OYSTERS AND METHODS OF OYSTER-CULTURE. 281 



rapidly in waters between 74° and 80° F., altliongh the mortality is 

 greater than at a slightly lower temperatnre. Under such conditions 

 the embryos reach the swimming stage in from 3 to 10 hours, a fact 

 which is, of course, advantageous to those undertaking artificial propa- 

 gation. When the temperature falls to below 65° F., development 

 almost ceases, and when it rises above 80° F. but few of the embryos 

 reach the swimming stage. Sudden changes are usually fatal, and cold 

 rains kill great numbers of the swimming fry. 



Dr. Ryder recommends "that the prevalent temperature of the water 

 during the spawning season shall range from 68 to 80° F." It is quite 

 possible that in other regions, with oysters native thereto, or even those 

 which have been acclimated therein, some other temperature may be 

 found more favorable, but no data bearing upon the matter have been 

 published. 



TEMPERATURE; PLANTED BEDS IN SAN FRANCISCO BAT. 



The temperature at San Francisco is usually not much higher in 

 surajner than in winter, but information upon the subject is limited. 

 Upon the oyster-beds at Millbrae it is said to vary from 58° to 60° F., 

 but at the extreme southern end of the bay it ranges from 67° to 74° F. 

 In October, 1890, Mr. 0. H. Townsend found 61° F. at Belmont; at San 

 Mateo, nearer the sea, 60° F., and at California city, 57° F. 



In midsummer the temperature was considerably higher; between 

 July 12, 1891, and September 7, 1891, it ranged from 67° to 74° F., the 

 means for 10-day periods during the same time being between 69.1° 

 and 72° F. As Mr. Townsend points out, there is, therefore, a con- 

 siderable period during the summer when the temperature, in portions 

 of the bay at least, is favorable for spawning of the planted eastern 

 oysters. The portions of the bay near the sea appear to have a tem- 

 perature several degrees cooler than in the southern portions. 



DENSITY OF WATER. 



Oysters are found living in water ranging in salinity from 1.002* to 

 1.025, but the lower densities are always injurious, and prolonged 

 exposure to their influence is fatal to oyster life. It is not possible to 

 profitably maintain oyster-beds in waters where the density falls below 

 1.007 for any length of time, the oyster, if not killed, becoming poor in 

 quahty, pale, watery, and tasteless. Heavy freshets, such as occur in 

 tlie rivers discharging into Chesapeake Bay and at various places on 

 the Gulf coast, frequently so lower the density of the water as to prac- 

 tically exterminate the oysters on certain beds. Experience apparently 

 indicates that the best oysters are grown in densities between about 

 1.011 and 1.022, the former being approximately the specific gravity 

 over the Tangier Sound beds, the latter that over the deep-water 

 oyster-grounds of Long Island Sound. 



*The figures represent the specific gravity as measured with the salinometer, that 

 of pure water bemg- 1.000. 



