SPAWNING AND SETTING OF OLYMPIA OYSTERS 
493 
be given most weight. In such an instance as that shown in the upper graph of 
figure 37 the first 3-hour set of plates caught few spat; the next one, planted 1 % hours 
later caught many; the third, still 1 % hours later caught few. It would seem safe 
to assume that most of this set occurred between 12 and 1:30 o’clock, or the third 
set would have caught a larger number. 
Summarizing these experiments it may be said that almost no spat are caught 
when the tide is low and the water in the dike is still. At this time the water is warmest, 
the salinity lowest, though only 1 to 2 per mille below that of the maximum, and the 
pH lowest, especially after the ground has been exposed for some time. As the tide 
comes in setting increases in intensity, most of it occurring when the water is 6 to 8 
feet deep over the beds on which the tests were made. At this time the pH and 
salinity are relatively high and the temperature low. In some cases there appears 
to be a correlation between rate of current and frequency of setting. There seems 
to be no obvious parallel with conditions observed by Prythercli (1929) in Connecticut. 
DEPTH OF SETTING 
Most of the favorable grounds for the collection of spat are relatively high, at 
a level of 3 to 8 feet above the average lower low tide. While usually not so good as 
producers of the best oysters for market the higher grounds are used almost entirely 
for the collection of seeds. To the practical oystermen, who have always believed 
that most of the larvae set when the tide is low and the water on the grounds clear 
and warm, the highest grounds offered the warmest water at low tide and for this 
reason were especially favorable for catching seeds. However, as was shown in a 
preceding section, setting takes place at a lower frequency at low tide than at any 
other time. 
Only when the tide is as much as half high are some of the best seed beds com- 
pletely covered, except for the few inches of water held by the dikes. Because they 
are covered by deep water so much less of the time and are closer to the warmer 
surface water the seed beds are best for obtaining rapid growth of the spat, while on 
lower grounds, where growth is slower, the oysters fatten better but not so many spat 
are caught. It is probably true that cultch becomes fouled with organic growth 
more quickly on the lower grounds, thus preventing a heavy set of spat, but it appeared 
possible that some other factor might be concerned in determining that higher 
grounds are so much more effective. 
Table 34. — Number of spat caught on shells in four series of wire baskets suspended at fixed distances 
from surface of water 
[A basket covered a depth of 5 inches] 
Depth 
(inches) 
Series 1 
number of 
spat 
Series 2 
number of 
spat 
Series 3 
Series 4 
Depth 
(inches) 
Number of 
spat 
Depth 
(inches) 
Number of 
spat 
0-5 
402 
228 
0-5 
338 
0-5 
182 
6-11 
1,029 
517 
6-11 
612 
8-13 
696 
14-19 
1,393 
640 
14-19 
662 
16-21 
604 
24-29 
779 
392 
24-29 
723 
24-29 
654 
39-44 
757 
417 
42-47 
624 
32-37 
544 
64-09 
718 
447 
71-76 
591 
99-104 
530 
495 
Several series of experiments were performed for the purpose of finding at what 
depths the larvae set most profusely. Wire baskets were constructed 12 by 12 inches 
wide and 5 inches deep, filled with clam shells, suspended in series one above the other, 
