SPAWNING AND SETTING OF OYSTERS 
73 
are rare and grew faster than average, for the individual variation is large. Appar- 
ently the oldest spat set soon after the shells were placed in the water. Then, since 
the growth rate may be considered constant, the abscissal axis representing size was 
divided up into 22 equal parts, as shown in the figure, each representing a day. 
On the basis of these divisions the size range of the spat for each day was recorded 
and the measurements regrouped according to the day of setting. These data were 
then plotted as in Figure 16, which shows the relative number of larvae which set 
each day. Th is is the approximate distribution of the 300 which were measured. 
Since there were 1,800 spat altogether, by multiplying each value by 6 the total 
number of larvae setting daily per half bushel of shells was obtained. 
It would be simple if it were possible to establish the age readily simply by 
referring to the age and size curve (fig. 14) ; but this would be less accurate because 
the spat grow more rapidly at some times and at some places than at others, due to 
depth of water and consequent temperature, etc. The method above described 
appears to be much more serviceable in indicating the approximate times of most 
profuse setting. 
Similar counts and calculations were made throughout the summer with samples 
of shells which received a set. These results are grouped in three series according 
to location of the reefs on which the shells were planted and are presented graphically 
on the charts (figs. 10, 11, and 12) which show also salinity and pH of the water. 
SETTING PERIODS DURING SEASON 
Shells in wire bags were planted at two stations in the bayou. (Fig. 5, Stations 
I and II.) The main reef, on which Station II was located, is on one side of the 
channel and is relatively free from the tidal currents which flow back and forth. 
The “middle ground,” however, (Station I), is an isolated shallow plot directly 
in the path of the currents and might be expected to be subject to relatively rapid 
changes in the water which flows in from West Bay during flood tide and out from the 
upper end of the bayou during ebb tide. This difference between the locations of 
the two bayou stations is important, for, in some cases, shells at one of the places 
received set while those at the other did not. 
It has been pointed out that spawning was taking place at the end of March. 
From the observations of Prytherch (1929) that the larval period is about 15 days, 
it was expected that small spat would be found by the middle of April. However, 
no set was obtained on any reef during the entire month of April. During about six 
days at the middle of May (fig. 10) a period of setting took place at both stations 
in the bayou, but not in West Bay (fig. 11). At about the same period, or a few days 
earlier, a light set occurred in East Bay on Hanna Reef. At the end of June a few 
days of light setting occurred in the bayou, on the middle ground reef, but not on the 
main reef. Neither in West Bay nor in East Bay did any setting take place at this 
time. 
Early in July a short period of relatively heavy setting occurred in West Bay. 
(Fig. 11.) The scale of ordinates on this figure is different from the others, and each 
unit is 12.5 times as great, in order to show the heavy setting which took place later. 
At the same time young spat were not found on shells in East Bay. However, there 
was a light set on the middle ground reef in the bayou but not on the main reef. 
At the end of July and beginning of August a 2-week period of setting occurred in 
East Bay, followed after a few days by continuous heavy setting in West Bay, but 
