, regardless of the 

 species of oysters present, certain major factors 

 are common to all oyster bottoms. 



It is a matter of historical interest that more 

 than SO years ago Mobius (1883) established the 

 concept of a biocenosis or a social community 

 using an oyster bank as an example. According 

 to his definition every oyster bed is to a certain 

 degree — 



... a communitv of living beings, a collection of species, 

 and a massing of individuals, which find here everything 

 necessary for their growth and continuance, such as suitable 

 soil, sufficient food, the requisite percentage of salt, and 

 a temperature favorable to their development. Each 

 species which lives here is represented by the greatest 

 number of individuals which can grow to maturity subject 

 to the conditions which surround them, for among all 

 species the number of individuals which arrive at maturity 

 at each breeding period is much smaller than the number 

 of germs produced at that time. The total number of 

 mature individuals of all the species living together in 

 any region is the sum of the survivors of all the germs 

 which have been produced at all past breeding or brood 

 periods; and this sum of matured germs represents a 

 certain quantum of life which enters into a certain number 

 of individuals, and which, as does all life, gains permanence 

 by means of transmission. 



Mobius further commented that a change in one 

 factor of a biocenosis affects other factors of the 

 environment and eventually changes the com- 

 munity character. Relative abundance of various 

 species constituting a bottom community is 

 affected by changes in estuarine environment 

 and by man's activities which alter the environ- 

 ment. Patterns of currents, salinity gradients, 

 and turbidity of water may be changed by dredg- 

 ing operations, construction of inshore installa- 

 tions, and other harbor and waterway improve- 



397 



