THE GERMAN CARP IN THE UNITED STATES. 593 



clear; and it has several rather important economical bearings besides 

 the mere fact that it usually accompanies or is accompanied by the 

 uprooting of the aquatic vegetation. The constant roiliness of a body 

 of water that has theretofore been clear must be an important ecological 

 factor in determining the quantity and character of both the vegetable 

 and, at least secondarily, the animal life inhabiting it. This will 

 readily be appreciated when we consider that it decreases the amount 

 of light that would reach to any given depth, thus depriving plants at 

 that depth of at least a part of one of the most important conditions 

 for their growth, while in the second place the settling of the sedi- 

 ment upon the stems and leaves of the plants acts as a mechanical hin- 

 drance to the ordinary processes of respiration. Where the balance 

 is once upset in this way in a body of water where things have become 

 adjusted to a certain set of conditions, it is difficult to predict just 

 what results will follow in the readjustment to new conditions; but it 

 is safe to assert that practically all the living organisms in the water 

 will be influenced to some extent. Even if the vegetation were not 

 uprooted b}^ the stirring up of the mud of the bottom, it is probable 

 that its abundance would be greatly reduced by the constant roiliness 

 of the water. This would in all likelihood affect the plankton or free- 

 swimming organisms as well, and thus greatly reduce the natural 

 food supply of the fish. In the large bodies of water these conditions 

 are ameliorated to a large extent, since by the movement of the fish 

 from place to place they are often absent from a given locality for 

 considerable periods, thus giving the sediment an opportunity to settle 

 and allowing the water to become clear; and even in smaller areas the 

 fish are not feeding all the time. But it must be admitted that where 

 there are a comparatively large number of carp in a pond the water 

 is kept in an almost constant state of roiliness. In the case of run- 

 ning waters there is a further tendency to impoverishment in the 

 carr3dng away of the rich mud while it is held in suspension in the 

 water. There has been no direct evidence collected, so far as I am 

 aAvare, to show to what extent this may be effective. It has even been 

 claimed by some that dikes and dams are weakened in this way, by 

 the destroying of the vegetation that held the mud in place, and the 

 loosening of the mud itself. 



The roiliness of the water caused b}' carp in supply reservoirs 

 has, in a number of instances, proved to be a serious problem, and is 

 one which has to be met with promptness. The only practicable rem- 

 ed}' is the removal of the fish. In some places this can be done with 

 comparative ease bv persistent seining; but more often, especiall}^ in 

 large reservoirs which present a diversit}^ of conditions, this method 

 is not feasible. In some cases it may even be necessary, where the 

 disturbance is very great, to withdraw the water and drain the 

 reservoir for the purpose of getting rid of the carp. An interest- 



F. C. 1904—38 



