THE GERMAN CARP IN THE UNITED STATES. 591 



lowering of the water level in the basin, might be the cause, though 

 it is difficult to see how that particular factor, even if it could be 

 proved to exist, would effect the vegetation as has been observed. It 

 would be expected as the result of such a lowering that the different 

 floral zones would not in most cases be destroyed, but would merely 

 reestablish themselves a little farther out from the original shores. 

 Since a similar decrease is being complained of in many parts of the 

 eountr}^, however, and within comparatively only very recent years, 

 we would expect to find the same cause in all cases, and would look 

 for some new factor coincident with the trouble. The planting and 

 astounding acclimatization and propagation of carp seems to have 

 introduced such a factor. 



Then, too, there must be examined the more direct evidence against 

 the carp. Vegetation has been rooted out of comparatively small 

 ponds and reservoirs, where close observations could be made, and 

 where apparently the only change in conditions that could account for 

 it is the introduction of carp. And, finally, we know that these fish 

 do root up many plants. In a pond where the carp were feeding in 

 large numbers I have seen the surface of the water quite well cov- 

 ered in places with the uprooted vegetation, among which were to be 

 seen whole plants of flags torn out bodily. In other places, when the 

 ValUsneria was still j^oung and did not reach nearly to the surface, I 

 have observed the leaves floating about, recently torn from the bottom. 

 Although it could not be determined with certainty in this case, it is 

 very probable that carp were responsible. The roiliness of the water 

 at the place served to strengthen the suspicion. 



One can not be too careful, however, in drawing conclusions of 

 this kind, since there are many opportunities to make mistakes. A 

 concrete example may serve to illustrate the point. I was wading 

 about in a little bay at the St. Clair Flats, where carp were abundant, 

 and noticed manj^ freshly torn up leaves of flags floating on the sur- 

 face. It looked very much at first as if this were the work of the carp, 

 but I later saw the agency at work — a muskrat, which dived to the 

 bottom, cut off' a leaf and brought it to the surface, floated there while 

 he ate the succulent lower end, and then left it, to go down after 

 another. These leaves were bitten off singly, however, w^hilo the flags 

 mentioned above as uprooted b}^ carp were torn up roots and all, prob- 

 ably not so much on account of direct pulling as hy having the mud 

 worked away from around the roots. '^ The male dog-fish {Amia calva) 



aUnfortuiiiitely it was found inexpedient to make an experimental test of the effect of carp upon 

 aquatic vegetation. Tliis could be done by having two similar ponds or enclosures in which con- 

 ditions are as nearly the same as possible. Into one of these should be introduced a certain number 

 of carp, while the other should be left without them. If this were done in the spring, for example, 

 an e.xact comparison could be made of the conditions in the two areas as the season advanced. The 

 greatest caution should be taken in seeing that all conditions, except the presence of the carp, should 

 be the same in the two enclosures. 



