26 AQUATIC PLANTS IN POND CULTURE. 
must be increased, and the method is then not applicable to fish culture 
because a solution of the necessary strength is fatal to most fishes. 
It is possible to retard the growth of plants in small ponds by keep- 
ing the mud thoroughly stirred up. The result is analogous to natural 
conditions in streams like the Potomac River during seasons of fre- 
quent heavy rains, when the water is almost constantly roily, and in 
consequence the growth of vegetation is very much less exuberant 
than in dry seasons, when the water is comparatively clear. In ponds 
where much mud is carried in and held for a considerable length of 
time in suspension the growth of both alge and the higher plants is 
rendered practically impossible. The same variations in vegetable 
growth are noticeable where suction dredges have discharged their 
mud into streams formerly clear. This means—roiling of the 
waters—has been used with success in small natural ponds main- 
tained for other purposes, but is not known to have been applied to 
pond culture. 
Experience at various pond culture stations shows carp to be quite 
efficient in checking the growth of vegetation if given access to it 
early in the spring before it becomes excessive. At the Fish Lakes 
station several carp were placed in one of the partitions of a bass pond 
containing Ceratophyllum demersum, Philotria canadensis, Pota- 
mogeton pectinatus, Potamogeton foliosus, Vallisnerta spiralis, and 
Nymphea. When the pond was drawn in the fall, the bottom in this 
partition was absolutely destitute of any kind of vegetation. The fol- 
lowing season carp were not introduced into this pond, and the 
aquatic growth became as abundant as formerly. Observations at 
the Erwin station in one of the large ponds where a number of adult 
carp were confined revealed a great scarcity of aquatic growth, al- 
though similar ponds adjoining, which contained bass and other fish, 
were well supphed. The plants most abundant in this pond were 
Philotria canadensis and Potamogeton crispus. The introduction of 
carp into breeding ponds with other fish is, however, inadvisable for 
various reasons, of which it is to the present purpose that carp work 
chiefly on the roots of plants and in mud-bottom ponds keep the water 
constantly roiled, a condition unfavorable to the breeding of all pond 
fishes with the possible exception of the crappie. It is very probable, 
moreover, that the roiliness of the water is itself partly responsible 
for the retardation of growth credited to the presence of carp. 
REMOVAL OF VEGETATION TO PERMIT SEINING. 
For the removal of vegetation in ponds preliminary to the period- 
ical seining operations, the pond culturist must depend upon mechan- 
ical methods of clearing away the foliage. It is customary to begin 
taking out the young fish for distribution soon after their yolk sac is 
absorbed, or after the fry have been feeding but two or three weeks. 
