348 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 
Recent summer temperatures taken by authority of the United States 
Fish Commissioner at the extreme southern end of the bay, through the 
codperation of the Morgan. Oyster Company, have yielded valuable 
information, the water of that part of the bay having been found to 
have a summer warmth amply sufficient for the propagation of the 
oyster. The important table of temperatures from this locality is con- 
densed to means of ten days from a lengthy series of daily observations 
at both high and low tide. 
Temperature at the oyster beds, 1 mile from Dumbarton Point, San Francisco Bay, 
July 12 to October 12, 1891. 
> 
Surface-water | |, 
Air temperature. pange of 
Date. | temper- ————————_] , water 
Bue High | Low agers 
tide. tide. are 
| 
oF, oF, OAH: OF, 
July 12 to 19...... 68. 69. 6 W1.9.|. 67 to 73 
July 20 to 29..... 68.9 70.9 71.9 | 69 to 74 
July 30 to Aug. 8 67.3 69.5 69.7 | 68 to 72 
Aarp. /97toL8: 22 see's 68. 4 70.3 70.7 | 68 to 72 
ALO TOFS sees asctememeclcas ceases 72.3 71.4 72.0 | 69 to 74 
AUP: ZOO SOD bal o--atce2 sears ote eee eee 66. 8 70.7 69.1 | 67 to 72 | 
Se) 0) ng GPE, eae Began ne See arimn Gee 66. 4 67.8 68.0 | 64 to 71 
NODES tO grusia. Wate ar coe tee ees nee 65. 7 66.1 67.3 | 64 to 70 
NODES iO OCU use aay eee eae arena 64.2 65. 6 62.9 | 58 to 70 
(Oko, Cimon Pi" Ba oon tee: SHosedoeceosns 61.9 63, 2 64.0 | 62 to 65 
Peculiar situation of the oyster beds. —There are at present no eastern 
oysters in San Francisco Bay, that are not laid upon tide lands, or 
so-called mudflats, completely exposed at the time of low tide. The 
principal reason for the selection of such situations is that the beds may 
be readily fenced in by closely set stakes to protect them from the dep- 
redations of the stingray (Myliobatis californicus), which enters the bay 
every spring and is the principal enemy of the oyster in these waters. 
In this complete dependence for oyster-growing upon tide lands, fre- 
quently left dry, is doubtless to be found one explanation of the slow 
increase of the species. The California summer is absolutely dry and 
rainless. It is a season of cloudless skies and regularly recurring heat 
in the daytime; therefore an oyster bed at this season, when the tide 
is out, is exposed not merely to the air, but to a heat sufficient to dry 
the moisture off from all the oysters in sight, and perhaps injure the — 
majority of the spat that might have been attached to their shells. 
If embryo oysters, set free on the beds, drift with the receding tides to 
deeper waters outside the stake-protected area of the flats, they are 
exposed to the stingrays when they have attained sufficient size. 
Stingrays, and the stake protection employed against them.—The Calli- 
fornia stingray (Myliobatis. californicus) enters San Francisco Bay in 
large numbers in the spring and remains until late in the fall. It is 
said to be as destructive to oysters in these waters as the starfish is on 
certain parts of the Atlantic coast. It has heavy flat teeth, arranged 
in a sort of pavement in each jaw, and is essentially a feeder on shell- 
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