— 49 — 



over 5 years old, i. e., fish over 35 cm. long, or the so-called "large" plaice, we get 

 a coefficient of mortality of 0.43, for plaice of over 6 years old (over 40 cm. long) 

 likewise 0.43. It is, unfortunately, impossible to make further estimates of any degree 

 of reliability, our knowledge as to the composition of the stock being insufficient. And 

 in particular, it is unfortunately impossible at present to estimate the coefficient of mor- 

 tality of that part of the plaice stock which includes fish of about 12 cm. long, or from 

 the completion of the second year of life; those which are taken by the ordinary trawl, 

 and upon which the fishing coefficient thus depends. We can only, with the aid of 

 the composition of the scientific hauls, get an approximate idea as to the height of 

 the coefficient of mortality for plaice of 20 cm. and upwards, i. e., plaice of an average 

 age of over three years, or all plaice from the beginning of the fourth year upwards. 

 This coefficient of mortality I estimate at about 0.70. It may be considered as certain 

 that the coefficient of mortality for that part of the stock which is subject to trawl 

 fishery, and which falls between the limits stated above, from the third year of life up- 

 wards, is considerably higher than 0.70, perhaps 0.80 or even more. 



These few possible determinations of the coefficient of mortality prove however, 

 with certainty, that the coefficient is higher, the more fish of the younger age-classes 

 are contained in the part in question of the stock; in other words, the small plaice 

 under four years old have a considerably higher mortality than the older fish. This 

 can to a certain extent be due to the fact that the small plaice have more enemies 

 than the larger fish, partly to the fact that they are fished with greater relative in- 

 tensity, or to both causes. The first reason appears to be sufficiently obvious, and is 

 very probably the case; the second is possible, but not yet supported by results of 

 marking experiments. In any case, the more or less reliable approximate coefficients 

 of mortality obtained can be regarded as the maximum possible fishing coefficient for 

 the same age-classes of the stock. We can thus probably say with certainty that the 

 fishing coefficient for the stock of plaice over four years old is in any case less than 

 0.47, and for plaice over three years old less than 0.70, for fish over two years of age 

 less than 0.80. It is however at present quite impossible to say how much less; we 

 do not know in what proportion the plaice which die a so-called natural death stand 

 to those which are destroyed by man, nor which of the two classes is the greater. As 

 long as we know nothing positive on this point, we may be allowed to consider botla 

 as equal. We thus obtain, for the stock of plaice over two years old (12 cm. long) 

 which are to be considered from a fishing point of view, a fishing coefficient of 0.40, 

 calculated from the coefficient of mortality 0.80. 



The various methods here employed for obtaining a reliable fishing coefficient, and 

 thus arriving at some calculation of tlie true size of the stock of plaice in the North 

 Sea give us, for the present, the following results: 



The North Sea trawl fishery is concerned with that part of the stock of plaice 

 which includes fish of two years old and upwards (about 12 cm.). Of tliis portion of 

 the stock between 20 and 40 "/o in number are probably taken each year by the trawl ; 

 on an average perhaps 30 or 33 %• This total trawling catch of North Sea plaice of 

 12 cm. and upwards we have already (see p. 43) estimated at 503 millions, of which 

 about 203 million fish are brought to market. The total stock, in tliose parts of the 

 North Sea which can be fished by the trawl, of plaice of the same sizes, i. e., from the 



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