394 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



be fully matured in his fields. He knows that the quantity of field aud 

 garden produce is increased by manure, which supplies the vegetables 

 with the necessary food. He knows exactly how many heads of cattle 

 his land can feed. It is very disagreeable to him that he has to wage 

 a constant war against weeds, which threaten to diminish the room and 

 food needed for his cultivated plants, and against hurtful animals 

 which injure his domestic animals. It can also be shown by examples 

 from the fresh-water fisheries that the quantity of fish raised is depend- 

 ent on the quantity of food found within a certain territory. 



The estate of llagen in the Probstei [a district of Holstein] has a carp- 

 pond, measuring upwards of 80 hectares, which is drained for each 

 alternate period of three years, during which time oats and clover 

 are cultivated in it. At the end of three years the water is again let 

 into the poud, and 30,000 one-year old carp are placed in it, which after 

 three years generally yield 20,000 kilograms of food-fish. In order to 

 obtain still better results more than 30,000 young carp were once placed 

 in the pond, ^fter three years had elapsed the number of fish was 

 greater than heretofore, but in all they did not weigh more than 

 20,000 kilograms. The quantity of carp-food, therefore, which this pond 

 produced in three years had produced the maximum quantity of carp 

 of which it was capable. 



As the food of carp consists of tender vegetable matter, of insects, and 

 other small aquatic animals, ponds with a heavy bottom developing 

 rich vegetation and animal life are particularly suitable for carp-cul- 

 ture. In carp-ponds where fish are to be developed for the market 

 in as short a time as possible, there should always be some pike. These 

 fish of prey, by devouring the young of other less valuable fish and 

 spawning carp, remove from the pond useless destroyers of good carp- 

 food, and thus, by their voracity, further the development of the carp. 



A carp-pond is, therefore, not merely a body of water inhabited by 

 carp, but a community of different plants and animals, which in that 

 water find all the conditions of their well-being, and each kind of which 

 is represented by the greatest number of individuals which, under these 

 conditions, can be developed, for in all kinds of animals the number of 

 individuals in each period of propagation which reach full maturity is 

 smaller than the total of seed or eggs produced. 



Like the carp pond, every coast-water of our Baltic and North Sea 

 coasts is a community of living beings, which in various ways are mu- 

 tually dependent upon each other, and which, combined, exhibit the 

 maximum quantity of life which can be developed under the given circum- 

 stances or conditions. Among these conditions are: the temperature, 

 saltness, and movements of the water, the depth and character of the 

 bottom, the degree of fecundity of the various animals and plants, and 

 the migrations of some kinds of animals. 



That the combined effect of all these causes in the bays of our Baltic 

 coasts produces in the course of each year the full number of animals 



