SPAWNING AND SETTING OF OLYMPIA OYSTERS 
463 
temperature occurs commonly during the higher high tide and this shows a striking 
relationship to the onset of spawning. Prvtherch’s (1929) conclusion that the high- 
tide temperature must be adequate before spawning will begin appears to apply 
equally well to this species. Judg- 
ing from these data, the critical 
temperature for spawning may be 
placed at about 13° C., possibly 
from 12.5° to 13° C. 
It has frequently been noted 
that spawning is most likely to 
begin during or shortly after a 
period of neap tides. In spring 
and early summer, as shown in 
figure 15, at such times the mini- 
mum temperature is at a relatively 
high level. During a period of 
spring, or extreme tides, the tide 
flats warm in the sunshine and 
raise the temperature of the water 
coming in with the flood tides. 
The great range of tide in this 
place, 18 to 19 feet during a spring 
tide period, causes the colder water 
of the deep channels down the bays 
to reach the oyster grounds at high 
tide, while the warmer water is 
forced toward the head of the bay. 
During the neap tides a week later, however, the range may be only 11 or 12 feet, 
permitting the relatively warm water, resulting from the preceding low tide period, 
to remain over the oyster grounds. 
30 
10 Ct 
JUNE. 
JULY 
Figure 17.— Daily average and minimum temperature in Mud Bay dur- 
ing summer of 1933 as related to frequency of spawning. Compare 
figures 16 and 18. 
While the highest temperature is to 
be found in the dikes at low tide 
during a period of spring tides, the 
highest high-tide temperature may 
frequently occur in the neap-tide 
period, thereby inducing spawning. 
It is probable that this estimate 
of the critical temperature for 
spawning is not out of harmony 
with the results of Hori (1933) and 
Coe (1931a), who stated that the 
water temperature during spawn- 
ing was at least 14° and 16°, 
respectively. Their measurements 
appear to refer either to the average 
temperature or to that indicated by more or less frequent readings. As shown 
above, the average temperature at the time of the initial spawning is generally 
14° to 16°. 
Figure 18.— Daily average and minimum temperature in Oyster Bay 
during summer of 1935 as related to frequency of spawning. 
