SPAWNING AND SETTING OF OLYMPIA OYSTERS 
501 
low, and the shallow water often reaches 30° C., while during winter low tides occur at 
night and a temperature as low as about — 2° C. has been recorded. 
3. Salinity of the water on the oyster beds at high tide varies, in Oyster Bay, 
between about 26 p. p. m. in winter and about 29 p. p. m. in summer; in Mud Bay the 
range is about 27 to 29.5 p. p. m. Salinity of the surface water, however, is subject to 
greater variation. 
4. Hydrogen-ion concentration varies throughout the year from a pH of 7.7 to 
7.8 in midwinter to about 8.4 in late spring. It is probable that prolific growth of 
algae in spring, in the presence of fertilizing substances brought in by the winter rains, 
accounts for the high pH at tins time. 
5. Market-size oysters bear broods of 250,000 to 300,000 larvae. The number of 
larvae per brood depends generally upon the size of the maternal oyster. 
6. Generally each oyster produces one brood per season, but in some years as 
many as 50 percent bear second broods while in other seasons as few as 75 percent of 
the individuals spawn as females. Abortions of embryos frequently occur, however. 
7. Spawning of functional females begins in the spring when the minimum, or 
high tide, temperature reaches 12.5° to 13° C. 
8. Most broods of larvae are produced during a period of about 6 weeks at the 
beginning of the spawning season, though an occasional gravid individual may be 
found as late as October. 
9. An average period of 10 days is required for development within the branchial 
chamber from the time the eggs (diameter, 100m to 105m) are extruded from the gonad 
until straight-hinge veliger larvae (length of valves, 180m) are discharged. 
10. As compared with oviparous species, development of the larvae of 0. lurida 
is very slow, and the age of the various stages may be stated approximately as follows: 
1 day, blastulae; 2 days, gastrulae; 3 days, trochopliores ; 4 days, first conchiferous 
larvae with incomplete valves; 5 days, straight-hinge veliger larvae completely en- 
closed by valves 110m-120m long; 10 day, veliger larvae with valves 180m~185m long. 
11. The free-swimming period is 30 or more days in length and varies from year 
to year, probably according to water temperature. 
12. Larvae set most frequently on an under horizontal surface, while fewest catch 
on upper horizontal surfaces. A definite relationship exists between angle of surface 
and number of spat caught. 
13. This setting behavior of larvae is not due to a directive influence of light but 
to the swimming position whereby the larval foot projects upward. 
14. A special type of manufactured spat collector, designed to take advantage of 
these habits, is now in 'use commercially. 
15. In Oyster Bay the setting season consists of two distinct periods, 6 to 8 weeks 
apart. Secondary periods of setting may occur between these two or after the second. 
16. Setting seasons in Oakland Bay and Skookum Inlet are similar to those in 
Oyster Bay. In Mud Bay seasons are shorter and maxima occur at different times. 
17. Times of maximum frequency of setting fall within periods of spring tides 
when tidal range is greatest. 
18. On cultch suspended from floats most spat are caught at a distance of 1 to 2 
feet from the surface. This appears to be one reason why high grounds catch the 
most seeds. Floats filled with cultch are now being employed commercially to take 
advantage of these results. 
19. Few spat are caught at low tide, most when the tide is about half high. 
Frequency of setting appears to be associated with swiftness of current. 
