OYSTERS AND METHODS OF OYSTER-CULTURE. 285 



fiilly set. Currents, even of considerable strength, do not prevent the 

 settling down of the larval oyster and its fixation upon a proj)er 

 surface. 



In the i^reparation of this surface the currents are also effective, 

 inasmuch as by their scouring action they prevent the deposit of sedi- 

 ment and slime, which soon render collectors unsuitable for the fixation 

 of the young oyster. Finally, where the fry are uniformly distributed 

 in a body of water a collector placed in a current will collect more spat 

 than one in quiet water, because a greater quantity of water and con- 

 sequently a larger number of fry will be brought into contact with it. 

 Points around which fry-charged water sweei)S with sufficient velocity 

 to prevent the deposit of sediment are good places for the location of 

 collectors. 



Freshets, for several reasons, usually have a bad effect upon the 

 oyster-beds. When the volume of fresh water is large, the oysters 

 suffer from the decrease in the density. Large quantities of mud and 

 sediment are brought down by the floods and often deposited on the 

 beds, covering up the cultch and smothering the young spat, and, if 

 the amount of sedimentation is very great, even injuring or killing 

 the adults. 



DEPTH OF WATER. 



The vertical range of the cultivated oyster beds is from the shore 

 line to a depth of 15 fathoms. In New Jersey, Chesapeake Bay, South 

 Carolina, and other places, there are beds which are partially exposed 

 at low water, while in Long Island Sound successful oyster-culture 

 is carried on in depths as great as 15 fathoms, the average over planted 

 grounds in that region, however, being from 5 to 6 fathoms. In most 

 places, however, the planting is done in shallow bays and coves. 



WEATHER CONDITIONS — STORMS^ GALES, AND ICE. 



Gales rarely have any influence upon adult oysters in deep water, 

 but they sometimes seriously aflfect shallow-water beds. Heavy surf 

 occasionally carries away the oysters and throws them upon the beach, 

 or they may be buried in situ by the sand and seaweeds which the 

 waves lodge upon the beds. Sometimes, after the lapse of a short time, 

 the beds are again uncovered by the eroding effects of currents, but in 

 many cases they are practically destroyed, both old and young being 

 smothered by the overlying deposits. 



In winter, ice often grounds upon the beds during gales and does 

 considerable damage. The oyster appears also to be temi^orarily 

 affected by the mere freezing of the waters, and it is said that, in the 

 Chesapeake, oysters on the deeper beds are more affected than those in 

 sho;il and brackish water, becoming dark, slimy, and worthless for the 

 market. Ten days or a fortnight must elapse after the disappearance 

 of the ice before they become again fit for use. 



The fry are more affected by the weather than are the adults. Dr. 

 Eyder found that in the swimming stage they were killed by thunder- 



