of the Fishery Board for Scotland. 



239 



decrease in growth-rate should exist in all the groups until the fishes have 

 become full grown. 



Another point of interest brought out by these experiments is the 

 growth of the young plaice at the various seasons of the year. The 

 average sizes in the A, B, and C groups in Table III. will also serve as a 

 measure in this respect. Group A represents the first year's growth , and 

 a comparison of the average sizes in the various series shows the gradual 

 growth from July till the following April. In the B group the growth 

 during the second year can be followed ; and the C group shows the pro- 

 gress during the third year at the various seasons. In all three groups it 

 will be noticed that the increase in length is greatest between May and 

 October, when it gradually diminishes and almost comes to a standstill 

 during the cold part of the year, from November to March. At that time 

 the growth rate begins to increase again till it reaches its height in the 

 middle of the summer. 



In Plate I. is given a graphic illustration of the gradual growth of the 

 plaice, during thirty-two months, from a size of 40'7 to 229*1 mm. when 

 the fishes w T ere about thirty-four months old. The black line (No. 1) 

 combines the average sizes for the various dates, as shown in Tables II. 

 and III. ; and the three yearly groups — A, B, and C — have been con- 

 nected in succession. The small irregularities in this line are caused by 

 undue fluctuations in the averages calculated from the captured fishes, and 

 have already been referred to. But the main curvature of the line is clearly 

 indicated, and has been represented by the red line in the Plate, which is 

 an approximate representation of the gradual growth of the plaice from 

 month to month for almost three years; and it shows clearly the alternate 

 increase and decrease in the growth-rate according to the season. 



The sum of the results of these observations may be said to be (1) 

 that the plaice during the first year of its life may be separated with 

 considerable accuracy from the previous year's fish, (2) that distinction 

 is also possible between the second and the third year's fish, although the 

 limits here are less marked, and (3) that the growth-rate of the plaice is 

 greatest in the summer, increasing in the spring and decreasing in the 

 autumn, while it is almost arrested during the winter. It rises and falls 

 very much in correspondence with the temperature of the sea on the 

 coast. 



The conclusions arrived at in the previous pages regarding the ijate of 

 growth of the plaice, particularly during the first and second years, are 

 based upon the results of systematically conducted fishing. As the 

 average sizes that have been quoted are deduced from the measurements 

 of well-defined groups, there are strong reasons why my results, relating 

 to the first two groups at any rate, ought to be fair'y correct represen- 

 tations of the actual facts. But as these results vary considerably from 

 those attained by Dr. Petersen regarding the growth-rate of the plaice in 

 Danish waters, I have found it necessary to subject his records to a 

 somewhat close examination. 



Dr. C. G. Joh. Petersen* has conducted some extensive researches 

 in the Kattegat in connection with the plaice, and he has furnished 

 valuable information as to the distribution of this fish and the minimum 

 size at which it becomes mature in these waters. By examination of a 

 large number of specimens Petersen has shown that the plaice in the 

 Kattegat spawns at a comparatively small size. Petersen has estimated 

 the average size of the mature plaice in the Baltic to be about 10 inches, 

 in the Lesser Belt about 17 inches, and in the Kattegat 12-13 inches at 

 least (p. 3). On the East Coast of Scotland the average size of the mature 



*Report of the Danish Biological Station IV., 1893. 



