SEA-FISHERIES LABORATORY. 249 
the increment is 9 inches, or 23 cms., very approxi- 
mately three times that of a single season’s growth. 
In the case of these fish which have undergone two or 
three seasons’ growth, the number of cases is too small 
for the calculation of a probable error. 
The Growth Rate from Month to Month. 
A knowledge of the above is of some importance, 
and I have made an attempt to deduce it from the data 
at hand. In dealing with these data it has been assumed 
that there is no difference between the plaice caught 
from the various regions of the Irish Sea with respect 
to their growth rate. Since the plaice originally caught 
on any one local fishing ground may migrate into any 
other, one is justified in making this assumption and in 
treating the statistical material as homogeneous. It is 
further assumed that all plaice lberated after the end 
of September and before the beginning of April may be 
considered together as constituting the material of one 
experiment, that is to say, that the amount of growth 
during those months is so small that it may be neglected. 
There is, of course, some growth during the last three 
months of the year, and it is probable that this amount 
of growth may be variable from year to year—for the 
length-weight co-efficient / does not vary in this 
manner—but we may, without serious error, proceed on 
this assumption. 
The following table gives the data and results of 
this estimation, and the method of calculation may 
briefly be described. 
