72 
BULLETIN OF THE BUREAU OF FISHERIES 
AGE Of SPAT IN DAYS 
station. While not accurate to the day these results may be considered significant 
within plus or minus two or three days. In order to obtain figures of the rate of 
growth of the shells of spat, measurements of the length of 128 living spat were 
made, and the shells to which they were fixed placed back in the water in wire bags. 
These spat were of diverse sizes at the start. After four to six days they were taken 
up and measured again, and the 
amount of growth divided by the 
number of days was taken as the 
growth per day. Since the original 
spat ranged from 1 to 15 millimeters 
in length, the method suffices to show 
the relative amount of shell growth 
per day with regard to the total size 
of the shell. The results are present- 
ed in Figure 13. The abscissae rep- 
resent the length of the shells of the 
spat at the beginning in groups of 1 
millimeter range, while the ordinates 
show the average growth in length. 
The amount of growth daily in- 
creases with size of the spat. Be- 
cause of the individual variation the 
graph is not perfect, so a theoretical line was drawn to represent the increment 
in length per day. While not intended to be mathematically accurate this is of 
use in making it possible to gain an idea of the day on which the spat actually 
set. Calculating from this theoretical line the length of spat at different ages, a 
curve (fig. 14) is obtained, which for the spat up to about 40 days old is to all prac- 
tical purposes a straight line. Certainly in proportion to the entire growth period 
of the oyster this would not be the case, but we are concerned here chiefly with spat 
of 20 days old or less. For this 
purpose it is considered that the 
growth or daily increase in size is 
constant. 
0 0.* as 1.2 1.6 2.0 2.4 2.6 3.2 16 4.0 4.4 4.6 522 5.6 6.0 64 66 7.2 76 60 8.4 
SIZE OF SPAT IN MILLIMETERS 
Figure 15. — Size distribution of 300 oyster spat on shells which were 
in water 22 days. At top of figure, divisions show age of spat with 
reference to size groups shown on lower axis 
DETERMINING DATES OF SETTING 
If a bag of shells bore a very 
heavy set, the spat were counted 
and then 300 of them were meas- 
ured. To do this the shells were 
not selected and all spat on each 
shell used were measured. An ade- 
quate distribution was obtained 
therefore. One example is present- 
ed in detail below. 
There were 1,800 spat on the half bushel of shells, and 300 of these were meas- 
ured and their size distribution plotted. (Fig. 15.) The figure shows the predomi- 
nance of the younger spat. The bag of shells had been in the water for 22 days, and 
the largest spat, therefore, could not be more than 22 days old. Some of the largest 
spat, according to the age curve (fig. 14) would appear to be older than this; but these 
Figure 16.— Graph showing the data given in Figure 15, recalculated 
to indicate the date of setting of the 300 spat. Setting was rare in the 
middle of July but became more abundant up until the 1st of August 
