150 



and this is because the trawl-net employed (one of 6-inch mesh) 

 did not sample the fish of lengths 10 to 20 adequately, many 

 of the latter escaping through the meshes. If we had had 

 representative samples of plaice of Age-group II of this range 

 of lengths the mean lengths for June, July, and August would 

 have been reliable : as it is they are not reliable and have 

 not been stated. (This selective action of the nets used is a 

 troublesome source of error far too frequently neglected in 

 discussions such as this.) 



Age-group III, male and female, are, however, more 

 adequately sampled and perhaps we can depend on the mean 

 lengths. Allowing, then, for a certain growth in May (not 

 given in the data) we may conclude, from inspection of these 

 tables, that the mean growth-rate for Age-group III was about 

 7 cms. There is no reason for supposing that there is any 

 trustworthy evidence that plaice of Age-group III grew any 

 faster or slower in 1920 than they did in 1908-1915. If there 

 are distinct differences in the prevalent lengths of plaice 

 inhabiting Liverpool Bay in the period 1908-1915 and in the 

 year 1920 this cannot be said to be due to different rates of 

 growth. 

 Composition of the Plaice Stock as regards Age-groups. 



Now we consider the relative abundance of plaice belonging 

 to Age-groups II and III in all the fish measured, from the 

 Liverpool Bay region, in all the months, during various periods. 

 Sex is not distinguished. We have 



Age-group II Age-group III Ratio of II to III 



1908-1916 3,638 442 100 to 12 



1914-1916 630 73 100 to 12 



1920 928 802 100 to 86 



All these plaice were taken in the 6-inch mesh trawl-net and so 

 were the following ones, caught during 1909 and 1911 ; Age- 

 group I being also included in this series of measurements : — • 



