CHAPTER XVIII ENVmONMENTAL FACTORS AFFECTING OYSTER POPULATIONS Page food. Those living singly and widely dispersed Positive factors of envlronment_____ 399 Character of bottom______________ 399 are of no commercial value. Water movements " .--- 400 Salinity - -_-_- -_-_-_ 404 Descriptions of oyster bottoms found in the Temperature__________________________ _____________________ __ ______ 407 world literature combined with personal observa­ Food -_-_---_-_---------_-------------_-- 408 tions over the course of years in the United States, Negative factors of envlronment_____ _ 409 Sedimentatlon___ __ _________ ___________ _____________________________ 410 France, Italy, the West Indies, Cuba, Venezuela, Dlsease___ ____ __ __ __ _____________ ___________________________________ 415 Panama, Hawaii, and some of the South Pacific Malpeque Bay dlsease____________________________________________ 415 Dermocystidium marinum________________________________________ 415 islands have convinced me that, regardless of the Disease associated with Haplosporidium__________________________ 417 species of oysters present, certain major factors Shell dlsease___________ 417 Foot disease _ _______________ __________ __________ _______________ 418 are common to all oyster bottoms. Hexamita_ _____________________ __________ _________________________ 419 It is a matter of historical interest that more Nematopsu_ ___ __________ ______ _____________________________ ______ 419 Trematodes and parasitic copepods_______________________________ 419 than 80 years ago Mobius (1883) established the Commensals and competltors_______________________________________ 420 concept of a biocenosis or a social community Boring sponges_ __ __ 420 Boring clam___ ___ ______ ____ __ ____________ ___________ ________ _____ 421 using an oyster bank as an example. According Mud worms__________________________ 421 to his definition every oyster bed is to a certain Oyster crab_____ ____ __ ____ ___ __ __ _________ __ _______ _________ __ __ __ 425 Splrochaetes____ __ _________________ __ ____ _________________ __ __ __ __ 426 degree- Perforating algae__________ __ 426 Fouling organlsms________ 426 . a community of living beings, a collection of species, Predators______ ______ _____________________________ _________ ______ 430 and a massing of individuals, which find here everything Carnivorous gastropods___________________________________________ 430 necessary for their growth and continuance, such as suitable Starfish____ __ ________ _________ _________________________ _____ ____ __ 437 soil, sufficient food, the requisite percentage of salt, and Flatworms___ __ ______ __ _______________________ ____ _____ ______ _____ 438 Crabs__ __ ____ __ ___________ ______ ____ ___________________ ______ __ 439 a temperature favorable to their development. Each Mud prawns and fish_____________________________________________ 439 species which lives here is represented by the greatest Blrds__ ___ __ _____ __________ ____ _______ __ ______ _________________ 439 number of individuals which can ~row to maturity subject Man_____ __ ____ __ _________ __ ____ __________________________________ 440 to the conditions which surround them, for among all Pollution __ ____ _________________ _____ __________________ __________ 441 Domestic sewage_ _ 442 species the number of individuals which arrive at maturity Industrial waste______________ __ 443 at each breeding period is much smaller than the number Radioactive waste________________________________________________ 445 of germs produced at that time. The total number of Combined etrect of environmental factors_____________________________ 445 mature individuals of all the species living together in Bibliography 446 any region is the sum of the survivors of all the germs which have been produced at all past breeding or brood The various species of the family Ostreidae periods; and this sum of matured germs represents a inhabit the coastal waters within the broad belt certain quantum of life which enters into a certain number of the sea, limited by the latitudes 64° N. and 44° of individuals, and which, as does all life, gains permanence by means of transmission. S. Some large aggregations cover many square miles of bottom of littoral and intertidal zones; Mobius further commented that a change in one they also thrive above the bottom attached to factor of a biocenosis affects other factors of the rocks and underwater structures, branches and environment and eventually changes the com­ trunks of fallen trees, and miscellaneous objects. munity character. Relative abundance of various These aggregations of live oysters and empty shells species constituting a bottom community is are called oyster bottoms, oyster beds, oyster affected by changes in estuarine environment banks, or oyster reefs. The expressions are not and by man's activities which alter the environ­ well defined either biologically or in the legal ment. Patterns of currents, salinity gradients, sense and are used interchangeably. Only those and turbidity of water may be changed by dredg­ species of oysters which form large and dense ing operations, construction of inshore installa­ populations are important to man as a source of tions, and other harbor and waterway improve- FISHERY BULLETIN: VOLUME 64, CHAPTER XVIII 397 :733--8510--64-26 ments. Commercial dredging removes substantial The inferences are in aCl?ord with observations portions of natural oyster grounds; planting of made by European ecologists and summarized seed oysters for growing increases artificially the by Hedgpeth (1953). In the Elbe estuary the population densities. Inadvertent introduction weight of all invertebrates per square meter of of foreign species, competitors, and predators bottom decreases from 6,068 g. in the area of full disturbs the established biological balance. Final­ oceanic salinity to only 37 g. in brackish water. ly, excessive discharge of domestic sewage and A similar decrease in the weight of community trade wastes causes irreparable damage to pro­ biomass is found along the northern coast of ductive oyster bottoms. Germany, although the difference is much smaller The productivity of a sea bottom may be ranging from 304 g. per square meter of sea measured by determining the sum of weights of bottom to 16 g. in the inshore areas. The decrease all animals and plants in a unit of area. The in the biomass cannot be attributed to a single value, called community biomass, is of consider­ factor of the environment since other conditions able theoretical interest to the marine ecologist such as rate of water movements, sedimentation, engaged in the study of oceanic productivity. It and food content are associated with the salinity has, however, no practical application in deter­ changes. minations of the productive capabilities of a There are many well-documented cases of community dominated by a single species such destruction of productive oyster bottoms by as oyster, clam, or scallop. The species produc­ human activities. Mobius (1883) cites formerly tivity of any bottom may be materially reduced rich oyster beds of Cancale, Rochefort, Marennes, by competitors, predators, and other conditions and Oleron on the West Coast of Europe in which that may suppress the reproduction and growth the oyster populations were replaced by cockles of commercially utilizable organisms while not and mussels. The newcomers were present in affecting or even sometimes encouraging the small numbers while the oysters flourished but growth of noncommercial forms. greatly increased in abundance when the removal Descriptions of oyster bottoms usually provide of oysters left more space for them to settle. information regarding their location, type of Many well-documented examples may be cited bottom, depth and salinity of water, the principal of the destruction of oyster bottoms by sand and species associated with oysters, and the abundance mud stirred up by dredging operations in nearby or absence of predators. This type of description areas. One incidence of this nature occurred in is found in the papers of Dean (1892) on South 1935 to 1938 near the Buzzards Bay entrance to Carolina grounds; Moore on the condition and the Cape Cod canal, Mass., where valuable oyster extent of oyster grounds in Texas (1907), Louisi­ grounds were buried under 8 to 12 inches of ana (1899), Mississippi Sound (1913), James material that was disturbed by dredging and then River, Virginia (1910), Delaware Bay (1911); settled on the oyster grounds. Three to four Pearse and Wharton (1938) on oysters of Apala­ years later the area was repopulated by quahogs and continues to remain highly productive, chicola Bay; Frey (1946) on oyster bars in the although the species composition has been Potomac River; Hagmeier and Kandler (1927) completely changed. on oyster banks in North Freisland shoals, Discounting minor local variations, the basic Germany; Joubin on the coast of France (1906, requirements of the oyster are identical regardless 1908), and many others.' of the location of the oyster bottoms. The Because of the great diversity in the kind suitability of a bottom area for the development and number of species forming an oyster com­ of a productive oyster community can,. therefore, munity only a few generalizations can be drawn be evaluated if the effects of different environ­ from descriptive data: 1) in common with other mental factors are estimated. bottom communities, oyster grounds of the warm Principal factors favorable for the propagation,