13 — 



APPENDIX E: HEINCKE 



opaque, and the new annual rings appearing become thinner and thinner, so that the deter- 

 mination of the age from the otoliths becomes more uncertain, even when sections are 

 made. As the majority of all the plaice taken in the North Sea are, however, not over 6 

 years old, this difficulty is not much met with in practice. 



It is especially remarkable, that the investigation of the otoliths almost always permits, 

 with sufficient certainty, the separation of the 0-group of plaice, i. e. those which have not 

 yet completed one full year and are thus in the year of their birth, from the plaice of 

 the I-group and later years. 



The results of our method in analysing a catch of plaice according to number, size 

 and age, and the value of this analysis for the determination of the age can be seen from 



Pig. 1. Analysis according to size and age, of a catch of plaice made with the 90-toot trawl 

 on Sylt inner-ground on March 19th 1904 (1024 specimens) 



the accompanying curves (Fig. 1). The .otoliths of all the plaice represented here (1024), 

 the result of a single catch, have been examined for the number of white rings. It will 

 be observed that, (1) groups of 5 years occur together, one however greatly predominant 

 in number, (2) the curves of sizes of the separate groups greatly overlap, (3) the maxima 

 of the curves 1, 2 and 3, representing the age, have exactly the same position as the 3 

 maxima of the curve of sizes and (4) the average difference in length between the I- 

 and Il-groups is about 6 cm., that between the II- and Ill-groups about 8 cm. From such 

 differences we may perhaps learn, in what year of life the plaice of the North Sea grows 

 most in length; to judge from this case, it is about 8 cm. on the average in the third 

 year against 6 cm. in the second year. 



In other flatfishes, especially however in the gadoid food-fishes, the otoliths do not 



