DISEASE CONTROL 



tank or concrete pond, and dis- 

 solved by mixing. In such cases 

 it should be added at a rate of 1 

 p. p. m. Water containing mala- 

 chite green should be gradually 

 replaced with fresh water. If the 

 flow of water is such that the con- 

 centration of the dye wnll be re- 

 duced in the pond by about 50 

 percent during the first hour, no 

 toxic effect on fish should be no- 

 ticeable. The use of malachite 

 green requires further investiga- 

 tion. 



Dipping 



The third basic method of con- 

 trolling diseases and parasites in 

 bait fishes, hand dipping, could be 

 the subject of several volumes. 

 Suffice it to say that it is a danger- 

 ous treatment, for the solutions 

 used are powerful and relatively 

 concentrated. Hence, the differ- 

 ence between an effective dose and 

 a killing one is exceedingly nar- 

 row in view of our present lack 

 of knowledge concerning this very 

 common method of treatment. 

 When applied with extreme care, 

 dipping undoubtedly may be of 

 great value in controlling infesta- 

 tions of external parasites and in- 

 fections caused by certain types of 

 bacterial diseases, such as fin rot 

 and the eastern type of gill dis- 

 ease. Certainly, hand dipping 

 should never be applied to any 

 large number of fish unless there 

 is valid reason to believe that 

 some external parasite is causing 

 the losses. In the absence of such 

 reason, a dip should be applied to 



a small number of fish as an ex- 

 periment. If the percentage loss 

 in this experimental group does 

 not fall significantly below that of 

 the rest of the affected fish within 

 2 days after the experimental 

 treatment w as given, hand dipping 

 should not be used. 



Hand dipping, in the opinion of 

 the authors, is best done in a dip- 

 ping box. This apparatus con- 

 sists of a solidly constructed, water- 

 tight, w^ooden box— in cross sec- 

 tion about 2 inches narrower than 

 the hatchery troughs, about half 

 again as deep, and approximately 

 o feet long. The height reached 

 by a known volume of water is 

 legibly marked on the box. In the 

 dipping box is slung an inner com- 

 partment, with four ears which 

 are sufficiently wide to rest on the 

 top of the dipping box, yet suffi- 

 ciently narrow to permit the com- 

 partment to slip into the hatchery 

 troughs. This inner compartment 

 has two wooden sides which round 

 vertically at the ends, and the bot- 

 tom is covered with a galvanized, 

 coarse wire mesh. If a dip bath 

 is used that contains copper sul- 

 fate or acetic acid, the galvanized 

 screen should be painted with as- 

 phalt, since even traces of dissolved 

 zinc may be toxic to the treated 

 fish. The galvanized mesh, in turn, 

 is covered on the inside with bob- 

 binet, that is caught to the wire 

 mesh at a sufficient number of 

 points to keep it from floating off. 



The desired quantity of disinfect- 

 ant is w^eighed out and dissolved 

 in the dipping box which has pre- 



53 



