232 BULLETIN OF THE BUREAU OF FISHERIES 
Table 16. — Occurrence of oyster larvae in 'plankton samples collected in Onset Bay during the period July 
20— August 28, 1927 1 
Surface 
Middle 
Bottom 
Number of samples in which oyster larvae were found . .. . _ 
16 
19 
14 
Maximum number of oyster larvae found in the sample. - 
75 
80 
35 
Minimum number of oyster larvae found in the sample _ - 
1 
1 
1 
Total number of oyster larvae found in all the samples. ._ ... 
143 
298 
102 
Average number of oyster larvae per sample - . ... .. . . . .. 
9 
15 
7 
1 74 samples that contained no oyster larvte are not included. 
The results of the examination of plankton samples made in 1927 are presented 
in Table 16, which shows the number of samples containing the oyster larvae and the 
maximum and minimum number of the larvae in the sample. All the samples are 
grouped in three groups according to the level of water from which they were taken. 
Of 123 samples collected by Perkins during the period from July 20 to August 28 and 
examined by the senior author, oyster larvae of various sizes were found in 49 samples. 
Altogether, only 543 oyster larvae were found in all the samples — the maximum 
number in one sample being 80. It is interesting that, in spite of the scarcity of 
the larvae in the plankton samples, setting, as will be shown later, was fair and in 
the zone between 0.5 and 2 feet above low water varied from 4,200 to 8,000 spat on 
a bushel of shells. Because of the small number of the larvae found in plankton, it 
was impossible to study their vertical distribution and to correlate their occurrence 
with the physical or chemical factors of the environment. An examination of the 
table shows, however, that the greatest number of larvae was found in the middle 
zone. 
Full-grown larvae were very scarce; out of 48 of them collected between August 3 
to 10, 34 were found in the bottom samples. (Table 17). The first spat was observed 
on shells on August 1, but setting continued until August 15. The heaviest setting 
apparently occurred on or about August 10. 
Table 17. — Occurrence of umbo larvae in plankton samples taken between August 8-15, 1927 
In the summer of 1928 quantitative plankton samples were taken by McMillin 
at different stations in the harbor. The first larvae were noticed on July 2; they had 
already passed the straight hinge stage and probably were from 4 to 6 days old. 
On July 12, when the temperature of the water was about 23° C., a distinct change 
was noticed in the character of the gonads of the adult oysters, indicating that they 
had released part of their spawn, but very few oyster larvae of various sizes were 
found in plankton until the end of July. A clear idea of the scarcity of the oyster 
larvae in plankton samples can be gained from an examination of Table 18, which 
contains the resxdts of the quantitative plankton collection made at frequent inter- 
