V THE INTERNATIONAL FISHERIES EXHIBITION 203 



destroying the smaller and more sickly individuals of 

 the carp-stock, and so prevent valuable vegetable and 

 insect-food from being consumed by individuals who 

 would either not survive for the three years, or would 

 show no growth proportionate to their consumption of 

 food. On the other hand, too large a number of pike 

 would reduce the total weight of carp, and leave much 

 of the minute food in the pond unconsumed, whilst a 

 large portion of it would have been converted into 

 pike-flesh instead of remaining as carp. 



In the limited area of the carp-pond there are a 

 great number of processes going on, which contribute 

 to the ultimate production of the 20,000 kilos, of carp- 

 flesh. The minute vegetable organisms are continually 

 feeding on the carbonic acid absorbed by the water 

 from the atmosphere, and on the nitrogen partly ex- 

 isting originally as nitrates and ammonia therein, 

 partly returned to the water by the excretion and 

 decay of its animal inhabitants. Minute worms and 

 Crustacea are feeding on these plants, and other larger 

 insects are feeding on these ; finally, the carp nourish 

 themselves on all these living things, and are to some 

 extent preyed upon by the pike. Definite physical 

 conditions, such as the presence or absence of a stream 

 in the pond, the extreme heat and cold of summer and 

 winter, and the presence of saline constituents in the 

 water, determine the excess or the absence of one or 

 another of the lower forms. If only one of these con- 

 ditions be varied, the whole balance may be upset. 

 An excessive growth of some minute plant, such as 

 an Oscillatoria, favoured by heat or by the destruction 



