20 " U. S. BUREAU OF FISHERIES. 



water with copper sulphate in the proportion of 8 pounds to 1,000,000 

 gallons of water. In limestone regions, however, or where the water 

 contains a large amount of organic matter, the proportion of copper 

 must be increased, and the method is then not applicable to fish cul- 

 ture because a solution of the necessary strength is fatal to most 

 fishes. It is doubtful if any fish-culturist has attempted to retard the 

 growth of plants by this method. 



It is possible to retard the growth of plants in small ponds by keep- 

 ing the mud thoroughly stirred up. Submerse plants require light 

 in order to thrive. Tlie result is analogous to natural conditions 

 in streams like the Potomac River during seasons of frequent heavy 

 rains, when the water is almost constantly roily and in conse- 

 quence the gi'owth of vegetation 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 ren- 

 dered 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 a 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 erato'phyllum demersion^ Philotria canadensis^ Pota- 

 mogeton 'pectin/itus^ Potamogeton foUosus, Vallisnei^i spiralis, and 

 NymphaBa. 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. 



At the Cold Spring Harbor State fish hatcheiy on Long Island 

 the water supply is taken from a long, narrow^ pond which collexits 

 the springs in the immediate vicinity. For many years a nmnber of 

 carp have been confined in this supply pond for the purpose of keep- 

 ing it free from vegetation, especially algse, with very satisfactoiy 

 results. In this instance it was particularly objectionable because it 

 clogged the screens in the hatching troughs. 



The introduction of carp into breeding ponds with other fish is, 

 however, inadvisable for various reasons, the one of present concern 

 being that carp work chiefly on the roots of plants and in mud- 

 bottom ponds keep the water constantly roiled, a condition unfavor- 

 able 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 vesretable 

 growth now credited to the presence of carp. 



