8 MAINTAINING FISHES 



Stance that represses reproduction (Swingle, 1956). 

 We may thus consider this material as a water pol- 

 lutant. This phenomenon is well enough recognized 

 that in hatcheries it is utilized as a means of pre- 

 venting goldfish from spawning too early in the spring. 

 The fish are crowded during the early spring and 

 moved to a less crowded condition when the operator 

 desires them to commence spawning. 



It has been shown (Rose, 1959) that tadpoles re- 

 lease into the water a material which inhibits the 

 growth of other tadpoles, and there is some evidence 

 that such a material may be produced by fishes. If 

 such is the case, the matter should prove to be of 

 great interest to aquarists. In order to illustrate the 

 possible nature of such substances we might review 

 some of the characteristics of the material that Rose 

 worked with in tadpoles. 



When tadpoles are crowded, one or two of a group 

 will greatly exceed the others in growth. 



The larger tadpoles produced something that in- 

 hibited the growth of the smaller ones. 



If small tadpoles in a separate tank were exposed 

 to water from a tank containing large tadpoles the 

 growth of the smaller ones was inhibited. 



Inhibiting is prevented by heating the water to 

 60° to 70° C, by filtering through fine filter paper or 

 by aging the water for a month. 



The presence of plants and other animals reduces 

 the inhibiting effect. 



The inhibitive agent is apparently produced only by 

 rapidly growing tadpoles. It is thus said to be a 

 feed-back phenomenon. 



The inhibiting agents are specific although inhibi- 



