COUNCIL— APRIL 1912 — APPENDIX D — 74 — 



and captured of between 32 — 41 cm. was in all probability smaller than the per- 

 centage of those between 25 — 31 cm. and 42 — 50 cm. Dr. Johansen mentioned 

 the following: When specimens have reached a size of ca. 30 cm., a very consider- 

 able proportion of them move to other areas than those where fishing is carried 

 on with the greatest intensity, viz. the areas A and B, and migrate to other areas: 

 G, D and E. When proportionally more specimens of 42—50 than of 32—41 are 

 caught, this may be due to the fact that the former, which are nearly all mature, 

 move to certain spawning grounds, where the fishing is intense, while the group 

 of those from 32 — 41 do not send so great a contingent to the spawning grounds, 

 as a large number of them are still immature. 



Prof. D'Arct Thompson demonstrated that several tables in Prof. Heincke's report 

 indicated the decrease per cent in number of plaice from cm. to cm. as highly 

 variable in the different areas of the North Sea. In many areas it appeared that 

 the decrease per cent within certain limits of size was negative, which meant that 

 a. migration of specimens of certain sizes to the area in question had taken place. 



The form of a frequency curve is to a very great extent dependent upon 

 immigration to and emigration from the area concerned. 



When dealing with market measurements, due attention should be paid to 

 the character of the stock in the respective areas, as well as their extent. 



Prof. Thompson doubted very much whether Prof. Heincke had succeeded 

 in giving, by means of the English market measurements, a reliable view of the 

 plaice stock for the whole of the North Sea. 



Prof. Heincke stated that the table on p. 148 of the English edition did not 

 apply to the stock; tables 14, 15, 16 probably do not give correctly the conditions 

 of the stock. It is asked: is there at least given an exact picture of the fish landed? 

 If so, then the plaice from 30—40 cm. decrease, as regards the fish landed, more 

 than those from 40 — 50; this could however be a false picture, viz. if too few spe- 

 cimens had been measured of the medium sized plaice. It was difficult to decide. 



Prof. Heincke thought, however, that the plaice had been measured in fairly 

 correct proportions, i. e. in the proportions in which they were represented among 

 the fish landed. Errors in this respect no doubt exist but would in all probability 

 be found to cancel each other. In reply to Prof. D'Arcy Thompson, Dr. Heincke 

 stated that it was impossible to demonstrate the actual proportional decrease in 

 stock for each individual area, but only for the North Sea as a whole. In so 

 doing, it would, however, be necessary to take a certain month, and the month 

 selected should be one in which plaice of all sizes are actually to be caught, that 



