466 BULLETIN OF THE BUREAU OF FISHERIES 
Table 18. — Number of spat caught on plane glass surfaces as determined by the angle of the surfaces: 
0°, under horizontal; 90° , vertical; 180°, upper horizontal 
Average 
Angle of 
Area square 
Number of 
number of 
surface 
inches 
spat 
spat per 2,400 
square inches 
0° 
2, 400 
1, 195 
1, 195 
>45° 
1, 200 
42 
} 181 
2 45° 
1, 200 
139 
>90° 
2, 400 
6 
} * 
2 qo° 
2, 400 
16 
‘135° 
1,200 
1 
} 3 
2 135° 
1,200 
2 
180° 
2,400 
1 
1 
1 Perpendicular. 2 Parallel to general direction of current. 
A correct estimate of spawning activity of functional females was obtained by 
opening 100 adults two or three times weeldy throughout the season on selected typical 
beds. This mode of sampling has been carried on in two bays during 4 consecutive 
years. In 1931, the work was not begun in time to permit exact determination of the 
entire duration of the spawning season, but in table 13 the results of miscellaneous 
samples are given for comparison with later years. The table indicates, however, 
that after the end of May the proportion of adults bearing larvae became continuously 
less, until in July and August gravid specimens were only occasionally found. 
More complete data were obtained during the years 1932 to 1935 in both Oyster 
Bay and Mud Bay. Tables 14 to 17 summarize the results. In Oyster Bay two 
grounds were employed for sampling, one well up the bay, the other some distance 
below, or one high ground (dike 5) and one low ground (dike T). In Mud Bay samples 
were taken from dike B, a ground which is closely similar in all respects to most of 
those in the bay. The oysters were opened at the beds to eliminate the possibility 
of confusion due to the occurrence of spawning or abortion during transportation. 
In the tables the gravid specimens are divided into two groups, according to whether 
they bear unshelled embryos (E) or conchiferous larvae (C), in order to indicate more 
exactly the rate of spawning. W hi le the complete data are given in the tables, a 
more significant picture of spawning activity may be obtained from figures 15 to 18, 
in which the percentage of gravid specimens on each day is shown as a column, the 
shaded portion of which represents conchiferous larvae, the open portion the embryos 
up through the trochophore stage. The tables and figures are not quite complete in 
that they do not include the very occasional gravid specimens that may be found as 
late as October, but these are too few to warrant any attention. 
In the tables it may be noted that the time when spawning begins varies over a 
period of about a month, depending upon climatic conditions which control the 
temperature of the water. While, in Oyster Bay, the first oysters bearing larvae were 
found at about the middle of May in 1932 and 1933, in 1934 spawning started a full 
month earlier. In Mud Bay the oysters generally start spawning some time later 
than in Oyster Bay, even early in June during some years. 
Spawning goes on at a significant rate for a period of about 6 weeks, although 
larvae may be found in some adults for as long as 5 or 6 months. The reproductory 
activity is best shown in figures 15 to 18, which indicate that after it once begins, the 
frequency of spawning slowdy increases to a maximum, then gradually diminishes until 
gravid individuals are found only occasionally. During most years, the rate of spawn- 
ing may be represented with fair accuracy by a simple symmetrical distribution curve, 
though sometimes (fig. 15) there is a later, secondary wave of spawning. In 1932, 
in Oyster Bay this later spawning was considerable, though not as important as the 
