SPAWNING AND SETTING OF OLYMPIA OYSTERS 
499 
By planting cultch periodically throughout each season and allowing each unit to 
remain in the water for only a few days it was possible to obtain a picture of the fre- 
quency of setting at all times. The results, of course, represent the potential catch 
at any time, rather than the number of larvae setting on the commercial beds, for in 
the experimental work new shells were planted twice weekly to provide clean cultch 
at all times, while the older shells on the grounds are usually fouled and unfavorable. 
In this manner it was shown that in most oyster-producing bays the setting season 
is not limited to a short period early in the summer, as thought by many oyster 
growers, but is of several months duration. This information has resulted favorably 
for the growers for they no longer plant cultch well in advance of the beginning of 
setting, as was the previous practice. The fact that after setting starts there is still 
a week or more before the time of maximum setting gives them sufficient time for the 
planting of cultch. 
The setting season consists of several distinct periods which in certain bays are 
remarkably uniform from year to year. The first period of the season is followed by 
a second major setting period 6 to 8 weeks later. There is a marked parallel between 
tidal periodicity and periodicity in the setting of larvae. The peak of a setting period 
coincides generally with the maximum tidal range of a run of spring tides. Therefore, 
after setting begins, one may determine from the tide tables the time of the following 
extreme tides when the rate of setting will be at a maximum. It is probable that the 
total tidal range is not so much the important factor, but the incidence of extreme low 
tides without regard to the height of the following high tides. Of practical importance 
is the very prolific late setting period, which follows the first on the next third and 
fourth spring tide periods; for oyster growers are able to plant cultch at tins time, 
also, thereby improving their chance of obtaining a satisfactory catch of seeds. 
The exact reason for the control of setting by tidal periods is not now definitely 
known. The beginning of spawning, however, is associated with the tides, for the 
water warms more rapidly during spring tides. After the minimum water temperature 
reaches the critical level for spawning there appears to be no connection between 
further spawning and tides. Orton’s (1926) observation that a maximum of spawning 
occurred at about the time of fidl moon may in some instances apply also to the 
Olympia oyster, but analysis of data on spawning during several years indicates that 
maxima of spawning, as judged from the findings of newly spawned eggs or young 
embryos, occur during neap tides as well as during full-moon and new-moon tidal 
periods. The relation between setting of larvae and tidal periods appears not to be 
traceable to a similar correlation between spawning and tidal periods. 
It appears most likely that Prytherch’s (1934) work on the effect of copper brought 
into the bays with land drainage may be applicable to the Olympia oyster, also. He 
reached the conclusion, from both laboratory experimentation and field observation, 
that precipitation of copper from solution in the inflowing river water permits the 
mature larvae to absorb this substance which is required for setting and metamor- 
phosis. For this reason natural oyster beds are always found in relatively enclosed 
bodies of water which receive a considerable inflow of land drainage. A period of 
extreme low tides permits a more effective mixing of the fresh water with the sea 
water, providing the required mineral for the larvae. He found that most larvae 
attached during low and early flood tides in the surface layer of the water when the 
salinity was lowest and the rate of current very slow. 
In the present work it was found that the best set of spat was caught, on floating 
cultch, within about 2 feet of the surface of the water, and that with increasing depth 
the frequency of attachment became less and less, Although during summer there is 
