SPAWNING AND SETTING OF OLYMPIA OYSTERS 
477 
description because it was in the water during a period of abundant setting, an average 
of 358 spat per shell were caught. 
During the first season the method employed was somewhat different. Bags of 
shells were left in the water for various lengths of time, the spat counted, and a great 
many of them measured for the purpose of determining from their size distribution 
the tune at which setting took place. The method of measurement was discarded 
because of the large error, due chiefly to differences in rate of growth during periods 
of spring and neap tides, which affect the low-tide temperature, and also probably to 
specific peculiarities of the different spat. The records for 1931 are therefore not 
as exact as those for the following years. 
Experimental series of this kind were carried on in the two principle oyster- 
producing bays near Olympia during five consecutive seasons. In addition, two other 
bays were studied for one season. The results of the analyses are given in detail in 
the following section. The time of beginning of setting was determined by daily 
examination of shells on the grounds until spat were found. 
SETTING SEASONS, OYSTER BAY 
SEASON OF 1931 
-I.,? 
3 
s 
Figure 23. — Average number of spat caught daily on bags of shells left in 
dike 5 (Oyster Bay) for different periods during 1931. Daily range of 
tide is also shown. 
Observations were not begun in 1931 in tune to permit obtaining of complete 
data on spawning, though it is evident (table 13) that most of the larvae were dis- 
charged during May and early June, for after this time few gravid specimens were to 
be found. Bags of shells, however, were left in the water for various periods through- 
out the season. The first spat 
were found on June 12, and they 
were of the size of mature larvae, 
with no new growth, so that this 
date may be considered as the 
time when larvae began to attach. 
The results of counts of spat 
on the shells are given in table 
21. Only two grounds were stud- 
ied in detail, and the samples from 
only one of these (dike 5) were 
thoroughly analyzed. The other 
series was checked sufficiently to show that the course of the setting season was iden- 
tical on the two grounds. Figure 23 gives a better picture of the abundance of larvae 
setting during different portions of the season. After setting had started on June 12, 
it continued for a period of about two weeks. Comparison of the graph (fig. 23) with 
table 21 is necessary for reaching an understanding of the season as a whole. The first 
bag of shells which caught spat was in the water for 15 days, from June 12 to 27, and 
caught a total of 6,065 spat, or an average of 404 per day. The bag which was in the 
water from June 30 to July 24 caught a negligible total of only 35 spat, and none at all 
was caught between July 10 and 18. The actual significant period of setting, there- 
fore, was between June 12 and 30, and during the last 10 days of this time the average 
daily catch was very slight. 
After this period, during which almost no larvae attached, a very profuse set 
began to take place. The table shows that the beginning of the second setting period 
was probably just after July 24, since the shells removed on that date bore a few spat. 
Between this date and August 3, when shells were brought in again, a great amount of 
setting occurred. The bags which were iu the water from July 30 to August 1 1 caught 
