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 Eiver 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 algae 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 C eratofliyTlum demersum^ Philotria canadensis, Pota- 

 mogeton pectinatus, Potamogeton foliosus, Vallisneria spiralis, and 

 Nymphcea. 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 supplied. 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 w^ork 

 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. 



