APPENDIX E: HEINCKE _ 12 — 



90 foot head-line, as used on the German steam-trawlers. We vary the experiments with 

 this net, employing different kinds of ground-rope and different cod-ends (with different 

 sizes of mesh). Further, we employ a smaller otter-trawl of only 50 foot head-line, 

 the so-called Hjort's trawl from data given by Hjort. We have not yet used a beam 

 trawl on the Poseidon. On the motor boat of the Biological Station at Heligoland, we 

 employ mostly a small otter-trawl of 50 foot head-üne, also a small beam trawl. 



B. The method of determining the age of fishes 



We begin here with the determination of the age of the plaice and other flat-fish 

 by means of the otolith-rings. It was first pointed out by Eeibisch, that the otoliths of 

 the plaice showed alternating dark and white rings in addition to a white kernel by 

 reflected light, and that a dark and white ring together denoted a year's growth. 



Along with the Kiel naturalists, we have extended these investigations on the otoliths 

 to other fishes, and have deepened the entire investigation and examined the otoliths of 

 many thousands of fishes. 



If we examine, in the summer months, the young plaice of about 30 to 50 mm. in 

 length, which have been born without any doubt in the beginning of the same year and 

 therefore belong to the 0-group, according to the notation used by C. G. J oh. Petersen 

 and adopted by us, we find that the otolith by reflected light shows an inner white kernel 

 and an outer dark more transparent ring. The inner kernel consists frequently, further, of 

 an inner small, particularly white, kernel point, round this a thin darker ring — the inter- 

 mediate kernel ring, and a broad white ring — the kernel ring which forms the chief 

 mass of the kernel. If we now examine other young plaice of about 100 to 150 mm. in 

 length, caught on the same ground and at the same period of the summer when the 30 

 to 50 mm. plaice are taken, we find the otoliths very different; they consist now of white 

 kernel, dark ring, white ring, dark ring. Such plaice belong to the so-called I-group of 

 Petersen, that is, they have lived one full year. In plaice of 180 to 220 m. in length, 

 examined in the summer, the rings of the otoliths of the I-group are increased, by another 

 white and dark ring; we have here the Il-group of over two full years and so on. 

 Consequently, the number of the completed years a plaice has lived, is 

 indicated by the number of the white otolith-rings which are present 

 outside the kernel. 



This method of determination is, according to our thorough investigations, always 

 reliable. We find further , that the white rings contain more organic substance than the 

 dark, the latter more inorganic; also, that the former are formed in spring, the latter in 

 summer and autumn; and that the growth of the otoliths ceases probably for a time in 

 winter, from which arises a sharper boundary between an inner dark ring and the next 

 following white ring. 



In the plaice, the first 4 to 6 white rings succeeding the kernel, can usually be very 

 clearly distinguished. In larger and older plaice, the otolith becomes thicker and more 



