ments. Commercial dredging removes substantial 

 portions of natural oyster grounds; planting of 

 seed oysters for growing increases artificially the 

 population densities. Inadvertent introduction 

 of foreign species, competitors, and predators 

 disturbs the established biological balance. Final- 

 ly, excessive discharge of domestic sewage and 

 trade wastes causes irreparable damage to pro- 

 ductive oyster bottoms. 



The productivity of a sea bottom may be 

 measured by determining the sum of weights of 

 all animals and plants in a unit of area. The 

 value, called community biomass, is of consider- 

 able theoretical interest to the marine ecologist 

 engaged in the study of oceanic productivity. It 

 has, however, no practical application in deter- 

 minations of the productive capabilities of a 

 community dominated by a single species such 

 as oyster, clam, or scallop. The species produc- 

 ti\'ity of any bottom may be materially reduced 

 by competitors, predators, and other conditions 

 that may suppress the reproduction and gi-owth 

 of commercially utilizable organisms while not 

 affecting or even sometimes encom'aging the 

 growth of noncommercial forms. 



Descriptions of oyster bottoms usually provide 

 information regarding their location, type of 

 bottom, depth and salinity of water, the principal 

 species associated with oysters, and the abundance 

 or absence of predators. This type of description 

 is found in the papers of Dean (1892) on .South 

 Carolina grounds; Moore on the condition and 

 e.xtent of oyster gi-ounds in Texas (1907), Louisi- 

 ana (1899), Mississippi Sound (1913), James 

 River, Virginia (1910), Delaware Bay (1911); 

 Pearse and Wharton (19.38) on oysters of Apala- 

 chicola Ba}^; Frey (1946) on oyster bars in the 

 Potomac River; Hagmeier and Kandler (1927) 

 on oyster banks in North Freisland shoals, 

 Germany; Joubin on the coast of France (1906, 

 1908), and many othei's.' 



Because of the great diversity in the kind 

 and number of species forming an oyster com- 

 nmnity only a few generalizations can be drawn 

 from descriptive data: 1) in common with other 

 bottom communities, oyster grounds of tlie warm 

 southern waters support a gi-eater \ariety nf 

 species than do the colder waters of the nortliern 

 latitudes, and 2) the variety of plant and animal 

 species is less in waters of low salinity than in 

 adjacent areas of higher salt concentrations. 



The inferences are in accord with observations 

 made by European ecologists and summarized 

 by Hedgpeth (1953). In the Elbe estuary the 

 weight of all invertebrates per square meter of 

 bottom decreases from 6,068 g. in the area of full 

 oceanic salinity to only 37 g. in brackish water. 

 A similar decrease in the weight of community 

 biomass is found along the northern coast of 

 Germany, although the difference is much smaller 

 ranging from 304 g. per square meter of sea 

 bottom to 16 g. in the inshore areas. The decrease 

 in the biomass cannot be attributed to a single 

 factor of the envu"onment since other conditions 

 such as rate of water movements, sedimentation, 

 and food content are associated with the salinity 

 changes. 



There are many well-documented cases of 

 destruction of productive oyster bottoms by 

 human activities. Mobius (1883) cites formerly 

 rich oyster beds of Cancale, Rochefort, Marennes, 

 and Oleron on the West Coast of Europe in which 

 the oyster populations were replaced by cockles 

 and mussels. The newcomers were present in 

 small numbers while the oysters floui'ished but 

 greatly increased in abundance when the removal 

 of oysters left more space for them to settle. 



Many well-documented examples may be cited 

 of the destruction of oyster bottoms by sand and 

 mud stirred up by dredging operations in nearby 

 areas. One incidence of this nature occurred in 

 1935 to 1938 near the Buzzards Bay entrance to 

 the Cape Cod canal, Mass., where valuable oyster 

 grounds were bm-ied under 8 to 12 inches of 

 material that was disturbed by dredging and then 

 settled on the oyster grounds. Three to four 

 years later the area was repopulated by quahogs 

 and continues to remain highly productive, 

 although the species composition has been 

 completely changed. 



Discounting minor local variations, the basic 

 requirements of the oyster are identical regardless 

 of the location of the oyster bottoms. The 

 suitability of a bottom area for the development 

 of a productive oyster community can, therefore, 

 be evaluated if the effects of different environ- 

 mental factors are estinaated. 



Principal factors favorable for the propagation, 

 growth, and general welfare of an oyster com- 

 munity are character of bottom, water movements, 

 salinity of water, temperatm'e, and food. The 

 unfavorable or destructive factors that tend to 

 inhibit the growth and productivity of a com- 



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PISH AND WILDLIFE SERVICE 



