- 48 - 



area covered in a single haul to the whole of the experimental ground, and thus also 

 the proportion of the plaice actually caught to those which should be present in the 

 area fished. This proportion is then, according to the analogy of the well-known ballot- 

 box experiment, the desired catch-coefficient for single hauls. According to the few 

 experiments which have been carried out in this direction (in Germany near Heligoland) 

 the coefficient of the ordinary trawl appears to be on an average about 0.25. It must 

 however be borne in mind that the difficulties in the way of obtaining reliable average 

 values by means of these experiments are manifold and serious, and we must reckon 

 with at least as wide a margin to either side as in the case of the fishing coefficient 

 calculated from the marking experiments. 



If it were possible to determine how often, on an average, each single area of 

 workable ground in the North Sea is fished by the trawl; whether once, twice or more 

 often, we should then have a means of calculating, from the catch-coefficient of single 

 hauls, a coefficient for the whole of the fishery. It is however, unfortunately impossible 

 as yet to calculate the extent of the fishing of the North Sea grounds with any degree 

 of reliability. The remarkable similarity between the size of the catch and fishing coef- 

 ficients would seem to suggest that each piece of practicable fishing ground in the North 

 Sea is only once fished by the net in the course of a year; since this would give a 

 catch coefficient equal to the fishing coefficient. On the other hand, the enormous num- 

 ber of first class trawlers now working the North Sea, and which may probably be 

 reckoned as fully 2,000, and the great number of trawling hours they lepresent per 

 annum, tend to show that each part of the North Sea is fished more than once a year 

 by the net, possibly twice or even more often. In such case the fishing coefficient would 

 be higher than the catch coefficient. Closer investigations upon this point are very 

 much to be desired; in the meantime I am inclined to suppose that the fishing coef- 

 ficient is probably higher than the catch coefficient, but not more than twice as high. 



4. Deteimination of the coefficient of mortality of the plaice stock. This coefficient of mor- 

 tality is to be considered as represented by that percentage of the stock of plaice which 

 dies each year. By "dying" is here understood, being caught, eaten, dying of illness, or by 

 any other natural means. The coefficient of mortality will thus in any case be higher than 

 the fishing coefficient, and its height will always form the maximum limit of the latter. The 

 coefficient of mortality of the stock could be calculated, if the composition in point of num- 

 ber of same, as regards plaice of the different age-classes, were known. Supposing that the 

 size of the whole stock, and its composition, remain unaltered for several years in succes- 

 sion, then as many plaice must die each year as there are annually born. For a por- 

 tion of the stock, e. g., for a series of successive age-classes, the coefficient of mor- 

 tality corresponds to the percentual number of the first of these age-classes; if, for 

 instance, the plaice in their fifth year amount to 47 % of all the plaice over 4 years 

 old in the whole stock, then the coefficient of mortality of these plaice over 4 years 

 old is 0.47. According to the explanation given on p. 45, we still lack the greater 

 part of the knowledge as to the composition of the plaice stock, which is necessary for 

 such calculations; we can only estimate it approximately for the classes of over 25 cm. 

 long and over 3V3 years old, and this, for the present, only on the basis of the Eng- 

 lish market measurements. From these we have the coefficient of mortality just quoted, 

 0.47 for plaice over four years old. For that part of the stock which includes plaice 



