148 



a preserved reoion, where all forms of trawling are prohibited 

 by the Scottish Fishery Board. 



Relative Proportions of the Age-grovps of Plaice in the various 

 years. 



Hitherto it has been rather suggested that plaice are 

 bigger or smaller in one year than in another because they grow 

 more or less rapidly. No doubt there are differences of such 

 a kind in various years, and, no doubt also, these differences 

 depend on a greater or less food supply. Just yet we have 

 very little information about variations in the food supply : 

 the subject is a very important one and it is being investigated, 

 for the North Sea, by the English Ministry of Agriculture 

 and Fisheries. What we have to point out here, however, 

 is that the differences in prevalent length between the plaice 

 of a certain ground from year to year (such differences as are 

 represented by the length-frequency distributions given in this 

 report) depend 2)rincipally upon the composition of the shoals 

 offish as regards the age-groups. This is the argument, worked 

 out very ingeniously by Dr. Hjort, for the Norwegian cod 

 fisheries. 



Consider, first, the lengths of the plaice belonging to 

 Age-group III (over three and under four years old) and taken 

 in Liverpool Bay in the years 1908-15. The sexes are not 

 separated, and the numbers of fish measured in each of the 

 months June to November (the Roman numerals at the heads 

 of the columns) are given separately. The mean lengths of the 

 plaice in each month are given at the bottom of the colmiins, 

 and we see that this increased from 23-0 cms. in June to 29-5 

 cms. in November. Thus we have a mean increase in size, 

 during the growing season of the year, of about 7 cms. (for we 

 must suppose that there was some growth in April and May, 

 for which months we have no data). 



