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from July and August), rather than those for the whole year, will give us the best 

 relative view of the stock of plaice in the North Sea; the various parts of the North 

 Sea being at this time of year more or less equally fished by English vessels. There 

 is however, no essential difference between the composition of the landings for the 

 whole year and these summer landings. In this way the descending part of the 

 curve of the English landings (see Fig. i) should give us a picture, very inaccurate, no 

 doubt, but to some degree approximate, of the composition of the stock of plaice in 

 the North Sea, as far as concerns such parts thereof as can be fished with the trawl, 

 and excluding the northern North Sea. In any case, we have at present no better. 



As regards the probable absolute size of the plaice stock in the North Sea, various 

 means may be considered as possible in order to arrive at an approximately accurate 

 knowledge of same. In any case, it is here a question of determining the so-called 

 fishery coefficient, i. e., that fraction of the plaice stock annually removed by fishing 

 from the North Sea. The various methods here adopted are as follows: 



1. The method of determining, by quantitative catches with the vertical net, the 

 number of drifting plaice eggs annually spawned in the North Sea. From the number 

 of eggs annually spawned by the female plaice we arrive at the number of females, and 

 from the number of mature females we can again, with the help of the composition of 

 the plaice stock with regard to males and females of various sizes and degrees of ma- 

 turity, (which is presumed to be known) arrive at the absolute number of plaice of a given 

 length which are annually to be found in the North Sea. This method, which has been 

 employed by Hensen, appears theoretically feasible, but presents in practice so many 

 and serious difficulties, that no positive results of real value have as yet been obtained. 



2. Determination of the fishing coefficient by means of marked plaice. The experiments 

 with marked plaice, which have been extensively carried out in the North Sea since 

 1902, can be compared to experiments made with a ballot-box, containing a very large 

 number of white balls and a very small number of black (or marked) all mixed together. 

 If the latter are equally distributed among the white balls, and many lots of balls then 

 taken at random from the box, the proportion between the white and black balls shown 

 by the average of all the samples will be same as the proportion in the total contents 

 of the box. In other words, the white balls drawn will stand in the same proportion 

 to all the white, as the black balls drawn to all the black. The total of white balls 

 here represents the entire plaice stock of the North Sea, the number of white balls 

 drawn answering to the number of plaice caught each year, while the total number of 

 black balls corresponds to the number of marked plaice set free at the beginning of 

 the year, the black balls drawn representing those recaptured at the end of that period. 

 The former divided into the latter gives the percentage of marked plaice recaptured in 

 the course of a year, and this again is equal to the percentage of all those plaice in 

 the North Sea which can be taken by the trawl, which are caught in the course of a 

 year: this is the fishing coefficient. 



Experiment has shown, that of about 27,000 marked plaice set free during the 

 years 1902 to 1908 in the North Sea, about 6,000 plaice, or roughly 22 %, were 

 recaptured within a year of their liberation. Thus the average fishing coefficient in the 

 North Sea for the years 1902 to 1908 can be taken as 0.22. 



The value of this fishing coefficient, obtained from the marking experiments of all 



