SPAWNING AND SETTING OF OLYMPIA OYSTERS 
487 
abundant setting the bags of shells caught an average of only about 50 spat per day. 
There was some later set after the middle of August, but it was of small significance 
and samples taken are not complete enough to be included in the table and graph. 
Mud Bay is different from Oyster Bay in time of spawning, time of beginning of 
setting, time of maximum intensity of setting, and duration of the setting season. 
While Oyster Bay, Little Skookum, and Oakland Bay are closely similar with respect 
to setting seasons, Mud Bay is entirely different, and the results have to be 
presented separately. 
PERIODICITY OF SETTING 
In the foregoing account it was described, particularly with reference to Oyster 
Bay, that several periods of setting may occur during each season. Attempts to 
correlate these periods with conditions of salinity, pH, or temperature have resulted 
in no significant relationship. Local weather conditions appear to have little or no 
influence upon the setting of larvae, save in their effect upon water temperature which 
controls spawning and rate of larval development. A period of setting occurs generally 
as a matter of many days duration, seldom less than 2 weeks. In this locality it is 
not concentrated within a few days, as described by Prytherch (1929) for Long Island 
Sound. Ordinarily, only a few spat are found when a setting period begins, but 
during the following days the larvae attach more and more abundantly. 
The oyster growers have the problem of deciding when to plant cultch so that it 
will not be silted over or covered with organic growth before the larvae are able to 
attach. The system has always been in use to plant the cultch in advance of the 
time of setting, after it is known that spawning has started, so as to be certain that 
the cultch is in the water when setting begins. Naturally, it frequently occurred 
that cultch was planted far too early and the maximum catch of seeds was not obtained. 
During 1931, an opportunity was afforded to test roughly the depreciation in efficiency 
of cultch after being in the water for some time. 
Table 32. — Loss in 'percentage of efficiency of cultch after 9 days 
Bag number 
Date 
planted 
Date re- 
moved 
Number of 
spat 
Difference 
Dike 1 
3578 
July 18 
July 27 
July 18 
July 27 
Aug. 3 
Aug. 3 
Aug. 3 
Aug. 3 
22, 320 
40, 520 
28, 071 
39, 040 
Percent 
} 44. 91 
} 28.09 
3515 
Dike 6 
3577 
3516 
36. 50 
During the middle of the summer the water is typically relatively free of silt and 
organic growth, as compared with spring and early summer. In 1931 the second 
major period of setting began between the 25th and the 28th of July. Two bags of 
shells had been planted on July 18 and two on the 27th. All were removed on August 
3. The counts of spat on the two groups are given in table 32. It is assumed, for 
convenience, that the bags planted July 27 were placed in the water just at the correct 
time to obtain the maximum catch, though they may have been a little too late to 
get all of it. The other bags were planted 9 days earlier. When all bags were removed 
at the same time and the number of spat counted there was found to be a remarkable 
difference. One bag planted in dike 1 on July 18 caught a total of 22,320 spat, while 
the bag planted beside it on the 27 caught 40,520 spat. Similarly, in dike 5 the 
earlier bag caught 28,071 spat, and the later, 39,040. The average difference between 
