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countries, is only qualified. It would be greater, the more numerous were the marked 

 plaice liberated, and especially, the better these were distributed over the whole of the 

 North Sea, and that in a way corresponding to the local and periodical density of the 

 plaice stock. This has however, only to a certain extent been the case: the greatest 

 number of marked plaice were, it is true, set free in those areas having the greatest 

 density of plaice population, i. e., in the southern shore areas, but this was done, for 

 the most part, in large numbers only at certain points in a very large district, and in 

 quantities which were infinitesimal when compared with the great mass of the plaice 

 stock; moreover, they were often liberated at places and times at which a very inten- 

 sive fishery was being carried on. The regular distribution of the marked plaice 

 throughout the North Sea, as demanded by the nature of the experiment, and so neces- 

 sary for obtaining reliable average values, thus suftered considerably. It is probable that 

 in most cases, and especially where large numbers of plaice have been liberated at once 

 at a certain spot, they have been found, when recaptured, in too great relative density. 

 This would make the fishing coefficient appear higher than it really is, and the figures 

 0.22 are in this respect probably too high. On the other hand, there are a whole 

 series of circumstances which can and will reduce the fishing coefficient to below the 

 normal value: first of all the fact, that a certain number of marked plaice, although 

 recaptured, are not delivered up as recaptured fish, and do not appear in the list of 

 marked fish retaken; also, as can be shown, a marked fish may sometimes lose its 

 mark, and finally, a certain percentage of the marked plaice liberated die sooner or 

 later in the course of the first year, without being caught; their death being no doubt 

 to some extent due to the fact that they have suffered somewhat from being taken in 

 the trawl in the first instance, and have less vitality when set free. The loss thus caused 

 among the liberated plaice by these circumstances which prevent their recapture must 

 be regarded as considerable; it is even probable that the coefficient of mortality is 

 greater than the percentage of fish recaptured. It is certain, that from this point of 

 view the fishing coefficient of 0.22 must be regarded as a minimum, and will in reality 

 be higher; how much, it is difficult to say. I am of opinion that the average fishing 

 coefficient of the North Sea, i. e.. for the entire stock of plaice, can be presumed to lie 

 between the limits 0.20 and 0.40, with the most probable value 0.30 or 0,33. It must 

 however be borne in mind, that this fishing coefficient only applies to those sizes of fish 

 which are caught by the ordinary trawl, and as used for the marking experiments, i. e., 

 from abt. 12 cm. long and upwards. 



3. Determination of the so-called catch coefficient of the ordinary traivl, i. e., that per- 

 centage of the plaice actually to be found on any part of the sea-floor, which is taken 

 by a single haul of the net over that ground. This investigation is a modification of 

 the determination of the annual fishing coefficient of the North Sea. A large number 

 of marked plaice are distributed, as equally as possible, over a small, measured and 

 limited plaice-ground (of from one to two nautical square miles in extent), i. e., mixed 

 with the plaice already présent on the ground. The experimental ground is then im- 

 mediately fished by a steamer using tlie ordinary trawl, moving at the rate of speed 

 usual for fishing vessels, and working the ground, as equally as possible, in a series of 

 hauls. Presuming that the trawl here fishes each hour an approximately equal area of 

 the sea-floor (area of normal trawling hour) we can then calculate the proportion of the 



