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 
country, 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. Ina pond where the carp were feeding in 
large numbers I have seen the surface of the water quite well coy- 
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 
Vallisneria was still young 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 many 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, while the flags 
mentioned above as uprooted by carp were torn up roots and all, prob- 
ably not so much on account of direct pulling as by having the mud 
worked away from around the roots.¢ The male dog-fish (Ama calva) 
aUnfortunately it was found inexpedient to make an experimental test of the effect of carp upon 
aquatic vegetation. This 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 exact comparison could be made of the conditions in the two areas as the season adyanced. 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. 
