18 THE OYSTER AND THE OYSTER INDUSTRY. 



It sometimes happens that a freshet of unusual severity, while 

 disastrous in its immediate effects, results eventually in an increased 

 productiA^eness of the beds. If the disaster be due to a prolonged 

 freshening of the water without an undue deposit of silt, the shells 

 are often left in a much-improved condition. This is apparently 

 due in part to the more active scouring action of currents of more 

 than usual velocity, but mainly to the destruction of the organic 

 slime, which often covers the shells in sea water, and the cultch is 

 thereby left in a more favorable condition for the attachment of 

 spat carried from more or less distant beds. The fresh water also 

 exterminates the drills wliich feed on the little oysters, and, as Dr. 

 Moore's observations of improved sets under the conditions described 

 indicate that sets usually occur in waters of rather high normal 

 salinity, where the drill ordinarily thrives, it is probable that this 

 action of the fresh water is no unimportant beneficial factor. The 

 oysters, from the nature of their reproductive and developmental 

 characteristics, are able to reestablish themselves much more rapidly 

 than their enemies. 



Gales, to have an effect on adult oysters in moderately deep water, 

 must be of extraordinary severity, but they frequently do great 

 damage or exterminate beds irl shoal water. The waves sometimes 

 pick up the o^^sters and throw them on the beach, but more fre- 

 quently they are destroyed by being buried in situ by sand, sea- 

 weeds, and debris piled up by the sea. Cases arc known of where 

 well-established beds have been overwhelmed by such deposits and 

 others in which thick strata of sand between layers of old shells indi- 

 cate a succession of such disasters in the more or less remote history 

 of the beds. 



Sometimes the eroding effect of currents and waves will uncover 

 the buried oysters and shells, and the beds will again reestablish 

 themselves through the attachment of young; but in other cases the 

 beds are permanently destroyed. The former is the usual result 

 when the reefs rise rather abruptly from the surrounding bottom, 

 and the latter is frequent when they are but little elevated above 

 the general floor of the sea. Planted beds, which usually lie at the 

 general level of the bottom, are usually permanently covered. 



Gales are sometimes agents in the establishment of new beds, 

 carrying oysters and shells to surrounding barren bottoms, where 

 they form a nucleus that gradually develops into economic impor- 

 tance. Certain productive beds at the eastern end of Mississippi 

 Sound, by character and by repute, appear to have been so estab- 

 lished. 



The free-swimming larvae arc more susceptible to the weather 

 conditions than are the adults, and cold rain storms, which would 

 have no effect on the latter, undoubtedly kill large numbers of the 

 swimming young. This was first noticed by Ryder and has been 

 amply corroborated," 



Ice is occasionally destructive to oyster beds quite independently 

 of the factor of temperature. When heavy ice grounds at extrernely 

 low tides, it sometimes crushes the oysters or presses them into 



a The author and J. S. Outsell, during the study of the occtirrence of free-swimming oyster laryse in 

 Great South Hav, 1919, found that the average number in 50 gallons of water was 8,339 on July 8. A vio- 

 lent squall and rain followed, together with a drop in temperature of 5" F. On July 11, as soon as collec- 

 tions could be made, the average number had dropped to 3,558 larvte per 50 gallons of water. 



