COUNCIL — APRIL 1912 — APPENDIX G — 118 — 



catches of the trawlers as are brought to land. In winter, on the other hand, the 

 number of small, unmarketable plaice taken is almost nil, probably due to the 

 fact that the small plaice hibernate buried in the ground, and are thus not caught 

 in the nets. This useless destruction of millions of undersized plaice, and its 

 possible prevention, must be considered as the real root of the plaice question. 



A third very important division of the General Report will deal with the 

 possibility of determining the so-called Fishing-coefficients, i. e. the question of what 

 percentage of the stock of plaice in the North Sea is annually captured by fishing. 

 The present annual quantity of plaice landed from the North Sea can be estimated 

 at about 200 million, ranging from 18 cm. and upwards. Is this a third, or a 

 half, or even more, of the total stock of plaice? It is important to be able to 

 determine this exactly, as also how far the quantity of fish annually 

 removed by capture is or is not replaced by natural increase. There are several 

 ways of finding this out. 1) The Hensen method, of determining by the catches 

 of the egg-nets the number of plaice eggs laid in the North Sea in the course of 

 a year ; then calculating therefrom the number of females that have laid these eggs, and 

 from this obtaining, with the help of the number of mature females estimated as 

 landed in a year, the fishing-coefficient. 2) The calculation of the fishing-coefficient 

 from the results of the so called marking experiments, the percentage of marked 

 plaice recaptured by fishery in the course of a year being considered equal to the 

 fishing coefficient. 3) Calculation of the catch-coefficient of the usual trawls, i.e., 

 that percentage of the plaice actually on the grounds which is caught by the 

 trawl in one unit of fishing power, (trawling hour, etc.) ; then estimating the number 

 of all the fishing units of the trawlers in the North Sea during one year, also 

 estimating the surfaces of the North Sea bottom which can be fished by trawl, 

 and how often one and the same stretch of ground is thus fished. 4) Calculation 

 of the so-called coefficient of mortality of the plaice in the North Sea, by deter- 

 mination of age, and the decrease per cent of the plaice from year to year in the 

 series, of the catches landed. The coefficient of mortality must be greater 

 than the fishing-coefficient. 



All the four methods of determining the fishing-coefficient here mentioned 

 have been tried, and the results will be dealt with in the General Report. As 

 regards the catch-coefficient of the nets I have myself undertaken investigations, 

 and also with regard to the coefficient of mortality from year to year, based 

 upon the fish brought to land; further details as to this are given in the now 

 completed portion of the General Report, (p. 148 of the English and 155 of the 

 German Edition). It has not yet proved possible to determine the fishing-coefficient 

 with the desired degree of exactness; it appears to lie between 20°/o and 50°/o. 



