496 
BULLETIN OF THE BUREAU OF FISHERIES 
themselves from many of the unfavorable environmental factors it must be considered 
that the chances of survival are greater than in the case of oviparous species, which cast 
the unprotected eggs into the open water. 
Table 35 . — Dates on which first spawning and first setting occurred 
Oyster Bay 
Mud Bay 
Date 
Spawn 
Spat 
Number of 
days 
Spawn 
Spat 
Number of 
days 
1931 
June 12 
June 16 
1932 
May 17 
June 26 
40 
May 25. 
July 7 
43 
1933 
do 
July 3 
47 
June 4 
July 25 
51 
1934 
Apr. 17 
June 4 
48 
Apr. 24 
June 16 
53 
1935 
May 5 
June 17 
44 
May 13_ 
June 29 
47 
The only accurate estimate available of the duration of the free swimming period 
of larval life is that obtained by Hori (1933) who grew the larvae in the laboratory by 
feeding them macerated sea lettuce ( Ulva ). He removed black larvae from the brood 
chambers and kept them in dishes of seawater at a temperature of about 20° C. and 
found that they 
reached full size 
and attached after 
22 days. The tem- 
perature of the 
water in Puget 
Sound is generally 
considerably lower 
and it is to be ex- 
pected that devel- 
opment o f larvae 
would proceed 
more slowly. 
Field observa- 
tions on the time 
when the first larvae and the first spat were found each season are summarized in table 
36. In Oyster Bay the interval varied during 4 years from 40 to 48 days, wliile in 
Mud Bay the extremes were 43 and 53 days, or about 4 days longer each year. Avail- 
able data do not permit an exact statement of the total time from spawning until 
setting, for it is most probable that natural conditions may cause it to vary from year 
to year. Water temperature necessarily is concerned in determining rate of growth 
and it is probable that development may be affected by the abundance of food material. 
Hori (1933) was able to grow larvae of Ostrea gigas by feeding them Chlorella pacifica, 
but larvae of 0. lurida did not thrive on this alga. The experiments of Amemiya 
(1926) indicated that salinity, also, is an important factor in the development of larvae 
of several species. It has been described that during the first 10 days larval develop- 
ment takes place within the maternal brood chamber and the free swimming period in 
Oyster Bay is therefore some 30 or more days in length. 
Although tables and graphs of both spawning and setting activities have already 
been described, a complete picture of the season of propagation is better presented, 
as in figures 40 and 41, by including measurements of both spawning and setting on the 
same graph. In figure 40, referring to Oyster Bay in 1932, the frequency of occurrence 
of gravid adults is shown as a histogram while the time and abundance of larvae set- 
ting is indicated by a trend line derived from the results obtained by sampling with 
Figure 40. — Frequency of spawning and setting during season of 1932 in Oyster Bay. Setting is 
indicated by a trend line derived from values given in figure 24. Tidal range is also shown. 
