APPENDIX E: HEINCKE _ 20 — 



the same region where the plaice eggs were found in March. It may be considered, that 

 a spawning ground has thus been discovered W. and N.W. from Heligoland between this 

 place and the 40 m. line. 



At the end of September, we caught a number of large plaice up to 60 cm. in length 

 on the Great Fisher Bank and north of this on the Fladen ground. Amongst these, the 

 spawn was running from one female of 60 cm. in length, and several males were nearly ripe. 



We have examined a large number of plaice with regard to the degree of matu- 

 rity of the sexual products. This is a method partly of distinguishing between the 

 ovaries and testes which have never spawned and those which have spawned at least once, i. e. it 

 can show the limits between the unripe and ripe plaice. Hitherto, this examination of the 

 sexual products has been the only sure way to determine ripeness; the signs given by 

 C. 6. Job. Petersen of the beginning of maturity — the appearance of rough scales on 

 the gill-covers and fin-rays in the males, and in both sexes the appearance of white rings 

 round the red spots — are uncertain. According to our observations, it is true, the 

 really ripe males always have rough scales to a more or less extent on the gill-covers 

 and fin-rays, in contrast to the quite smooth females, and this roughness is as a rule 

 quite absent from many young and still obviously unripe males; it is clear, therefore, that 

 this secondary sexual character, as a matter of fact, does develop at the age of so-called 

 puberty (first on the fins, then on the cheeks) , but it does not make its appearance at a 

 sufficiently well-defined period, but so gradually, that a sharp distinction of the ripe from the 

 unripe males is impossible by its means alone. The appearance of the white rings round 

 the red spots is quite useless for the determination of first-maturity; they occur rarely, it 

 is true, before this period, but they are also very often absent or extremely slightly deve- 

 loped in plaice distinctly mature for a long time, even as large as 60 cm. Probably these 

 rings are more strongly developed in the main, only at the actual spawning time. 



The investigation of the sexual organs enables us also, to ascertain the interval which 

 elapses between the end of one spawning period and the beginning of another, and the 

 rapidity with which the formation of new sexual products proceeds. We have observed, 

 that the most developed sexual products in the middle of July have reached stage IV on 

 our scale, the majority however, only II— III or III, that is, the eggs were clearly 

 recognisable with the naked eye or at most already flattened against one another. On 

 the 8*'' of July 1904, we caught 10 large plaice of 40 to 56 cm. in length, north from the 

 Dogger Bank, the sexual products of most of which were in stage III— IV, but one female 

 of 53 cm. and one male of 41 cm. were in stage VII, i.e. had not quite spawned, as 

 numerous ripe eggs were present in the mostly emptied ovary of the one, and some milt 

 still ran from the other. 



The migrations of the plaice in the German portion of the North Sea. 

 Direct positive results with regard to the migrations of the plaice, are only given by the 

 marking of the fish and setting them out again into the water. In the 2 years from the 

 2b^ of September 1902 to the 24^^ of September 1904, we have put out 4015 marked 

 plaice in all, 1764 with aluminium rings and 2251 with marks of vulcanised india-rubber. 

 To obtain the percentage of fish returned, we must deduct from these, 800 which were 

 only set out on the 23''^ of September 1904. Of the remaining 8215 plaice, 372 were 

 recaught, i.e. 11 •6% of the number set out. These numbers are much too small to give 

 a satisfactory basis for further conclusions, even with regard to the German Bight. We 



