SPAWNING AND SETTING OF OLYMPIA OYSTERS 
491 
darkness. The two high tides in this case were almost identical and setting was 
almost equally heavy on the two floods, one day, the other night. 
In the next experiment, about 2 weeks later, made on a different ground, the 
first high tide was more than 4 feet higher than the second. Whether the different 
picture obtained (fig. 35) is due to the large difference between the two tides is un- 
certain though it would not seem unlikely that the greater flow of water in the former 
may account for the heavier set. On neither day was a single spat caught when the 
dike was exposed. Setting was about equally profuse on flood and ebb of the first 
tide, and it is suggested that the very low tide after midnight, which almost left the 
ground exposed, caused rate of current or other factors on the ebb and flood tides to 
be similar. The following secondary high tide produced few spat, most of which set 
during the flood. 
Except when the ground was exposed the temperature varied only slightly. It 
is of interest that the most prolific setting took place when both pH and salinity were 
quite high. The water in the dike at the end of the period of exposure had a pH of 
7.8, while about an hour 
later it was 8.4. At the 
same time the salinity rose 
from a little over 26 to more 
than 29 parts per mille. 
In an attempt to reach a 
more definite conclusion a 
similar experiment was 
made in a later year with 
the additional help of an 
electric current meter. 
The boat was anchored in 
the channel near the oyster 
ground and the current 
meter suspended from a 
framework directly in the 
dike. The meter hung just 
above the oysters, at the level of the panes of glass, and the transmission line was 
led to the boat where counts of revolutions were made. The results are shown in 
figure 36. Spat were caught only at three well separated times: During the first 
flood and the following small ebb, and during the major ebb of the next day. The 
first and the last coincide definitely with the times of swift current. In the second 
case the current was not particularly rapid, but it is true that the heaviest setting 
took place about half way between high and low tide, when the current during this 
ebb was swiftest. Why no spat were caught on the second flood is not known, 
though it must be realized that in an experiment of this kind, necessarily carried on 
over a limited area and with a relatively small amount of cultch, chance is a large 
factor in determining whether the water in the particular place happens to contain 
larvae. For this reason the error involved in the tests is considerable. Nevertheless, 
the results are, within certain limits, of great interest. 
The final series to be described here is shown in figure 37. The experiment 
consisted of two parts. One set of plates was in the water for an entire tide, from 
high to low or low to high. In the other group two overlapping series of plates 
were used, each set being left in the water 3 hours. The lower portion of the graph 
represents the first group. Although spat were caught during all four tides, only 
-v 15 12 OQ 
Figure 36. — Number of spat caught hourly per unit of cultch with relation to stage of 
tide, rate of current, temperature, salinity, and pH. (Oyster Bay, Made Ground, 
June 26 and 27, 1934.) 
