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grounds. Correspondingly the largest and oldest plaice are found, as a rule, farthest 

 away from land, but also these occasionally make more or less regular retrograde move- 

 ments towards the coast, partly in autumn, in order to reach the spawning grounds 

 lying nearer land, and partly in spring, after the spawning is over, in order to seek out 

 richer feeding-grounds. 



From these regular movements and migrations of the plaice in the North Sea, 

 which stand in distinct relation to the growth and reproduction of the fish, we obtain a 

 very characteristic form for distribution, which can be briefly expressed in the fol- 

 lowing rule, or, one might say, the following law of distribution for the Plaice. "■The 

 size and age of the plaice in a definite part of the North Sea are inversely proportional 

 to the density of their occurrence, but directly proportional to the distance of the locality 

 from the coast., and to its depth". That is to say: the younger and smaller the plaice, 

 the nearer they live to the coast, and for the most part, the shallower the water in 

 which they are found. Moreover, the younger fish being naturally more numerous than 

 those of greater age^ the greater is their density: the more so, as migrations of any 

 extent to the open sea involve considerable spreading of the shoals. This law of 

 distribution for the plaice is, however, not to be taken too literally. The unequal growth 

 of fish of the same age, the interruption of the general seaward migration by hiber- 

 nation and retrograde movements, crossings of direction, and various other local causes, 

 render the rule or law, as in all similar cases, strictly applicable only to the average 

 age and average size. Then also there appears the extremely characteristic phenome- 

 non, that wherever plaice occur, different ages and sizes are invariably found mixed 

 together, and this in greater degree the farther out to sea they appear. The great 

 mass of the whole local stock at any one place is however, invariably found to consist 

 of a single, or some few adjacent age classes. In the shallow zones of the southern 

 North Sea, at a depth of from lO to 25 metres, we find for instance, plaice of from 

 the first to the fifth year of life, with a length of from 6 to 40 cm., the great majority 

 however, being from 2 to 3 years old, and from 10 to 20 cm. long. Their density, cal- 

 culated according to the number taken by an ordinary trawl in the space of an hour, 

 may amount, on an average, to 300, varying according to time and place from 30 to 

 several thousand. On the Dogger Rank, on the other hand, far out at sea, plaice of from 

 16 to 70 cm. in length, and 3 to 20 years old and upwards, are taken: the majority 

 however, amounting to about 75 "/o, are more than 30 cm. long and over four years 

 old: the density per trawling hour varies from i to 12, and may average from 5 to 7. 



In spite, however, of the fact that the plaice of the North Sea are continually mo- 

 ving, migrations of very great extent, such as for instance from one side of the sea to 

 the other, or from the southern coast to the northern deeps never, or only quite excep- 

 tionally occur, at any rate, as far as is has been possible to judge from the numerous 

 marking experiments made. Of all the marked plaice recaptured, fully 90 "/o have been 

 retaken at a distance of not more than 50 nautical miles, and 96 Vo at not more than 

 100 miles from the spot where they were set free. This bears out the supposition that 

 there exists in all probability a considerable number of local forms of the plaice, which 

 are distinguished by morphological and physiological peculiarities and inhabit different 

 regions of the sea. In the North Sea in particular we can perhaps distinguish between two 

 slightly, though very characteristically varying local forms, differing for instance in their 



