462 
BULLETIN OF THE BUREAU OF FISHERIES 
Figure 15. — Daily average and minimum temperature in Oyster Bay during summer 
of 1932 as related to the frequency of spawning and range of tide. Open portions of 
columns refer to white larvae or embryos, shaded portions to conchiferous larvae. 
In Oyster Bay in 1933 (fig. 16) the graph has a somewhat different appearance 
because spawning was relatively light. On May 18, 2 percent of the oysters bore 
newly spawned eggs, though the minimum water temperature was only about 12° C. 
On the 24th or 25th, how- 
ever, spawning started at a 
considerable rate and con- 
tinued thereafter. At this 
time the minimum temper- 
ature was about 13° C. and 
the average about 14° C. 
It is remarkable that the 
few oysters which spawned 
early did not retain the 
larvae, for it may be noted 
on the graph that the first 
conchiferous larvae, of 
about 5 days development, 
were not found until the 
31st. It is probable that 
a few oysters were able to 
discharge eggs but subse- 
quent low temperature 
caused abortions. Though 
this record is not as clear 
as could be desired, it in- 
dicates definitely that the first successful spawning took place when the minimum 
temperature reached approximately 13° C. 
During the same year, in Mud Bay, the beginning of spawning may be more 
closely correlated with a rise in minimum temperature from about 12° to over 13° C. 
(see fig. 17). The average 
temperature, at the same 
time, was about 14° C. 
The spring rise in tempera- 
ture in Mud Bay is charac- 
teristically late in compari- 
son with Oyster Bay and 
the entire breeding season is 
therefore later. In 1935, in 
Oyster Bay (fig. 18), spawn- 
ing started just after the 
minimum temperature 
reached a level of 13° C., 
although the average was 
between 15° and 16° C. 
These four series are selected for reproduction because they represent the most 
complete data at hand referring to specific localities. It is certain that it is not the 
low-tide temperature which initiates spawning in spring, for on many days preceding 
the time of beginning of spawning the water in the dikes would warm to 20° or 25° C. 
and remain so for several hours. It may be noted that, in the four cases presented, 
the average temperature varied over a wide range at the critical time. The minimum 
MAY JUNE JULY AUGUST 
Figure 16.— Daily average and minimum temperature in Oyster Bay during sum- 
mer of 1933 as related to frequency of spawning. Compare figures 15 and 18. 
